An active ingredient (AI), is the substance in a pharmaceutical drug or a pesticide that is biologically active. Terms in similar use include: active pharmaceutical ingredient (API) and bulk active in medicine, or in pesticide formulations active substance may be used. Some medications and pesticide products may contain more than one active ingredient. The traditional word for the API is pharmacon or pharmakon (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. Because homeopathic products no longer have any biologically active ingredients, their list of ingredients refers to the original ingredients used in their preparation and the finished product no longer contains any active ingredients.
Patients often have difficulty identifying the active ingredients in their medication, and are often unaware of the notion of an active ingredient. When patients are on multiple medications, active ingredients can interfere with each other, often resulting in severe or life-threatening complications. There now exist online services which can identify the active ingredient of most medications, such as the Medicine Name Finder developed by the National Prescribing Service.
In phytopharmaceutical or herbal medicine, the active ingredient may be either unknown or may require cofactors in order to achieve therapeutic goals. This leads to obvious complications in labeling. 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 John's wort 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.
Homeopathic products are prepared starting with active ingredients which are then serially diluted to the point where the product should no longer contain any molecules of those ingredients. Thus the product actually contains no "active" ingredients since there is no scientific evidence of any lingering biological activity. The list of ingredients refers to the original ingredients used in their preparation and the finished product no longer contains any active ingredients.