The chief alkaloid (see ALKALOIDS) upon which the action of OPIUM depends. A traditional and invaluable opioid analgesic (see ANALGESICS) used to control severe pain, it is the standard against which other opioid analgesics are measured. Used widely in patients with post-operative pain or those in PALLIATIVE care who have severe pain, it produces a sense of EUPHORIA. A serious side-effect is that morphine can cause nausea and vomiting. The drug may also cause DEPENDENCE. Morphine is a Class A controlled drug and is classified in Schedule II of the Misuse of Drugs Regulations 1985 (see CONTROLLED DRUGS; MEDICINES; MISUSE OF DRUGS).