Heroin: Pharmacology

 

When taken orally, diacetylmorphine undergoes extensive first-pass metabolism via deacetylation, making it a prodrug for the systemic delivery of morphine. 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. 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. 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.

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. Cessation of diacetylmorphine use results in a set of extremely uncomfortable symptoms including pain, anxiety, muscle spasms, and insomnia called the opioid withdrawal 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. The contribution of these receptors to the overall pharmacology of heroin remains unknown.