Drug Allergy

Drug allergy vs side effect vs intolerance

Recording side effects as allergies can lead to less effective or more expensive alternative treatments, so it is worth clarifying.

Types of allergic drug reactions

Immediate reactions (within 1 hour)

Driven by IgE antibodies: hives, itching, flushing, angioedema (deep swelling), wheezing, vomiting and anaphylaxis, a rapid, life-threatening whole-body reaction.

Delayed reactions (hours to weeks)

• Maculopapular rash ("morbilliform" or measles-like), the most common drug rash, appearing 4 to 14 days into treatment
• Fixed drug eruption: a round, dark patch that recurs in the same spot with each exposure
• Serum sickness-like reaction: rash, joint pain and fever
• DRESS (drug reaction with eosinophilia and systemic symptoms): rash, fever, swollen glands and organ inflammation 2 to 8 weeks after starting drugs such as allopurinol, anti-seizure medicines or sulfa drugs
• Stevens-Johnson syndrome and toxic epidermal necrolysis: rare, severe blistering and skin loss with mucosal sores; a medical emergency

Common causes

• Penicillins and other beta-lactam antibiotics: the most commonly reported drug allergy
• Cephalosporins: cross-reactivity with penicillin is lower than once thought (around 1% to 2% for most)
• Sulfonamide antibiotics such as trimethoprim-sulfamethoxazole
• NSAIDs and aspirin, which can trigger hives, swelling or asthma flares by a non-allergic mechanism
• Anti-seizure medicines: carbamazepine, phenytoin, lamotrigine
• Allopurinol
• Chemotherapy and biologic medicines
• Contrast dye and anesthesia drugs (usually non-IgE reactions)

Some severe reactions are linked to specific genes. For example, people with HLA-B*15:02 (more common in some Asian populations) have a high risk of SJS with carbamazepine, so testing is advised before starting it in at-risk groups.

Diagnosis

An allergist will ask:

• what drug, dose and route
• how long after starting the reaction began
• exactly what the reaction looked like, and how it was treated
• whether you have taken the drug or related drugs since

Tests include:

• Skin prick and intradermal tests for penicillin and some other drugs
• Oral drug challenge: a supervised test dose. For low-risk penicillin histories, a direct oral challenge without skin testing is now often used.
• Blood tests such as tryptase taken soon after an anaphylactic reaction
• Patch tests for some delayed reactions

Tests are not done for severe blistering reactions or DRESS; the drug is simply avoided.

Treatment

• Stop the suspected medicine (with your prescriber's advice).
• Antihistamines for hives and itching.
• Epinephrine for anaphylaxis, followed by emergency care.
• Steroids for some widespread or delayed reactions.
• Hospital care for SJS, TEN or DRESS.
• Drug desensitization: when no alternative exists (for example, penicillin for syphilis in pregnancy or certain chemotherapy), an allergist can give gradually increasing doses under close supervision to allow temporary safe use.

Living with a drug allergy

• Tell every doctor, dentist and pharmacist about your allergy and the reaction you had.
• Wear a medical alert bracelet for severe reactions.
• Keep a written record of the drug name and reaction.
• Ask about allergy testing if your label is old or uncertain, especially for penicillin.

Frequently asked questions

What are the symptoms of a drug allergy?

Immediate reactions cause hives, itching, swelling of the face or throat, wheezing and, in severe cases, anaphylaxis. Delayed reactions usually cause a rash days into treatment, sometimes with fever. Rarely, severe delayed reactions cause blistering skin or organ inflammation.

Can you suddenly become allergic to a medicine you have taken before?

Yes. Allergies can develop after previous safe use, because the immune system becomes sensitized with earlier exposures. A reaction can occur the next time you take the drug, even if you tolerated it before.

Am I really allergic to penicillin?

Probably not. About 10% of people report a penicillin allergy, but about 90% of them can tolerate it after testing, partly because sensitivity often fades over 10 years. An allergist can test you with skin tests or a supervised oral challenge.

What is the difference between a drug allergy and a side effect?

A drug allergy is an immune reaction, such as hives or anaphylaxis, that is not predictable and means the drug should be avoided. A side effect, such as nausea or diarrhea, is a known effect of the drug and usually does not prevent future use.