Definition:
An unwanted or unexpected negative reaction to a medication or treatment that is used in an approved manner.
Adverse reactions can be mild, such as a simple rash, or severe and life-threatening in nature. They can occur immediately when a treatment is started, or develop over time. Some adverse reactions are common, and can be anticipated by your doctor, whereas others occur very rarely.
Examples:
Evelyn developed hives when she began the new medication to help with her lung cancer. Her oncologist told her this was an adverse reaction that some people have, and recommended a different medication.
