penicillin, taken to treat a minor infection, instead gave me an itchy rash all over my body. So I respond automatically, and call out the common antibiotic. But I recently learned that this diagnosis could be wrong. Penicillin sensitivity can disappear over time, a fact researchers have known for years. So why hasn't my doctor told me to go get an official test? It could be because she doesn't actually know the allergy can fade.