Heroin, or diacetylmorphine (INN), also known as diamorphine (BAN), is a semi-synthetic opioid drug synthesized from morphine, a derivative of the opium poppy. It is the 3,6-diacetyl ester of morphine (di(two)-acetyl-morphine). The white crystalline form is commonly the hydrochloride salt diacetylmorphine hydrochloride, though often adulterated thus dulling the sheen and consistency from that to a matte white powder, which however heroin freebase typically is.^[1]

As with other opioids, heroin is used as both a pain-killer and a recreational drug and has an extremely high potential for abuse. Frequent and regular administration is associated with tolerance, moderate physical dependence, and severe psychological dependence which develops into addiction.

Internationally, heroin is controlled under Schedules I and IV of the Single Convention on Narcotic Drugs.^[2] It is illegal to manufacture, possess, or sell diacetylmorphine without a licence in Belgium, Denmark,Germany, Iran, India, the Netherlands, the United States, Australia, Canada, Ireland, Pakistan, the United Kingdom and Swaziland.

Under the name diamorphine, it is a legally prescribed controlled drug in the United Kingdom. It is available for prescription to long-term addicts in the Netherlands, the United Kingdom, Switzerland, Germany andDenmark.^[3][4]

The opium poppy was cultivated in lower Mesopotamia as long ago as 3400 BC.^[8] The chemical analysis of opium in the 19th century revealed that most of its activity could be ascribed to two alkaloids, codeine and morphine.

Diacetylmorphine was first synthesized in 1874 by C. R. Alder Wright, an English chemist working at St. Mary’s Hospital Medical School in London. He had been experimenting with combining morphine with various acids. He boiled anhydrous morphine alkaloid with acetic anhydride for several hours and produced a more potent, acetylated form of morphine, now called diacetylmorphine. The compound was sent to F. M. Pierce of Owens College in Manchester for analysis. Owens told Wright:

Wright’s invention did not lead to any further developments, and diacetylmorphine only became popular after it was independently re-synthesized 23 years later by another chemist, Felix Hoffmann. Hoffmann, working at the Aktiengesellschaft Farbenfabriken (today the Bayer pharmaceutical company) in Elberfeld, Germany, was instructed by his supervisor Heinrich Dreser to acetylate morphine with the objective of producing codeine, a constituent of the opium poppy, pharmacologically similar to morphine but less potent and less addictive. Instead the experiment produced an acetylated form of morphine one and a half to two times more potent than morphine itself.

From 1898 through to 1910 diacetylmorphine was marketed under the name heroin as a non-addictive morphine substitute and cough suppressant. Bayer marketed heroin as a cure for morphine addiction before it was discovered that it rapidly metabolizes into morphine. As such, heroin is essentially a quicker acting form of morphine. The company was embarrassed by the new finding, which became a historic blunder for Bayer.^[10]

In the U.S.A. the Harrison Narcotics Tax Act was passed in 1914 to control the sale and distribution of “heroin” and other opioids, which allowed the drug to be prescribed and sold for medical purposes. In 1924 the United States Congress banned its sale, importation or manufacture. It is now a Schedule I substance, which makes it illegal for non-medical use in signatory nations of the Single Convention on Narcotic Drugs treaty, including the United States.

Later, as with Aspirin, Bayer lost some of its trademark rights to “heroin” under the 1919 Treaty of Versailles following the German defeat in World War I.^[11]

Pharmacology

When taken orally, diacetylmorphine undergoes extensive first-pass metabolism via deacetylation, making it a prodrug for the systemic delivery of morphine.^[12] 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.^[13] Once in the brain, it then is deacetylated into 6-monoacetylmorphine (6-MAM) and morphine which bind to μ-opioid receptors, resulting in the drug’s euphoric, analgesic (pain relief), and anxiolytic (anti-anxiety) effects; diacetylmorphine itself exhibits relatively low affinity for the μ receptor.^[14] Unlike hydromorphone and oxymorphone, however, administered intravenously, diacetylmorphine creates a larger histamine release, similar to morphine, resulting in the feeling of a greater subjective “body high” to some, but also instances of pruritus (itching) when they first start using.^[15]

Both morphine and 6-MAM are μ-opioid agonists which bind to receptors present throughout the brain, spinal cord and gut of all mammals. The μ-opioid receptor also binds endogenous opioid peptides such as β-endorphin, Leu-enkephalin, and Met-enkephalin. Repeated use of diacetylmorphine results in a number of physiological changes, including decreases in the number of μ-opioid receptors.^{[citation needed]} These physiological alterations lead to tolerance and dependence, so that cessation of diacetylmorphine use results in a set of extremely uncomfortable symptoms including pain, anxiety, muscle spasms, and insomnia called the opioidwithdrawal syndrome. Depending on usage it has an onset 4 to 24 hours after the last dose of diacetylmorphine. Morphine also binds to δ- and κ-opioid receptors.

There is also evidence that 6-MAM binds to a subtype of μ-opioid receptors which are also activated by the morphine metabolite morphine-6β-glucuronide but not morphine itself.^[16] The contribution of these receptors to the overall pharmacology of heroin remains unknown.

A subclass of morphine derivatives, namely the 3,6 esters of morphine, with similar effects and uses includes the clinically-used strong analgesics nicomorphine (Vilan), and dipropanoylmorphine; there is also the latter’s dihydromorphine analogue,diacetyldihydromorphine (Paralaudin).