Pharmacokinetics

Most of the opioids are well absorbed from the GI tract in addition to being absorbed following transcutaneous administration. As described previously, the first-pass effect on drugs like morphine, which have a free hy-droxyl group in position 3, is glucuronidation by the liver. In the case of morphine, the conjugation to glu-curonide decreases the oral bioavailability of the drug. Following absorption, the drugs distribute rapidly to all tissues, although the distribution is limited by their lipophilicity. Fentanyl (highly lipophilic) distributes to the brain rapidly but also remains in fat, which serves as a slow-releasing pool of the drug. Certain of the drugs, notably methadone and fentanyl, have long half-lives inconsistent with their duration of action. This discrepancy is due to accumulation in various tissue and plasma reservoirs and redistribution from the brain to these reservoirs. Heroin passes readily into the brain. Codeine passes into the brain more readily than morphine, which is slow in crossing the blood-brain barrier. The drugs cross readily into fetal tissues across the pla centa and therefore should be used with care during pregnancy and delivery. Moreover, glucuronidation by the fetus is slow, increasing buildup of the drugs and increasing their half-life in the fetus.

The majority of their metabolites are inactive with a few notable exceptions, such as morphine-6-glucuronide, which produces an analgesic effect; normeperidine and norpropoxyphene, which produce excitatory but not analgesic effects; and 6-p-naltexol, which is less active than naltrexone as an antagonist but prolongs the action of naltrexone. Excretion of the metabolites requires adequate renal function, since excretion by routes other than the urine are of minor importance.

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