The self-reported prevalence of penicillin allergy is about 10%. The actual prevalence of true anaphylaxis is what percent?

• A. 0.02-0.04%
• B. 0.2-0.4%
• C. 2-4%
• D. 0.001-0.005%

Answer: (A)

The important idea is that many people who think they’re allergic to penicillin are not actually at high risk for a severe reaction when given penicillin. True IgE‑mediated anaphylaxis is very rare, even among those who report an allergy.

The best estimate for true anaphylaxis from penicillin exposure is about 0.02–0.04%, which is roughly 2–4 cases per 10,000 exposures. This shows how much smaller the real risk is compared with the self‑reported rate of penicillin allergy (about 10%).

The other options imply much higher or much lower rates than what is typically observed. A rate around 0.2–0.4% or 2–4% would correspond to tens to hundreds of reactions per ten thousand exposures, which is not supported by data. A rate as low as 0.001–0.005% would be far rarer than what studies have found.