Heroin is a semi-synthetic opiate drug that is synthesized from morphine, a derivative of the Asian opium poppy plant. Heroin usually appears as a white or brown powder or as a black sticky substance ("black tar heroin").

Heroin can be injected, snorted or smoked. Upon crossing the blood-brain barrier, which occurs soon after introduction of the drug into the bloodstream, heroin is converted into morphine, which mimics the action of emdorphins, creating a sense of euphoria. Frequent and regular administration is associated with tolerance and physical dependence, which may develop into addiction.

Also known as: smack, junk, chiva or H.