A drug allergy is the reaction of the immune system to a medicine. Any medicine — over-the-counter, prescription or herbal — can trigger a drug allergy. However, a drug allergy is more likely with certain medicines.
The most common symptoms of drug allergy are hives, rash and fever. But a drug allergy also may cause serious reactions. This includes a severe, life-threatening condition known as anaphylaxis.
A drug allergy is not the same as a medicine side effect. A side effect is a known possible reaction to a medicine. Side effects to medicines are listed on their labels. A drug allergy also is different from drug toxicity. Drug toxicity is caused by an overdose of medicine.
Symptoms of a serious drug allergy often happen within an hour after taking a medicine. Other reactions, particularly rashes, can happen hours, days or weeks later.
Drug allergy symptoms may include:
Anaphylaxis is a rare, life-threatening drug allergy reaction that causes widespread changes in the way body systems function. Symptoms of anaphylaxis include:
Less common drug allergy reactions happen days or weeks after exposure to a medicine and may last for some time after you stop taking the medicine. These include:
Call 911 or emergency medical help if you experience signs of a severe reaction or suspected anaphylaxis after taking a medicine.
If you have milder symptoms of a drug allergy, see a healthcare professional as soon as possible.
A drug allergy happens when the immune system mistakenly identifies a medicine as a harmful substance, such as a virus or bacterium. Once the immune system detects a medicine as a harmful substance, it develops an antibody specific to that medicine. This can happen the first time you take a medicine, but sometimes an allergy doesn't develop until there have been repeated exposures.
The next time you take the medicine, these specific antibodies flag the medicine and direct immune system attacks on the substance. Chemicals released by this activity cause the symptoms associated with an allergic reaction.
You may not be aware of your first exposure to a medicine, however. Some evidence suggests that trace amounts of a medicine in the food supply, such as an antibiotic, may be enough for the immune system to create an antibody to it.
Some allergic reactions may result from a somewhat different process. Researchers believe that some medicines can bind directly to a certain type of immune system white blood cell called a T cell. This event causes the release of chemicals that can result in an allergic reaction the first time you take the medicine.
Although any medicine can cause an allergic reaction, some medicines are more commonly associated with allergies. These include:
Sometimes a reaction to a medicine can produce symptoms that are virtually the same as those of a drug allergy. However, a drug reaction isn't triggered by immune system activity. This condition is called a nonallergic hypersensitivity reaction or pseudoallergic drug reaction.
Medicines that are more commonly associated with this condition include:
While anyone can have an allergic reaction to a drug, a few factors can increase someone's risk. These include:
If you have a drug allergy, the best prevention is to avoid using the problem medicine. Steps you can take to protect yourself include the following:
An accurate diagnosis is essential. Research has suggested that drug allergies may be overdiagnosed and that patients may report drug allergies that have never been confirmed. Misdiagnosed drug allergies may result in the use of less appropriate or more-expensive medicines.
A healthcare professional typically does a physical exam and asks you questions. Details about when symptoms started, the time you took the medicine, and improvement or worsening of symptoms are important clues for helping your health professional make a diagnosis.
Your health professional may order more tests or refer you to an allergy specialist, called an allergist, for tests. These may include the following.
With a skin test, the allergist or a nurse administers a small amount of a suspect medicine to the skin with a tiny needle that scratches the skin, a shot or a patch. A positive reaction to a test often causes a red, itchy, raised bump.
A positive result suggests that you may have a drug allergy.
A negative result isn't as clear-cut. For some medicines, a negative test result usually means that you're not allergic to the medicine. For other medicines, a negative result may not completely rule out the possibility of a drug allergy.
A healthcare professional may order blood tests to rule out other conditions that could be causing symptoms.
While there are blood tests for detecting allergic reactions to a few medicines, these tests aren't used often because of the relatively limited research on their accuracy. They may be used if there's concern about a serious reaction to a skin test.
After looking at your symptoms and test results, a healthcare professional can usually reach one of the following conclusions:
These conclusions can help when making future treatment decisions.
Treatments for a drug allergy can be divided into two general strategies:
The following treatments may be used to treat an allergic reaction to a medicine:
If you have a confirmed drug allergy, a healthcare professional likely would not prescribe the medicine that causes a reaction unless it is necessary. Sometimes — if the diagnosis of drug allergy is uncertain or there's no other treatment — your health professional may use one of two strategies to give you the suspect medicine.
With either strategy, your health professional provides careful supervision. Supportive care also is available in the event of an adverse reaction. These treatments are generally not used if medicines have caused serious, life-threatening reactions in the past.
If the diagnosis of a drug allergy is uncertain and a healthcare professional judges that an allergy is unlikely, a graded drug challenge may be an option. With this procedure, you receive 2 to 5 doses of the medicine, starting with a small dose and increasing to the desired dose, also called the therapeutic dose.
If you reach the therapeutic dose with no reaction, then your health professional may recommend that you take the medicine as prescribed.
If it's necessary for you to take a medicine that has caused an allergic reaction, your care professional may recommend a treatment called drug desensitization. With this treatment, you receive a very small dose and then increasingly larger doses every 15 to 30 minutes over several hours or days. If you can reach the desired dose with no reaction, then you can continue the treatment.
See a healthcare professional if you have symptoms that may be related to a medicine you recently started taking or take regularly. Be prepared to answer the following questions. These details are important in helping determine the cause of your symptoms.
You may want to take pictures of any condition, such as a rash or swelling, to show your healthcare team. These may help if symptoms have eased by the time of your appointment.
