The basic structure of morphine (Fig. 26.1) can be altered in rather minor ways that drastically change the effects of the drug. Acetylation of the hydroxyl groups leads to the synthesis of heroin (diacetylmorphine), which has a much greater ability to pass the blood-brain barrier. In the brain, however, heroin is converted to morphine and monoacetyl morphine. Some researchers attribute heroin's potent analgesic effects and rapid onset of action solely to its conversion to morphine. Others contend that heroin produces analgesic effects distinct from the conversion to morphine and thus should be considered as a therapeutically useful analgesic. Heroin is not approved for medical use in the United States, although it is used therapeutically in other nations.
Morphine is glucuronidated in the liver at the phenolic hydroxyl group (C3). Protection of that group with a methyl group, as occurs in codeine and other codeine derivatives such as oxycodone, renders the molecule less susceptible to glucuronidation and decreases the first-pass effect in the liver. It is for this reason that codeine and its derivatives retain activity following oral
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