[Heroin, part III: the pharmacology of heroin]

The major pharmacological effects of heroin can be traced back to some structural properties of the morphine molecule. The analgesic effects of heroin derive from the two active metabolites, 6-O-acetylmorphine and morphine, which bind specifically to the mu-opioid receptors of the central nervous system. mu-receptors also mediate other pharmacological actions of heroin i.e. respiratory depression, euphoria and physical dependence. Heroin is more potent and faster acting than morphine as an analgesic drug. Presently, there is a considerable dispute whether heroin should be legalized for the treatment of pain in terminal cancer. Chronic administration of heroin results in the development of tolerance. It is characterized by a shortened duration and decreased intensity of the analgesic, euphoric, sedative and other CNS-depressant effects. Tolerance to opioids is due to increased adaptation of the cells which changes their receptor sites after chronic exposure to the drug. Toxicity of heroin depends markedly on the route of administration in animal studies. Variable composition of street heroin or sudden loss of tolerance can cause fatal heroin overdose of addicts. Respiratory arrest is the most common reason for death among heroin addicts.