An active ingredient is any ingredient that provides biologically active or other direct effect in the diagnosis, cure, mitigation, treatment, or prevention of disease or to affect the structure or any function of the body of humans or animals.
The similar terms active pharmaceutical ingredient (abbreviated as API) and bulk active are also used in medicine. The term active substance may be used for natural products.
Some medication products can contain more than one active ingredient. The traditional word for the active pharmaceutical agent is pharmacon or pharmakon (from , adapted from pharmacos) which originally denoted a magical substance or drug.
The terms active constituent or active principle are 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), since the word "ingredient" can be taken to connote 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. For example, petrolatum and mineral oil are common vehicles. The term inactive should not, however, be misconstrued as meaning inert.