Penicillin allergy

About 1 in 10 people in Canada carry a penicillin allergy label, meaning that an allergy to this antibiotic is recorded in their medical file or reported in their medical history. However, research consistently shows that the vast majority do not have a true allergy — and that removing an inaccurate label can improve treatment options and contribute to better health outcomes.

Why is this label so often wrong?

• The reaction happened long ago: True penicillin allergy fades over time. Most people who had a genuine reaction will lose their sensitivity within 5 to 10 years.
• Side effects were recorded as allergy: Certain adverse reactions such as nausea, stomach upset, or diarrhea are sometimes reported as allergic reactions, although they are in fact common side effects rather than true allergies.
• A viral rash and not a drug reaction: Children who develop a rash while taking amoxicillin are often reacting to the underlying infection, not the antibiotic. This is especially common with viral illnesses and is not in itself a sign of allergy. However, some skin reactions can represent a true allergic reaction and require allergy evaluation to confirm this.
• No testing was ever done: In most cases, the label was based on a reported symptom alone, with no formal allergy testing to confirm it.

You may have a low-risk penicillin allergy if your past reaction involved a mild rash during childhood, a rash affecting only a small area of the body, or an unexplained reaction that occurred more than 10 years ago. Most of these reactions are lost over time or may not have been caused by penicillin in the first place.

Why can this label have an impact on your health?

When you carry a penicillin allergy label, doctors must use alternative antibiotics that are often less effective, more expensive, and broader in spectrum. This can increase your risk of side effects — including serious gut infections such as C. difficile — and contributes to antibiotic resistance. Patients with this label also tend to have longer hospital stays and higher rates of surgical site infections.

What is "delabeling" and how does it work?

Delabeling is the process to safely determine whether you are truly allergic to penicillin and whether you actually need to avoid it. It is carried out by an allergist (or another trained health care professional) and is individualized to your history. It may involve:

• A review of your reaction history: often the first step, and sometimes sufficient on its own.
• A supervised oral challenge: you swallow a small amount of penicillin and are monitored. If there is no reaction, you do not have an allergy. This test is safe, has been performed in thousands of patients worldwide, and is the gold standard for low-risk cases.
• Skin testing: small amounts of penicillin applied to the skin. This is reserved for specific situations, such as a history of severe skin reaction, anaphylaxis or a recent reaction.

If your test shows you no longer have a penicillin allergy, your doctor will have more antibiotics to choose from — making it more likely you receive the most effective treatment and recover more quickly.

What if I truly am allergic?

If testing confirms a true allergy, your doctor will clarify exactly which antibiotics to avoid and advise on safe alternatives. Many related antibiotics (such as cephalosporins) are well-tolerated, as cross-reactivity is lower than previously believed.

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Dr. Ana-Maria Copaescu, MD FRCPC

Updated 06/2026