Patient education: Drug allergy (The Basics)

What is a drug allergy? — 

This is a bad reaction to a drug or medicine. It happens when the body's immune system responds to a medicine as if it were a dangerous invader and tries to fight it off. It is also called "drug hypersensitivity."

The immune system's job is to fight off infection. Normally, it should not react to a medicine as though it were an invader, but some medicines can cause that response in some people.

The symptoms of a drug allergy can include a skin rash, itching, swelling, or (in severe cases) trouble breathing or a drop in blood pressure that can cause a person to pass out.

A drug allergy is not the same as a drug side effect. Side effects are unintended or unwanted effects that medicines can cause. Side effects can affect anyone who is given enough of a medicine. Drug allergies affect only a small number of people.

What are the symptoms of a drug allergy? — 

There are a few different types of drug allergy. Each has its own set of symptoms.

One serious type of allergy, called an "immediate" allergy, starts quickly after a medicine is taken (usually within an hour or so). It usually happens with medicines a person has taken before without any problem. Symptoms can include:

●Hives, which are raised patches of skin that are usually very itchy (picture 1 and picture 2)

●Itchy skin

●Flushing, which is when the skin turns red and feels hot

●Swelling of the face, hands, feet, or throat

●Throat tightness, hoarse voice, wheezing, or trouble breathing

●Nausea, vomiting, or belly pain

●Feeling lightheaded

Immediate allergies are serious because the symptoms can get worse if the person keeps taking the medicine. It can turn into a life-threatening, whole-body allergic reaction, called "anaphylaxis."

Another type of drug allergy, called a "delayed" allergy, is much more common. It is not very serious and usually causes a rash that begins after a few days of taking a medicine. The rash often spreads over much of the skin (picture 3). Sometimes, it is itchy. This type of drug allergy does not involve swelling, trouble breathing, throat tightness, or the other symptoms listed above. It doesn't usually get worse or affect anything besides the skin. (Stevens-Johnson syndrome is a rare but severe skin reaction to a medicine and requires immediate medical attention.)

Should I see a doctor or nurse? — 

It depends on what kind of reaction you have.

Call for an ambulance (in the US and Canada, call 9-1-1) if you start a new medicine and develop any of these symptoms:

●Wheezing or trouble breathing

●Chest tightness or pain

●Passing out, or feeling like you might pass out

●Swelling of your face, lips, tongue, or throat

Call your doctor or nurse if you start a new medicine and develop any of these symptoms:

●Hives

●High fever

●Painful skin

●Skin blisters

●Pain and irritation of the pink, moist tissue that lines the eyes, mouth, vagina, and other organs

How is a drug allergy treated? — 

Severe reactions are treated in the hospital. There, your health care team will try to get your body back to normal. They might give you medicines to calm your allergic response.

After your symptoms are managed, your doctor will decide what to do about the allergy. If possible, they might try to switch you to a different medicine that is less likely to cause the same reaction. If that is not possible, they might send you to an allergist (allergy doctor) to decide what to do next.

Will I need tests? — 

Maybe. If your allergist suspects you have an immediate drug allergy, they might do allergy tests. The most common test is called skin testing. The doctor puts a drop of the medicine you might be allergic to on your skin and makes a tiny prick in your skin, or injects a very tiny amount. Then, they watch your skin to see if it turns red or bumpy.

If you have a reaction that is only a rash (the type that is not serious), your allergist might want to do a "drug challenge test." You will take a small amount of the medicine that caused the rash while they observe you. That way, the doctor can find out if you will have the same reaction twice. Doctors usually do drug challenge tests only with certain medicines that are used a lot and can't easily be replaced with a different medicine. For example, amoxicillin is the best antibiotic to treat many common infections. Because of this, it's important people only avoid it if they are truly allergic.

Can a drug allergy be prevented? — 

If you already know you have a drug allergy, you can reduce your chances of having problems again if you:

●Tell all your doctors and nurses, and anyone who might prescribe medicines for you, about your allergy. Tell your pharmacy, too. They will avoid giving you any medicines that could cause problems. Medicines sometimes have more than 1 ingredient and go by more than 1 name, so it won't always be obvious if you are being prescribed the medicine you are allergic to. Also, medicines are sometimes related to each other. If you are allergic to a specific medicine, you might also be allergic to others that are related to it.

●Wear a medical alert bracelet or necklace explaining your drug allergy.

More on this topic

Patient education: Penicillin allergy (The Basics)
Patient education: Sulfonamide allergy (The Basics)
Patient education: Contrast allergy (The Basics)
Patient education: Anaphylaxis (The Basics)
Patient education: Stevens-Johnson syndrome and toxic epidermal necrolysis (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: Medication safety (The Basics)
Patient education: Side effects from medicines (The Basics)
Patient education: Why taking your medicine as prescribed is important (The Basics)
Patient education: Taking medicines when you're older (The Basics)

Patient education: Allergy to penicillin and related antibiotics (Beyond the Basics)
Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)
Patient education: Significance of negative penicillin allergy skin testing (Beyond the Basics)