Exploring Drug Allergies

What is a drug allergy?

Reactions to drugs can occur with both prescription and non-prescription medications. Most reactions to drugs are expected side effects and only rarely are the reactions due to a true drug allergy. The two are commonly confused and misunderstood. Here are some helpful guidelines to aid in differentiating between an allergic and non-allergic reaction to a drug:

What Causes a Drug Allergy?

Drug allergies can be due to either your genetic makeup or the way your body metabolizes a drug. These reactions can occur after previously tolerating the drug, however they can also sometimes occur with the first dose. It is possible to outgrow an allergy to a drug over time.

Penicillin allergy is one of the most common drug allergies. About 10% of the population is labeled as being allergic to penicillin, which places a significant impact on health related costs and outcomes. Individuals who experience a reaction in response to penicillin will not necessarily react to related drugs. Sulfa-based drugs are another common cause of drug allergy, however most people with sulfa-based antibiotic drug allergies do not react to non-antibiotic sulfa-based drugs.

What are Symptoms of Drug Allergy?

Allergic reactions to a drug can include either immediate or delayed symptoms involving itching, rash, hives, wheezing, shortness of breath, swelling of body parts, anaphylaxis, and lung, kidney, or liver damage. Anaphylaxis is a life-threatening reaction which happens immediately after taking the drug. This reaction can involve swelling of your body parts, hives, lightheadedness, loss of consciousness, wheezing, shortness of breath, throat closure, nausea, vomiting, diarrhea, serum sickness, or shock.

How are Drug Allergies Diagnosed?

Drug allergies can be difficult to diagnose as they often mimic symptoms of other diseases and can be difficult to discern between allergic and non-allergic symptoms. Your allergist will take a detailed clinical history and perform a careful physical examination. Skin testing is available for penicillin, however it is difficult to test many other drugs with this method. Drug challenges can be helpful if your allergist determines that an office based challenge would be appropriate for you.

Penicillin Allergy Testing

Penicillin Allergy Testing involves pricking the skin with two forms of penicillins (major and minor determinants of penicillin) then an intradermal test. If these tests are negative, then a dose of an oral penicillin (like Amoxicillin) is given, followed by an observation period. If there is no reaction, the patient is not at risk of having any serious immediate reaction and penicillins can be used when medically indicated.

Oral Challenges

Oral drug challenges are the gold standard in determining a drug allergy. If your allergist feels this test is right for you, you will be given increasing doses of the drug in question and monitored between each dose for signs of any reaction. The goal of the drug challenge is to tolerate a full dose of the drug without any immediate or delayed reactions of up to 24 hours. The drug challenge typically lasts for about 2-3 hours in the office.

What are Treatment Options?

For a mild reaction, simply discontinuing use of the drug can stop the reaction. For serious, life-threatening reactions, your doctor may provide antihistamines, corticosteroids, or epinephrine. Antihistamines can be taken preemptively if use of the drug is necessary for medical care.

Your physician may prescribe alternative medication for future use. If the drug you are allergic to is essential for your medical care, desensitization to the drug may be considered. This involves taking the drug in small but increasing amounts until you can tolerate the full dose. In order for desensitization to be effective, you need to continue to take the drug every day, or else you may lose tolerance.

Weihong Zheng, M.D.