An active ingredient (AI), also active pharmaceutical ingredient (or API), is the substance in a drug that is pharmaceutically active. The term is similarly used in pesticide formulations where active substance is also used. Some medications may contain more than one active ingredient. The traditional word for the API is pharmacon (from Greek, adapted from pharmacos) which originally denoted a magical substance or drug.

In phytopharmaceutical or herbal medicine the active ingredient be a result of the interplay of a variety of constituents both acting on a pathogen and upon a variety of body systems involved in immunity. The API may be either unknown or may require cofactors in order to achieve therapeutic goals. One way manufacturers have attempted to indicate strength is to engage in standardization to a marker compound. However standardization has not been standardized yet: different companies use different markers, or different levels of the same markers, or different methods of testing for marker compounds.

The API may be either unknown or may require cofactors in order to achieve therapeutic goals. One way manufacturers have attempted to indicate strength is to engage in standardization to a marker compound. However standardization has not been standardized yet: different companies use different markers, or different levels of the same markers, or different methods of testing for marker compounds.

The API may be either unknown or may require cofactors in order to achieve therapeutic goals. One way manufacturers have attempted to indicate strength is to engage in standardization to a marker compound. However standardization has not been standardized yet: different companies use different markers, or different levels of the same markers, or different methods of testing for marker compounds.