Codeine is an alkaloid that occurs naturally in opium, but the amount present is usually too small to be of commercial importance. Consequently, most commercial codeine is prepared from morphine by methylating the phenolic OH group. It occurs as levorotatory, colorless, efflorescent crystals, or as a white crystalline powder. It is light sensitive. Codeine is a monoacidic base and readily forms salts with acids, with the most important being the sulfate and the phosphate. The acetate and methylbromide derivatives have been used to a limited extent in cough preparations.
The general pharmacological action of codeine is similar to that of morphine, but it does not possess the same analgesic potency. The equianalgesic dose of parenteral morphine 3 to 5 mg q 4 hours is codeine 30 to 65 mg q 4 hours. The equianalgesic dose of oral morphine 10 mg q 4 hours is codeine 60 mg q 4 hours. The decreased potency also leads to a lower addiction potential compared with morphine. Side effects include respiratory depression, mioisis, constipation, nausea, itching, dry mouth, and drowsiness. Approximately 5% of codeine is metabolized to morphine via O-demethylation. (Fig. 24.8) The enzyme responsible for the O-demethylation of codeine is cy-tochrome P450 (CYP) 2D6. This enzyme exhibits genetic polymorphism with an estimated 7% to 10% of Caucasians designated as poor metabolizers, and thus only able to form traces of morphine after codeine is administered.60 The analgesic component of codeine has long been assumed to be the O-demethylated metabolite, morphine. If codeine has no analgesic potency itself, then patients who lack this enzyme should have no analgesic effect to administered codeine. This has been shown not to be the case, so codeine itself may posses analgesic activity, or codeine-6-glucuronide may be the active analgesic.56,57
Codeine's role as an effective antitussive agent has been questioned. A Cochrane evidence-based review of the literature shows that codeine is no more effective than placebo for acute cough in children or adults.61 Codeine is available in several combination products with either aspirin, ibupro-fen, or acetaminophen for the treatment of moderate pain.
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