Heroin is an illegal, highly addictive drug. It is a white or brown powder or a black, sticky substance (black tar heroin). It can be sniffed, snorted, smoked, or injected into a muscle or vein. It is often mixed (cut) with other drugs or substances, such as sugar or powdered milk. It may also be cut with poisons, such as strychnine. Other names for heroin are smack, junk, H, and ska.