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What is a drug allergy? — A drug allergy is a bad reaction to a drug or medicine. It happens when the body's immune system responds to a drug as if it were a dangerous invader and tries to fight it off. This is also called "drug hypersensitivity."
The immune system's job 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 has 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 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 you keep taking the drug. 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 different and requires immediate medical attention.)
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, call 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 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 the symptoms listed below:
●Hives (raised patches of 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 health care 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.
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, 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 type of testing is called skin testing. 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, 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 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 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, the antibiotic amoxicillin is the best choice of antibiotic for many common infections. Because of this, it's important that 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. 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 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, they can try to avoid giving you any drugs that could cause problems.
●Wear a medical alert bracelet or necklace explaining your drug allergy.
Patient education: Anaphylaxis (The Basics)
Patient education: Side effects from medicines (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: What you should know about antibiotics (The Basics)
Patient education: Stevens-Johnson syndrome and toxic epidermal necrolysis (The Basics)
Patient education: Brand versus generic medicines (The Basics)
Patient education: Taking medicines when you're older (The Basics)
Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)
Patient education: Allergy to penicillin and related antibiotics (Beyond the Basics)
Patient education: Using an epinephrine autoinjector (Beyond the Basics)
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