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. One of the most common methods of heroin use is via intravenous injection. When taken orally, heroin 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 is deacetylated into 3- and 6-monoacetylmorphine and morphine, which bind to μ-opioid receptors. As with other opiates, heroin is used both as a pain-killer and a recreational drug. Frequent administration has a high potential for causing addiction and may quickly lead to tolerance.