The ingredients in the medicine that produce the therapeutic effect. Labels on over-the-counter products are required by law to identify all active ingredients and to identify and list quantities of certain ingredients, such as alcohol, whether active or not.
In botany, the specific chemical in a plant responsible for the drug action ascribed to the entire plant.
The main medicinal ingredient of an ointment or lotion, as opposed to the base.
The chemical component that gives a medicine its action.