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

A dosage form of a drug is traditionally composed of two things: The API, which is the drug itself; and an excipient, which is the substance of the tablet, or the liquid the API is suspended in, or other material that is pharmaceutically inert. Drugs are chosen primarily for their active ingredients.

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. For instance St. Johnswort is often standardized to the hypericin which is now known not to be the "active ingredient" for antidepressant use. Other companies standardize to hyperforin or both, although there may be some 24 known possible active constituents. Many herbalists believe that the active ingredient in a plant is the plant itself.^[1]

Herbalist and manufacturer David Winston points out that whenever different compounds are chosen as "active ingredients" for different herbs, there is a chance that suppliers will get a substandard batch (low on the chemical markers) and mix it with a batch higher in the desired marker to compensate for the difference.^[2] This could result in a product that does not have the full spectrum of properties that would come from a properly selected and harvested herbal source.

• Pharmakos
• Medication
• Counterfeit drug
• Regulation of therapeutic goods
• Standardization
• Herbal medicine