Active pharmaceutical ingredient (API) is the ingredient which is biologically active in pharmaceutical drugs (tablets, cream, capsules, injectables etc.) that produces intended effects. It is used in a finished pharmaceutical product (FPP), intended to carry certain pharmacological actions or otherwise have a direct effect in the diagnosis, cure, treatment, mitigation or prevention of various diseases, or in some cases in restoring, correcting or modifying human physiological functions.

Some drugs which can be used in combination therapies, have multiple active ingredients to treat different symptoms or act in different ways. In simple terms, it is the constituent that will have the desired pharmaceutical action or effect when administered.