Most drug allergies are caused by antibiotics, particularly penicillin, cephalosporins and sulfonamides. Penicillin is the most common cause of drug allergies, and is also the most common cause of drug allergy-induced anaphylaxis.
Most of the time, a negative reaction to a drug is not in fact a drug allergy, because it doesn’t involve the immune system. Though it may seem like an allergic reaction, it is really a non-allergic adverse reaction, and is either an undesired side effect or a sign of drug sensitivity. The most common causes of non-allergic adverse reactions are anticonvulsants, aspirin and NSAIDS, vaccines, diabetes meds and chemotherapeutic agents.