Heroin or diacetylmorphine (INN) is a semi-synthetic opioid. It is the 3,6-diacetyl derivative of morphine (hence diacetylmorphine) and is synthesised from it by acetylation. The white crystalline form is commonly the hydrochloride salt, diacetylmorphine hydrochloride. It has a high addiction potential, and frequent repeated ingestion causes a fast development of tolerance to it when compared to other substances, although occasional use without symptoms of withdrawal is also possible. 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 the United States but, under the name diamorphine, heroin is a legal prescription drug in the United Kingdom. Popular street names for heroin include dope, diesel, smack, skag, heron, black tar, horse, junk, jenny, brown, brown sugar, dark and H.
In the United Kingdom heroin is available on prescription, though it is a restricted Class A drug. According to the British National Formulary (BNF) edition 50, diamorphine hydrochloride may be used in the treatment of acute pain, myocardial infarction, acute pulmonary edema, and chronic pain. The treatment of chronic non-malignant pain must be supervised by a specialist. The BNF notes that all opioid analgesics cause dependence and tolerance but that this is "no deterrent in the control of pain in terminal illness". When used in the palliative care of cancer patients, heroin is often injected using a syringe driver. In comparison to morphine, it may cause less nausea, hypotension, sedation, euphoria and can be dissolved in a smaller quantity of liquid.
Heroin is also widely and illegally used as a powerful and addictive drug that produces intense euphoria, which often disappears with increasing tolerance. It is thought that heroin's popularity with recreational users, compared to morphine or other opiates, comes from its somewhat different perceived effects. This in turn comes from its high lipid solubility provided by the two acetyl groups, resulting in a very rapid penetration of the blood-brain barrier after use. Heroin can be taken or administered in a number of ways, including snorting it and injecting it. It may also be smoked by inhaling the vapors produced when heated from below (known as "chasing the dragon").
Once in the brain, heroin is rapidly metabolized into morphine by removal of the acetyl groups. It is the morphine molecule that then binds with opioid receptors and produces the subjective effects of the heroin high. Heroin is therefore a prodrug.
The onset of heroin's effects is dependent on the method of administration. Orally the heroin is totally metabolized in vivo into morphine before crossing the blood-brain barrier, so the effects are the same as morphine when taken by mouth. Snorting heroin results in onset within 10 to 15 minutes. Smoking heroin results in an adrenaline rush within 2-5 minutes. Intravenous injection results in rush and euphoria within 7 to 8 seconds, while intramuscular injection takes longer, having an effect within 5 to 8 minutes.
Heroin is a opioid (mu-opioid) agonist. It acts on endogenous opioid receptors that are spread in discrete packets throughout the brain, spinal cord and gut in almost all mammals. Heroin, along with other opioids, are agonists to four endogenous neurotransmitters. They are β-endorphin, dynorphin, leu-enkephalin, and met-enkephalin. The body responds to heroin in the brain by reducing (and sometimes stopping) production of the endogenous opioids when heroin is present. Endorphins are regularly released in the brain and nerves and attenuate pain. Their other functions are still obscure, but are probably related to the effects produced by heroin besides analgesia (antitussin, anti-diarrheal). The reduced endorphin production in heroin users creates a dependence on the heroin, and the cessation of heroin results in extreme symptoms including pain (even in the absence of physical trauma). This set of symptoms is called withdrawal syndrome. It has an onset 6 to 8 hours after the last dose of heroin.