Active ingredients are those ingredients in a drug, vitamin or dietary supplement which cause an intended effect in the body. Other ingredients such as flavoring, coloring, preservatives and other substances that are not related to the intended effect are called inactive ingredients.

For example, one tablet of a pain reliever may contain 200-500 mg of an active ingredient such as aspirin, ibuprofin or acetominophen. The remainder of the tablet (often much of the pill's volume) is made up of other inactive ingredients that are not involved in pain relief. It is only the amount of active ingredient, not the size of the tablet, which determines how much "medicine" is in the pill. Thus, two different brands of pain reliever which each contain 350 mg of ibuprofin per tablet contain the same amount of active ingredient, regardless of any differences in pill size or appearance.

The form a drug or supplement takes can affect how easily the active ingredient is absorbed into your body, with liquids and inhalants typically entering the bloodstream faster than pills which must dissolve in the stomach before being released into the bloodstream.

Some drugs and supplements contain inactive ingredients such as sugar, lactose, sodium and alcohol. Although these are not "active ingredients" in the sense of contributing to the intended effect of a drug or supplement, these substances can indeed affect the body, and it is important to be aware of inactive ingredients if you are under any kind of dietary or medical restrictions.

Both active and inactive ingredients should be listed on the product label.