[Pilot study of the metabolism of codeine to morphine and a possible modification by benzodiazepines]

After administration of high-dose codeine we found that in two cases 20-40% of the total morphine-equivalents in the 24-hour urine sample were free morphine. After another 24 hours the proportion of free morphine was up to 70% and after roughly 96 hours even went up to 96%. Although 10% of the administered codeine was eliminated as morphine. In contrast to the values reported in literature we found that even with codeine/morphine ratios greater than 0.5 and morphine concentrations of up to 2000 ng/ml urine one cannot naturally conclude that morphine/diamorphine has been consumed. The short half-life of codeine in serum was 2-4 hours, the final half-life was 9-11 hours. In urine we found a short half-life of 1-6 hours as well as a long half-life of 7-12 hours. When diazepam was administered simultaneously with codeine the expected half-life in serum was up to 1.5 times and in urine 2.5 times. The codeine/morphine ratios were hardly affected so far as evidence goes which they give of preceding drug-intake.