Definition: The active ingredient(s) in a drug or medicine are the chemicals that create the intended effects, and any side effects. You can find the name of the active ingredient on your medicine's label, just below "Drug Facts." In 2006, the FDA changed the label design for medications. The new design lists the purpose of the active ingredient on the same line. For example, the purpose of the active ingredient acetaminophen is listed on the label as "pain reliever/fever reducer."
The active ingredient in Tylenol is acetaminophen.