Heroin (INN: diacetylmorphine, BAN: diamorphine) is a semi-synthetic opioid synthesized from morphine, a derivative of the opium poppy. It is the 3,6-diacetyl ester of morphine (hence diacetylmorphine). The white crystalline form is commonly the hydrochloride salt diacetylmorphine hydrochloride, however heroin freebase may also appear as a white powder.

As with other opiates, heroin is used both as a pain-killer and a recreational drug. Frequent administration quickly leads to tolerance and dependence and has a very high potential for addiction. If sustained use of heroin for as little as three days is stopped abruptly, withdrawal symptoms can appear. This is much quicker than other common opioids such as oxycodone and hydrocodone.

One of the most common methods of heroin use is via intravenous injection (colloquially termed "shooting up"). When taken orally, heroin undergoes extensive first-pass metabolism via deacetylation, making it a prodrug for the systemic delivery of morphine.Sawynok J. "The therapeutic use of heroin: a review of the pharmacological literature." Canadian Journal of Physiology and Pharmacology. 1986 Jan;64(1):1-6. PMID 2420426 When the drug is injected, however, it avoids this first-pass effect, very rapidly crossing the blood-brain barrier due to the presence of the acetyl groups, which render it much more lipid-soluble than morphine itself.Klous MG, Van den Brink W, Van Ree JM, Beijnen JH. "Development of pharmaceutical heroin preparations for medical co-prescription to opioid-dependent patients." Drug and Alcohol Dependence. 2005 December 12;80(3):283-95. PMID 15916865. doi: 10.1016/j.drugalcdep.2005.04.008 Once in the brain, it is deacetylated into 3- and 6-monoacetylmorphine and morphine which bind to μ-opioid receptors, resulting in intense euphoria, decreased pain perception and anxiolytic effects (relief of anxiety).

Internationally, heroin is controlled under Schedules I and IV of the Single Convention on Narcotic Drugs. It is illegal to manufacture, possess, or sell heroin in Belgium, Denmark, Germany, the Netherlands, the United States, Australia, Canada, Ireland, United Kingdom and Swaziland. However, under the name diamorphine, heroin is a legal prescription drug in the United Kingdom. In the Netherlands, heroin is available for prescription as the generic drug diacetylmorfine to long-term heroin addicts. Popular street names for heroin include black tar, smack, junk, skag, horse, dope, chiva, "H", "Boy", and others.

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