Allergic to medications is caused by hypersensitive immune system. Body becomes sensitive when exposed first time to drug. Second time exposure or further will trigger immune reaction. Allergic reaction may be divided from side-effect or light irritation (skin rashes, irritation, and nausea) to the effect that can bring to the deaths such as anaphylactic shock.
Penicillin and related antibiotic are the ordinary cause of this allergy case. The best way to avoid this allergy is to avoid from using those medications.