An allergic reaction occurs when the immune system overreacts to a drug viewed as foreign. The first exposure allows antibodies to form, so reactions typically occur after repeated exposure, not the first. Common triggers are painkillers, antibiotics, anti-inflammatories, and antiseizure drugs. Symptoms range from mild skin reactions to life-threatening anaphylaxis. Penicillin is a frequent cause of drug allergy, with symptoms like hives, swelling, and asthma. Sulfa drugs can also cause allergic reactions like rashes, photosensitivity, and liver/kidney issues. Avoiding the culprit drug is key treatment for drug allergies.