Patient education: Drug allergy (The Basics)
Written by the doctors and editors at UpToDate
What is a drug allergy?
— A drug allergy, also called drug hypersensitivity, is a bad reaction to a drug that happens when the body’s immune system responds to a drug as if it were a dangerous invader and tries to fight it off. The job of the immune system is to fight off infection. Normally, it should not react to a drug as though it were an invader, but some drugs 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 drugs can cause. Side effects can affect anyone who is given enough of a drug. Drug allergies, on the other hand, 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 with its own set of symptoms.
One serious type of allergy is called an “immediate” allergy because it starts quickly after a drug is taken (usually within an hour or so). It usually happens with drugs that a person had taken before without any problem. Symptoms can include:
●Hives, which are raised, red patches of skin that are usually very itchy (picture 1)
●Flushing, which is when your skin turns red and feels hot
●Swelling of the face, hands, feet, or throat
●Throat tightness, hoarse voice, wheezing, or trouble breathing
●Nausea, vomiting, belly pain
This type of allergy is serious because it can get worse if the medicine is continued. 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. This type is not very serious and usually causes a rash that begins after a few days of taking a drug. The rash often spreads over much of the skin (picture 2). Sometimes it is itchy, but sometimes not. This type of drug allergy does not involve swelling, trouble breathing, throat tightness, or the other symptoms listed above. This type of allergy doesn’t usually get worse or affect anything besides the skin. (A rare but severe skin reaction to a drug, called Stevens Johnson syndrome, is discussed separately.)
Should I see a doctor or nurse?
— That depends on what kind of reaction you have.
Call for an ambulance (in the US and Canada, dial 9-1-1) if you start a new medicine and develop any of the symptoms listed below:
●Wheezing or trouble breathing
●Chest tightness or pain
●Passing out or feeling as if you will pass out
●Swelling of your face, lips, tongue, or throat
Call your doctor if you start a new medicine and develop any of the symptoms listed below:
●Hives (raised red welts on the skin that are usually very itchy)
●Feeling like you want to hurt or kill yourself
●Severe stomach ache or vomiting
●Pain and irritation of the pink, moist tissue that lines the eyes, mouth, vagina, and other organs
How is a drug allergy treated?
— If you have a severe reaction to a drug you will be treated in a hospital. There your healthcare team will try to get your body back to normal. To do that, they might have to give you medicines to calm your allergic response. They might also need to give you oxygen to breathe through a mask, and fluids through a tube (called an “IV”) that goes into a vein.
After your symptoms are managed, your doctor will explore what to do about the allergy. If possible, your doctor might try to switch you to a different drug that is less likely to cause the same reaction. If that is not possible, he or she might send you to an allergist to decide what to do next.
Will I need tests? — Maybe.
If your allergist suspects you have an immediate drug allergy, he or she might send you for allergy skin tests. The most common is called a skin prick test. For this test, the doctor puts a drop of the drug you might be allergic to on your skin and makes a tiny prick in your skin. He or she then watches your skin to see if it turns red and bumpy. This is to check if you have a true allergy.
If you have a reaction that consists only of a rash (the type that is not serious), your allergist might want to do a “drug challenge test.” For this test, the allergist will have you take a small amount of the drug that caused the rash while he or she observes you. That way the doctor can find out if you will have the same reaction twice. Doctors usually do drug challenge tests only with medicines that are very useful, such as the antibiotic amoxicillin. Amoxicillin is the best choice of antibiotic for many common infections, so it would be a problem if someone had to avoid it forever.
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 do the following:
●Tell all your doctors and nurse, and anyone who might prescribe medicines for you about your allergy. Medicines sometimes have more than one ingredient and go by more than one name, so it won’t always be obvious if you are being prescribed the problem drug. Also, drugs are sometimes related to each other. If you are allergic to a specific drug, you might also be allergic to others that are related to it. If you tell your doctor or nurse about your allergy, he or she can try to avoid giving you any drugs that could cause problems.
●Always wear a medic alert bracelet or necklace explaining your drug allergy.