Some people are allergic to some medications. Their bodies sense the drug as harmful and make antibodies to it. The antibodies bind to the drug to rid it from the body. That's a problem, because clumps of drug-bound antibody travel can harm body tissues or interfere with normal body function as they travel through the bloodstream.
Allergic reactions to drugs are often limited, causing skin rashes. But they can also cause problems in the kidneys, liver, joints, and blood. Some are severe enough to be deadly.
If you think you are having an allergic reaction, call your doctor right away. The most severe type of allergic reaction is anaphylactic shock. This is a life-threatening problem in which blood pressure drops and your airways become so narrow that you can't breathe. It requires emergency treatment.