Any substance which when absorbed into a living organism may modify one or more of its functions. The term is generally accepted for a substance taken for a therapeutic purpose, but is also commonly used for abused substances (medicine, pharmaceutical)., aine, mis organismi sisestatuna võib modifitseerida selle funktsioone (medikamendid, psühhotroopsed ained jms) või tekitada tugevat sõltuvust (narkootikumid)