• W T Beaver, S L Wallenstein, A Rogers, and R W Houde.
• J. Pharmacol. Exp. Ther. 1978 Oct 1; 207 (1): 92-100.

AbstractThe relative analgesic potency of oral and intramuscular codeine was evaluated in a double-blind crossover comparison of graded single doses in patients with chronic pain due to cancer. When both duration and intensity of analgesia are considered (total effect), oral codeine was 6/10 as potent as the intramuscular form. This is a high oral/parenteral analgesic relative potency ratio compared with morphine, metopon and oxymorphone and correlates well with the results of recent studies which have determined the oral vs. intramuscular bioavailability of codeine in man. Oral and intramuscular oxycodone were also compared in a similar patient group. Like codeine, oxycodone retained at least 1/2 of its analgesic activity when administered orally. We hypothesize that the high oral/parenteral relative potency ratios of codeine and oxycodone relative to morphine and its congeners are not due to more efficient absorption after oral administration, but rather that methylation at position 3 in codeine and oxycodone protects these drugs from rapid first-pass metabolism.

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