An active ingredient (AI) is the ingredient in a pharmaceutical drug that is biologically active. The similar terms active pharmaceutical ingredient (API) and bulk active are also used in medicine, and the term active substance may be used for natural products. Some medication 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.
The term active constituent is often chosen when referring to the active substance of interest in a plant (such as salicylic acid in willow bark or arecoline in areca nuts), because the word ingredient in many minds connotes a sense of human agency (that is, something that a person combines with other substances), whereas the natural products present in plants were not added by any human agency but rather occurred naturally ("a plant doesn't have ingredients").
In contrast with the active ingredients, the inactive ingredients are usually called excipients in pharmaceutical contexts. The main excipient that serves as a medium for conveying the active ingredient is usually called the vehicle. Petrolatum and mineral oil are common vehicles.
The dosage form for a pharmaceutical contains the active pharmaceutical ingredient (API), which is the drug itself, and excipients, which are the substances 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.
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 Medicines database providing information on medications available in Australia. harish 
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.
- Regulation of therapeutic goods
- Northeast of England Process Industry Cluster
- Lee, Dennis; Marks, Jay M. (ed.) (April 14, 2009). "Drug Interactions: Know the Ingredients, Consult Your Physician". MedicineNet.
- "Medicines". NPS MedicineWise. October 28, 2012.
- 1992, American Herbalism edited by Michael Tierra Crossings Press