A pharmacon or pharmakon (from Greek: (φάρμακον), adapted from pharmacos) is a biologically active substance. It is used more broadly than the term drug because it includes endogenous substances, and biologically active substances which are not used as drugs. Typically the word pharmacon includes pharmacological agonists and antagonists, but also enzyme inhibitors (such as monoamine oxidase inhibitors).  Spelt with a "k", is acceptable in English, but more common in German. In most cases pharmacon is the equivalent to active ingredient.