Heroin is an opioid analgesic synthesized by C.R. Alder Wright in 1874 by adding two acetyl groups to the molecule morphine, found in the opium poppy. It is the 3,6-diacetyl ester of morphine, and functions as a morphine prodrug, meaning that it is metabolically converted to morphine inside the body in order for it to work.
When used in medicine it is typically used to treat severe pain, such as that resulting from a heart attack or a severe injury. The name "heroin" is only used when being discussed in its illegal form. When it is used in a medical environment, it is referred to as diamorphine. The white crystalline form considered "pure heroin" is usually the hydrochloride salt, diacetylmorphine hydrochloride.
Illicit heroin is usually in freebase form, dulling the sheen and consistency to a matte-white powder. Because of its lower boiling point, the freebase form of heroin is also smokable. It is prevalent in heroin coming from Afghanistan, which as of 2004 produced roughly 87% of the world supply in illicit raw opium. However, production in Mexico has risen six times from 2007 to 2011, changing that percentage and placing Mexico as the second largest opium producer in the world.
Mexican cartels are also known to produce a third type of illicit heroin, commonly called black tar, which results from a simplified, quicker synthesis procedure and contains a high percentage of morphine derivates other than heroin, such as 6-monoacetylmorphine (6-MAM).
As with other opioids, diacetylmorphine is used as both an analgesic and a recreational drug. Frequent and regular administration is associated with tolerance and physical dependence. Internationally, diacetylmorphine is controlled under Schedules I and IV of the Single Convention on Narcotic Drugs. It is illegal to manufacture, possess, or sell diacetylmorphine without a license in almost every country.
Under the chemical names diamorphine and diacetylmorphine, heroin is a legally prescribed controlled drug in the United Kingdom, and is supplied in tablet or injectable form for the same indications as morphine is, often being preferred over morphine due to its lower side-effect profile. It is also available for prescription to long-term users as a form of opioid replacement therapy in the Netherlands, United Kingdom, Switzerland, Germany, and Denmark, alongside psycho-social care—in the same manner that methadone or buprenorphine are used in the United States or Canada—and a similar programme is being campaigned for by liberal political parties in Norway.