Numerous mechanisms are implicated in drug allergy and the incidence of immediate drug reactions (Type I) seems to be very low in comparison with allergy to more common allergens such as pollens and pets. The incidence of allergy to penicillins is 1/1000 administrations, i.e. 0.7 to 10% of treatments (3). Can you become allergic as an adult, even if you have never been so before?