Heroin is a morphine derivative and morphine is opium's most potent active ingredient. First synthesized in 1874, heroin was widely used in medicine in the early part of the 20th century, until its addictive potential was recognized.
Pure heroin is a powder with a bitter taste. Street heroin may vary in color from white to dark brown because of impurities or additives. There is a dark brown or black form of the drug, as dense as roofing tar or coal, known as "black tar."