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Phentermine: Pediatric drug information

Phentermine: Pediatric drug information
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For additional information see "Phentermine: Drug information" and "Phentermine: Patient drug information"

For abbreviations, symbols, and age group definitions show table
Brand Names: US
  • Adipex-P;
  • Lomaira
Brand Names: Canada
  • RHO-Phentermine
Therapeutic Category
  • Anorexiant;
  • Central Nervous System Stimulant;
  • Sympathomimetic
Dosing: Pediatric

Note: Dosing is presented in terms of phentermine hydrochloride salt, not as phentermine base. Phentermine is available as 2 products (Adipex-P, Lomaira) which have unique dosing.

Weight management, short term

Weight management, short term: Note: For short-term use (up to 12 weeks) as an adjunct to diet and exercise in patients who cannot take preferred agents and who have a BMI ≥30 kg/m2 or patients with a BMI ≥27 kg/m2 and ≥1 weight-associated comorbidity (eg, dyslipidemia, diabetes mellitus, hypertension). Dose should be individualized to lowest effective dose.

Adolescents ≥17 years:

Adipex-P: Capsule, tablet: Oral: Usual dose: 37.5 mg once daily before breakfast or 1 to 2 hours after breakfast; with the tablet dosage form, a dose of 18.75 mg (1/2 tablet) once or twice daily may be appropriate in some cases.

Lomaira: Tablet: Oral: 8 mg 3 times daily 30 minutes before meals; in some cases, 4 mg (1/2 tablet) may be appropriate.

Dosage adjustment for concomitant therapy: Significant drug interactions exist, requiring dose/frequency adjustment or avoidance. Consult drug interactions database for more information.

Dosing: Kidney Impairment: Pediatric

Adolescents ≥17 years:

Adipex-P: Capsule, tablet:

eGFR ≥30 mL/minute/1.73 m2: There are no dosage adjustments provided in the manufacturer's labeling; systemic exposure may be increased; use with caution.

eGFR 15 to 29 mL/minute/1.73 m2: Maximum daily dose: 15 mg/day.

eGFR <15 mL/minute/1.73 m2: Avoid use (has not been studied).

End-stage renal disease (ESRD) requiring dialysis: Avoid use (has not been studied).

Lomaira: Tablet: There are no dosage adjustments provided in the manufacturer's labeling (has not been studied); systemic exposure may be increased; use with caution.

Dosing: Liver Impairment: Pediatric

Adolescents ≥17 years: There are no dosage adjustments provided in the manufacturer's labeling (has not been studied).

Dosing: Adult

(For additional information see "Phentermine: Drug information")

Weight management, short term

Weight management, short term (alternative agent):

Note: Avoid in patients with heart disease or poorly controlled hypertension (Ref). For short-term use (up to 12 weeks) as an adjunct to diet and exercise in patients who cannot take preferred agents and who have a BMI ≥30 kg/m2 or patients with a BMI ≥27 kg/m2 and ≥1 weight-associated comorbidity (eg, dyslipidemia) (Ref). Some experts use long-term in select patients (Ref). Dosing is presented in terms of the salt, phentermine hydrochloride (not as phentermine base).

Capsule, tablet (excluding Lomaira): Oral: 15 to 18.75 mg once daily or 30 to 37.5 mg in 1 or 2 divided doses (Ref).

Tablet (Lomaira only): Oral: 8 mg 3 times daily 30 minutes before meals.

Dosage adjustment for concomitant therapy: Significant drug interactions exist, requiring dose/frequency adjustment or avoidance. Consult drug interactions database for more information.

Dosing: Kidney Impairment: Adult

Capsule, tablet (excluding Lomaira):

eGFR ≥30 mL/minute/1.73 m2: There are no dosage adjustments provided in the manufacturer's labeling; systemic exposure may be increased; use with caution.

eGFR 15 to 29 mL/minute/1.73 m2: Maximum dose: 15 mg/day.

eGFR <15 mL/minute/1.73 m2: Avoid use (has not been studied).

End-stage renal disease (ESRD) requiring dialysis: Avoid use (has not been studied).

Tablet (Lomaira only): There are no dosage adjustments provided in the manufacturer's labeling (has not been studied); systemic exposure may be increased; use with caution.

Dosing: Liver Impairment: Adult

There are no dosage adjustments provided in the manufacturer's labeling (has not been studied).

Adverse Reactions (Significant): Considerations
Cardiac effects

Data regarding the cardiac effects of phentermine alone are limited and conflicting; available evidence suggests that cardiac effects are rare (Ref). Phentermine alone may cause increased blood pressure and tachycardia; arrhythmias (including prolonged QT interval on ECG, supraventricular tachycardia, ventricular tachycardia [polymorphic], and ventricular fibrillation); cardiomyopathy (atrial fibrillation-induced); and acute myocardial infarction have been reported in patients with no history of cardiovascular disease (CVD) (Ref). One study suggests that phentermine may not increase blood pressure long-term (Ref). Historically, serious regurgitant valvular heart disease (primarily affecting the mitral, aortic, and/or tricuspid valves) has been reported in patients receiving a combination of phentermine and fenfluramine or dexfenfluramine (combination removed from market) (Ref); rare cases of valvular heart disease with the use of phentermine alone have been reported to the FDA, and the possibility of an association cannot be ruled out. In a descriptive analysis using the FDA Adverse Event Reporting System (FAERS) databases, cardiovascular adverse reactions (unspecified) with phentermine accounted for 31% (n=542) of the total adverse reactions reported among all anti-obesity mediations between 2013 to 2020; the baseline CVD risk of the patients associated with these reports is unknown. In addition, due to limitations related to reporting, these data do not prove cause but rather suggest potential risk (Ref). Conversely, one retrospective study showed no increased risk of incident CVD or death in patients with no history of CVD who were taking phentermine alone for >3 months (Ref).

Mechanism: Not clearly established; possibly due to coronary artery vasospasm related to increased adrenergic activity from phentermine, primarily via norepinephrine (Ref).

CNS effects

Phentermine (alone and in combination) has been associated with CNS effects, including delirium, mania, and psychosis (Ref). Literature has reported insomnia, irritability, and anxiety in 24% to 27% of users (Ref).

Mechanism: Dose-related; related to the pharmacologic action; stimulates release of norepinephrine and dopamine (Ref).

Onset: Varied; typically occurs within 1 week of initiation (Ref) but may occur several months after initiation (Ref).

Risk factors:

• High dose (112.5 to 150 mg/day) (Ref)

• Personal or family history of psychiatric disease (Ref)

Primary pulmonary hypertension

Primary pulmonary hypertension (PPH) has been reported in patients receiving a combination of phentermine and fenfluramine or dexfenfluramine (Ref). The possibility of an association between PPH and the use of phentermine alone cannot be ruled out; at least one case has been reported (Ref).

Mechanism: Time-related; related to the pharmacologic action; hypothesized to be due to an increased 5-HT and subsequent vascular remodeling (Ref).

Onset: Delayed. In a case report with phentermine alone, onset of PPH occurred 3 months after the use of phentermine for 5 weeks (Ref).

Risk factors:

• Duration of use >3 months (Ref)

Adverse Reactions

The following adverse drug reactions are derived from product labeling unless otherwise specified.

Frequency not defined:

Cardiovascular: Increased blood pressure, ischemia, palpitations

Dermatologic: Urticaria

Endocrine & metabolic: Change in libido

Genitourinary: Erectile dysfunction

Nervous system: Dysphoria, euphoria, overstimulation

Postmarketing:

Cardiovascular: Acute myocardial infarction (Ref), cardiomyopathy (atrial fibrillation-induced) (Ref), prolonged QT interval on ECG (Ref), supraventricular tachycardia (Ref), tachycardia (Ref), ventricular fibrillation (Ref), ventricular tachycardia (polymorphic) (Ref)

Gastrointestinal: Constipation (Ref), diarrhea (Ref), gastrointestinal distress (Ref), unpleasant taste (Ref), xerostomia (Ref)

Nervous system: Anxiety (Ref), cerebral hemorrhage (Ref), dizziness (Ref), headache (Ref), insomnia (Ref), irritability (Ref), psychosis (Ref), restlessness (Ref), tremor (Ref)

Renal: Acute interstitial nephritis (Ref)

Respiratory: Primary pulmonary hypertension (Ref)

Contraindications

Hypersensitivity or idiosyncrasy to phentermine, other sympathomimetic amines or any component of the formulation; history of cardiovascular disease (eg, arrhythmias, heart failure, coronary artery disease, stroke, uncontrolled hypertension); hyperthyroidism; glaucoma; agitated states; history of drug abuse; use during or within 14 days following MAO inhibitor therapy; pregnancy; breastfeeding.

Significant drug interactions exist, requiring dose/frequency adjustment or avoidance. Consult drug interactions database for more information.

Warnings/Precautions

Disease-related concerns:

• Cardiovascular disease: Avoid stimulants in patients with known serious structural cardiac abnormalities, cardiomyopathy, serious heart rhythm abnormalities, or other serious cardiac problems that could increase the risk of sudden death that these conditions alone carry. Use with caution in patients with mild hypertension and other cardiovascular conditions that might be exacerbated by increases in blood pressure or heart rate.

• Diabetes: Use with caution in patients with diabetes mellitus; antidiabetic agent requirements (eg, insulin or oral hypoglycemic agents) may be decreased with anorexigens and concomitant dietary restrictions.

• Renal impairment: Use caution in patients with renal impairment; an increase in exposure is expected. Avoid use in patients with eGFR <15 mL/minute/1.73 m2, including end-stage renal disease (ESRD) requiring dialysis (has not been studied).

• Seizure disorders: Avoid or use with caution in patients with history of seizures (Apovian 2015).

• Tourette syndrome: Use with caution in patients with Tourette syndrome; stimulants may unmask tics.

Special populations:

• Bariatric surgery: Presurgical assessment of the indication for use, symptoms, and goals of therapy should be documented to enable postsurgical assessment. Monitor for continued efficacy and tolerability after bariatric surgery and consider switching to an alternative medication if symptoms worsen.

• Older adult: Use caution in this age group due to the risk for causing dependence, hypertension, angina, and myocardial infarction.

Other warnings/precautions:

• Abuse potential: Phentermine is pharmacologically related to the amphetamines, which have a high abuse potential; prolonged use may lead to dependency. Prescriptions should be written for the smallest quantity consistent with good patient care to minimize possibility of overdose.

• Tolerance: Tolerance to the anorectic effect usually develops within a few weeks; discontinue use when tolerance develops, do not exceed recommended dosage in an attempt to overcome tolerance.

Dosage Forms: US

Excipient information presented when available (limited, particularly for generics); consult specific product labeling. [DSC] = Discontinued product

Capsule, Oral, as hydrochloride:

Adipex-P: 37.5 mg [DSC]

Generic: 15 mg, 30 mg, 37.5 mg

Tablet, Oral, as hydrochloride:

Adipex-P: 37.5 mg [scored; contains corn starch]

Lomaira: 8 mg [scored; contains corn starch, fd&c blue #1 (brilliant blue)]

Generic: 37.5 mg

Generic Equivalent Available: US

Yes

Pricing: US

Capsules (Phentermine HCl Oral)

15 mg (per each): $0.34 - $1.57

30 mg (per each): $0.20 - $1.32

37.5 mg (per each): $0.33 - $1.57

Tablets (Adipex-P Oral)

37.5 mg (per each): $2.77

Tablets (Lomaira Oral)

8 mg (per each): $0.77

Tablets (Phentermine HCl Oral)

37.5 mg (per each): $0.19 - $1.54

Disclaimer: A representative AWP (Average Wholesale Price) price or price range is provided as reference price only. A range is provided when more than one manufacturer's AWP price is available and uses the low and high price reported by the manufacturers to determine the range. The pricing data should be used for benchmarking purposes only, and as such should not be used alone to set or adjudicate any prices for reimbursement or purchasing functions or considered to be an exact price for a single product and/or manufacturer. Medi-Span expressly disclaims all warranties of any kind or nature, whether express or implied, and assumes no liability with respect to accuracy of price or price range data published in its solutions. In no event shall Medi-Span be liable for special, indirect, incidental, or consequential damages arising from use of price or price range data. Pricing data is updated monthly.

Dosage Forms: Canada

Excipient information presented when available (limited, particularly for generics); consult specific product labeling.

Capsule, Oral, as hydrochloride:

Generic: 15 mg, 30 mg

Controlled Substance

C-IV

Administration: Pediatric

Oral: Avoid late evening administration. Scored tablets may be cut in half to achieve dose.

Adipex-P: Administer before breakfast or 1 to 2 hours after breakfast.

Lomaira: Administer 30 minutes before meals.

Administration: Adult

Oral:

Avoid late evening administration. Most effective when combined with a low-calorie diet and behavior modification counseling.

Capsules, tablets (excluding Lomaira): Administer before breakfast or 1 to 2 hours after breakfast. Tablets may be divided in half and dose may be given in 2 divided doses.

Tablet (Lomaira only): Administer 30 minutes before meals. Tablets are scored and may be divided in half.

Storage/Stability

Store at 20°C to 25°C (68°F to 77°F).

Use

Adjunct short-term (few weeks) treatment for chronic weight management along with a reduced-calorie diet and increased physical activity in patients with either an initial BMI of ≥30 kg/m2 or an initial BMI of ≥27 kg/m2 in the presence of other risk factors (eg, dyslipidemia, diabetes mellitus, hypertension) (FDA approved in ages ≥17 years and adults).

Medication Safety Issues
Sound-alike/look-alike issues:

Phentermine may be confused with phentolamine, phenytoin

High alert medication:

The Institute for Safe Medication Practices (ISMP) includes this medication among its list of drugs (contraindicated in pregnancy) which have a heightened risk of causing significant patient harm when used in error (High-Alert Medications in Community/Ambulatory Care Settings).

International issues:

Fastin [Puerto Rico] may be confused with Fastin dietary supplement [US]

Metabolism/Transport Effects

Substrate of CYP3A4 (Minor); Note: Assignment of Major/Minor substrate status based on clinically relevant drug interaction potential;

Drug Interactions

Note: Interacting drugs may not be individually listed below if they are part of a group interaction (eg, individual drugs within “CYP3A4 Inducers [Strong]” are NOT listed). For a complete list of drug interactions by individual drug name and detailed management recommendations, use the drug interactions program by clicking on the “Launch drug interactions program” link above.

Note: Interacting drugs may not be individually listed below if they are part of a group interaction (eg, individual drugs within “CYP3A4 Inducers [Strong]” are NOT listed). For a complete list of drug interactions by individual drug name and detailed management recommendations, use the drug interactions program

Acebrophylline: May increase stimulatory effects of CNS Stimulants. Risk X: Avoid

Alcohol (Ethyl): May increase adverse/toxic effects of Phentermine. Risk C: Monitor

Alkalinizing Agents: May decrease excretion of Amphetamines. Management: Consider alternatives to using amphetamines and alkalinizing agents in combination. If these agents must be used together, patients should be monitored closely for excessive amphetamine effects. Risk D: Consider Therapy Modification

Amifampridine: Agents With Seizure Threshold Lowering Potential may increase neuroexcitatory and/or seizure-potentiating effects of Amifampridine. Risk C: Monitor

Amisulpride (Oral): Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Amisulpride (Oral). Specifically, the risk of seizures may be increased. Risk C: Monitor

Ammonium Chloride: May decrease serum concentration of Amphetamines. This effect is likely due to an enhanced excretion of amphetamines in the urine. Risk C: Monitor

Antihypertensive Agents: Amphetamines may decrease antihypertensive effects of Antihypertensive Agents. Risk C: Monitor

Antipsychotic Agents: May decrease stimulatory effects of Amphetamines. Antipsychotic Agents may increase adverse/toxic effects of Amphetamines. Risk C: Monitor

ARIPiprazole Lauroxil: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of ARIPiprazole Lauroxil. Specifically, the risk of seizures may be increased. Risk C: Monitor

ARIPiprazole: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of ARIPiprazole. Specifically, the risk of seizures may be increased. Risk C: Monitor

Asenapine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Asenapine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Atomoxetine: May increase hypertensive effects of Sympathomimetics. Atomoxetine may increase tachycardic effects of Sympathomimetics. Risk C: Monitor

Benperidol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Benperidol. Specifically, the risk of seizures may be increased. Risk C: Monitor

Blonanserin: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Blonanserin. Specifically, the risk of seizures may be increased. Risk C: Monitor

Bornaprine: Sympathomimetics may increase anticholinergic effects of Bornaprine. Risk C: Monitor

Brexpiprazole: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Brexpiprazole. Specifically, the risk of seizures may be increased. Risk C: Monitor

Bromperidol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Bromperidol. Specifically, the risk of seizures may be increased. Risk C: Monitor

BuPROPion: May increase neuroexcitatory and/or seizure-potentiating effects of Agents With Seizure Threshold Lowering Potential. Risk C: Monitor

Cannabinoid-Containing Products: May increase tachycardic effects of Sympathomimetics. Risk C: Monitor

Carbonic Anhydrase Inhibitors: May decrease excretion of Amphetamines. Risk C: Monitor

Cariprazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Cariprazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

ChlorproMAZINE: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of ChlorproMAZINE. Specifically, the risk of seizures may be increased. Risk C: Monitor

Clothiapine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Clothiapine. Specifically, the risk of seizures may be increased. Risk C: Monitor

CloZAPine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of CloZAPine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Cocaine (Topical): May increase hypertensive effects of Sympathomimetics. Management: Consider alternatives to use of this combination when possible. Monitor closely for substantially increased blood pressure or heart rate and for any evidence of myocardial ischemia with concurrent use. Risk D: Consider Therapy Modification

Dihydralazine: Sympathomimetics may decrease therapeutic effects of Dihydralazine. Risk C: Monitor

Doxofylline: Sympathomimetics may increase adverse/toxic effects of Doxofylline. Risk C: Monitor

DroPERidol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of DroPERidol. Specifically, the risk of seizures may be increased. Risk C: Monitor

Esketamine (Injection): May increase adverse/toxic effects of Sympathomimetics. Specifically, the risk for elevated heart rate, hypertension, and arrhythmias may be increased. Risk C: Monitor

Esketamine (Nasal): May increase hypertensive effects of CNS Stimulants. Risk C: Monitor

Fluorodopa F18: Coadministration of CNS Stimulants and Fluorodopa F18 may alter diagnostic results. Management: Discontinue medications used to treat Parkinson disease, including dopamine reuptake inhibitors and dopamine releasing agents, such as psychostimulants, 12 hours prior to fluorodopa F 18 administration if these medications can be safely withheld. Risk D: Consider Therapy Modification

Flupentixol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Flupentixol. Specifically, the risk of seizures may be increased. Risk C: Monitor

FluPHENAZine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of FluPHENAZine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Gastrointestinal Acidifying Agents: May decrease serum concentration of Amphetamines. Risk C: Monitor

Guanethidine: May increase hypertensive effects of Sympathomimetics. Guanethidine may increase arrhythmogenic effects of Sympathomimetics. Risk C: Monitor

Haloperidol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Haloperidol. Specifically, the risk of seizures may be increased. Risk C: Monitor

Iloperidone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Iloperidone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Inhalational Anesthetics: Amphetamines may increase hypotensive effects of Inhalational Anesthetics. Risk C: Monitor

Inhibitors of the Proton Pump (PPIs and PCABs): May increase absorption of Amphetamines. Specifically, the amphetamine absorption rate may be increased in the first hours after dosing. Risk C: Monitor

Iobenguane Radiopharmaceutical Products: Amphetamines may decrease therapeutic effects of Iobenguane Radiopharmaceutical Products. Management: Discontinue all drugs that may inhibit or interfere with catecholamine transport or uptake for at least 5 biological half-lives before iobenguane administration. Do not administer these drugs until at least 7 days after each iobenguane dose. Risk X: Avoid

Iobenguane Radiopharmaceutical Products: CNS Stimulants may decrease therapeutic effects of Iobenguane Radiopharmaceutical Products. Management: Discontinue all drugs that may inhibit or interfere with catecholamine transport or uptake for at least 5 biological half-lives before iobenguane administration. Do not administer these drugs until at least 7 days after each iobenguane dose. Risk X: Avoid

Ioflupane I 123: Coadministration of Phentermine and Ioflupane I 123 may alter diagnostic results. Risk C: Monitor

Iohexol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Iohexol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iohexol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic antiseizure drugs. Risk D: Consider Therapy Modification

Iomeprol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Iomeprol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iomeprol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic antiseizure drugs. Risk D: Consider Therapy Modification

Iopamidol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Iopamidol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iopamidol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic antiseizure drugs. Risk D: Consider Therapy Modification

Kratom: May increase adverse/toxic effects of Sympathomimetics. Risk X: Avoid

Landiolol: Sympathomimetics may decrease therapeutic effects of Landiolol. Risk C: Monitor

Levothyroxine: May increase therapeutic effects of Sympathomimetics. Sympathomimetics may increase therapeutic effects of Levothyroxine. Levothyroxine may increase adverse/toxic effects of Sympathomimetics. Specifically, the risk of coronary insufficiency may be increased in patients with coronary artery disease. Risk C: Monitor

Loxapine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Loxapine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Lumateperone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Lumateperone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Lurasidone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Lurasidone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Methenamine: May decrease serum concentration of Amphetamines. This effect is likely due to an enhanced excretion of amphetamines in the urine. Risk C: Monitor

Methotrimeprazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Methotrimeprazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Molindone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Molindone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Monoamine Oxidase Inhibitors: May increase hypertensive effects of Amphetamines. While linezolid and tedizolid may interact via this mechanism, management recommendations differ from other monoamine oxidase inhibitors. Refer to monographs specific to those agents for details. Risk X: Avoid

Multivitamins/Fluoride (with ADE): May decrease serum concentration of Amphetamines. More specifically, the ascorbic acid (vitamin C) in many multivitamins may decrease amphetamine concentrations. Risk C: Monitor

Multivitamins/Minerals (with ADEK, Folate, Iron): May decrease serum concentration of Amphetamines. Risk C: Monitor

Multivitamins/Minerals (with AE, No Iron): May decrease serum concentration of Amphetamines. Specifically, vitamin C may impair absorption of amphetamines. Risk C: Monitor

OLANZapine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of OLANZapine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Opioid Agonists: Amphetamines may increase analgesic effects of Opioid Agonists. Risk C: Monitor

Paliperidone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Paliperidone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Periciazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Periciazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Perphenazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Perphenazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Pimozide: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Pimozide. Specifically, the risk of seizures may be increased. Risk C: Monitor

Pipamperone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Pipamperone. Specifically, the risk of seizures may be increased. Risk X: Avoid

Polyethylene Glycol-Electrolyte Solution: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Polyethylene Glycol-Electrolyte Solution. Specifically, the risk of seizure may be increased. Risk C: Monitor

Prochlorperazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Prochlorperazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Promazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Promazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

QUEtiapine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of QUEtiapine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Quinolones: Amphetamines may increase cardiotoxic effects of Quinolones. Risk C: Monitor

RisperiDONE: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of RisperiDONE. Specifically, the risk of seizures may be increased. Risk C: Monitor

Selective Serotonin Reuptake Inhibitors (Strong CYP2D6 Inhibitors): Amphetamines may increase serotonergic effects of Selective Serotonin Reuptake Inhibitors (Strong CYP2D6 Inhibitors). This could result in serotonin syndrome. Selective Serotonin Reuptake Inhibitors (Strong CYP2D6 Inhibitors) may increase serum concentration of Amphetamines. Management: Monitor for amphetamine toxicities, including serotonin syndrome/serotonin toxicity (eg, hyperreflexia, clonus, hyperthermia, diaphoresis, tremor, autonomic instability). Initiate amphetamines at lower doses, monitor frequently, and adjust dose as needed. Risk C: Monitor

Serotonergic Agents (High Risk): Amphetamines may increase serotonergic effects of Serotonergic Agents (High Risk). This could result in serotonin syndrome. Management: Monitor for signs and symptoms of serotonin syndrome/serotonin toxicity (eg, hyperreflexia, clonus, hyperthermia, diaphoresis, tremor, autonomic instability). Initiate amphetamines at lower doses, monitor frequently, and adjust doses as needed. Risk C: Monitor

Sertindole: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Sertindole. Specifically, the risk of seizures may be increased. Risk C: Monitor

Sodium Phosphates: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Sodium Phosphates. Specifically, the risk of seizure or loss of consciousness may be increased in patients with significant sodium phosphate-induced fluid or electrolyte abnormalities. Risk C: Monitor

Solriamfetol: CNS Stimulants may increase hypertensive effects of Solriamfetol. CNS Stimulants may increase tachycardic effects of Solriamfetol. Risk C: Monitor

Solriamfetol: Sympathomimetics may increase hypertensive effects of Solriamfetol. Sympathomimetics may increase tachycardic effects of Solriamfetol. Risk C: Monitor

Sulpiride: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Sulpiride. Specifically, the risk of seizures may be increased. Risk C: Monitor

Sympathomimetics: May increase adverse/toxic effects of Sympathomimetics. Risk C: Monitor

Tedizolid: May increase adverse/toxic effects of Sympathomimetics. Specifically, the risk for increased blood pressure and heart rate may be increased. Risk C: Monitor

Thioridazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Thioridazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Thiothixene: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Thiothixene. Specifically, the risk of seizures may be increased. Risk C: Monitor

Tricyclic Antidepressants: May increase adverse/toxic effects of Amphetamines. Tricyclic Antidepressants may potentiate the cardiovascular effects of Amphetamines. Amphetamines may increase serotonergic effects of Tricyclic Antidepressants. This could result in serotonin syndrome. Management: Monitor for signs and symptoms of serotonin syndrome/serotonin toxicity (eg, hyperreflexia, clonus, hyperthermia, diaphoresis, tremor, autonomic instability, mental status changes) and increased cardiovascular effects when these agents are combined. Risk C: Monitor

Trifluoperazine: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Trifluoperazine. Specifically, the risk of seizures may be increased. Risk C: Monitor

Urinary Acidifying Agents: May decrease serum concentration of Amphetamines. Risk C: Monitor

Ziprasidone: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Ziprasidone. Specifically, the risk of seizures may be increased. Risk C: Monitor

Zuclopenthixol: Agents With Seizure Threshold Lowering Potential may increase adverse/toxic effects of Zuclopenthixol. Specifically, the risk of seizures may be increased. Risk C: Monitor

Dietary Considerations

Capsules, tablets (excluding Lomaira): Should be taken before breakfast or 1 to 2 hours after breakfast.

Tablet (Lomaira only): Should be taken 30 minutes before meals.

Reproductive Considerations

Phentermine was evaluated in patients with infertility and overweight/obesity. Patients in the single center retrospective study (n=55) used contraception during therapy and were instructed to stop phentermine 1 month prior to trying to conceive. Although patients treated with phentermine had significant weight loss, pregnancy and live birth rates were not increased between patients who lost <5% or >5% of their initial body weight (Chang 2020).

Obesity increases the risk of infertility. Optimal weight control prior to conception improves pregnancy outcomes. However, medications for weight loss are not recommended prior to pregnancy due to safety issues and adverse events. Weight loss medications should be discontinued prior to conception (ACOG 2021; Wharton 2020).

Pregnancy Considerations

Information related to the use of phentermine during pregnancy is limited (D'Adesky 2019; Jones 2002; McElhatton 2006).

An increased risk of adverse maternal and fetal events is associated with obesity. However, moderate gestational weight gain based on pre-pregnancy BMI is required for positive fetal outcomes in all pregnancies, including patients with overweight or obesity. Therefore, medications for weight loss therapy are not recommended during pregnancy (ACOG 2021; Wharton 2020). Use of phentermine is contraindicated during pregnancy (lack of potential benefit and possible fetal harm).

Monitoring Parameters

Weight, BMI; CNS effects (eg, delirium, mania, psychosis); signs/symptoms of primary pulmonary hypertension (eg, new-onset dyspnea, chest pain, syncope, lower extremity edema).

Mechanism of Action

Phentermine is a sympathomimetic amine with pharmacologic properties similar to the amphetamines. The mechanism of action in reducing appetite appears to be secondary to CNS effects, including stimulation of the hypothalamus to release norepinephrine.

Pharmacokinetics (Adult Data Unless Noted)

Absorption: Well absorbed.

Distribution: Vd: 348 L.

Protein binding: 17.5%.

Metabolism: Hepatic via p-hydroxylation (aromatic ring) and N-oxidation (aliphatic side chain); primarily metabolized by CYP3A4 (but does not show extensive metabolism).

Half-life elimination: ~20 hours.

Time to peak: 3 to 4.4 hours.

Excretion: Primarily urine (62% to 85% as unchanged drug).

Pharmacokinetics: Additional Considerations (Adult Data Unless Noted)

Altered kidney function: Exposure increases can be expected in patients with renal impairment.

Brand Names: International
International Brand Names by Country
For country code abbreviations (show table)

  • (AU) Australia: Alenami | Duromine | Metermine | Phentermine gh | Phentermine juno er | Phentermine sandoz | Phentodur;
  • (BE) Belgium: Ionamin;
  • (CH) Switzerland: Adipex | Ionamine | Normaform;
  • (CL) Chile: Compulxine | Duromine | Fentex | Figuran | Finapet | Obexol | Sentis;
  • (CZ) Czech Republic: Adipex | Mirapront;
  • (DO) Dominican Republic: Duromine | Obexol | Supress p | Terfamex;
  • (EC) Ecuador: Duromine | Novocetonil | Obexol | Sentis;
  • (FI) Finland: Mirapront;
  • (GB) United Kingdom: Duromine | Ionamin;
  • (HK) Hong Kong: Duromine | Panbesy | Phentermine | Redusa;
  • (HU) Hungary: Adipex;
  • (ID) Indonesia: Mirapront;
  • (IE) Ireland: Ionamin;
  • (IL) Israel: Novirasin | Razin;
  • (KR) Korea, Republic of: Adipex | Artmin | Bm zine | Bmzine | Daehwa nobse | Dietamin | Festin | Furimin | Hanbul phentermin | Hutermin | Hutermine semi | Lediphen | Lowcal | Metamax | Panbesi | Panbesy | Pender | Penimin | Penmin | Pentazen | Pentmin | Phedin | Phenkini | Phentazen | Phentazia | Phentemy | Phentermine | Ptm | Retis | Sinsphen | Slemin | Tintin | Weltmine;
  • (LT) Lithuania: Adipex | Mirapront;
  • (LU) Luxembourg: Ionamine;
  • (LV) Latvia: Adipex | Mirapront;
  • (MX) Mexico: Acxion | Acxion ap | Acxion c | Definitum | Disebsin | Fentermina | Ifa acxion | Ifa-reduccing s | Ina10 | Novagrit | Raamcaps | Reducap | Sinpet | Terfamex | Terfamex od;
  • (MY) Malaysia: Adipex | Axcel phentermine | Duromine | Ionamin | Panbesy;
  • (NO) Norway: Adipex-p;
  • (NZ) New Zealand: Duromine | Metermine | Umine;
  • (PA) Panama: Terfamex;
  • (PE) Peru: Obexol;
  • (PH) Philippines: Duromine | Ionamin | Phentermine;
  • (PL) Poland: Mirapront;
  • (PR) Puerto Rico: Adipex-p | Fastin | Ionamin | Lomaira | Phentermine HCL;
  • (PY) Paraguay: Obexol | Sentis | Terfamex;
  • (SG) Singapore: Duromine | Ionamin | Panbesy | Umine;
  • (SK) Slovakia: Adipex;
  • (TH) Thailand: Duromine | Ionamin | Mirapront | Panbesy | Phentermine;
  • (VE) Venezuela, Bolivarian Republic of: Mirubal;
  • (ZA) South Africa: Duromine | Minex | Minobese | Musalos | Panbesy
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