How do you know if you are allergic to a drug?

Itchiness, swelling: For a true drug allergy, look for signs of a growing rash anywhere on your body within an hour of taking your medication. These rashes can manifest as hives, itchiness, or swelling. The most concerning of the symptoms would be swelling, particularly in the upper body as it may constrict breathing. Discuss with your pharmacist about the likelihood of a certain drug to cause such a reaction.