An active ingredient (AI) is the substance in a pharmaceutical drug or a pesticide that is biologically active. The similar terms active pharmaceutical ingredient (API) and bulk active are also in medicine, and the term active substance may be used for pesticide formulations. 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.
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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.[1] 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.[2]
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 complications in labelling. One way manufacturers have attempted to indicate strength is to engage in standardization to a marker compound. However, standardization has not been achieved 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.[3]