This is any serious reaction that happens during or after treatment with a medication. An adverse event can be moderate to severe (such as a heart attack or an allergic reaction to the medication). It can be harmful if it’s not managed right away, and might involve stopping treatment with the medication. The medication label and/or the paper inserts that come with the medication list adverse events. Always talk to your doctor or pharmacist if you are concerned you’re having an adverse event caused by your medication.