The traditional word for the active pharmaceutical agent is pharmacon or pharmakon (from Greek: φάρμακον, adapted from pharmacos) which originally denoted a magical substance or drug.
[citation needed] 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").
The main excipient that serves as a medium for conveying the active ingredient is usually called the vehicle.
When patients are on multiple medications, active ingredients can interfere with each other, often resulting in severe or life-threatening complications.
[7] In phytopharmaceutical or herbal medicine, the active ingredient may be either unknown or may require cofactors in order to achieve therapeutic goals.