Pentazocine and codeine: effects on human performance and mood and interactions with diazepam.

Effects of two opioid analgesics on performance and their interactions with diazepam were studied double-blind and cross over in ten healthy students. At two-week intervals the subjects received first a single oral dose of placebo, codeine (100 mg) or pentazocine (75 mg). Then, 1 h 30 min later they were all given diazepam (0.25 mg/kg) orally. Lastly, naloxone (0.4 mg) was injected intravenously at 4 h. In addition to this, the subjects on pentazocine received a second 75 mg dose at 3 h. Codeine and pentazocine alone failed to affect performance in objective tests (body sway, digit symbol substitution, flicker fusion, Maddox wing, nystagmus) recorded at 1 h 30 min. Visual analogue scales showed subjective drug effects: pentazocine made the volunteers talkative, contented, interested and energetic, whilst codeine rendered them mentally slow. 75 mg of pentazocine and 100 mg of codeine produced comparable plasma opiate activity (determined in morphine equivalents) according to radioreceptor bioassay with [3H]-dihydromorphine as a ligand. Impaired performance was clear at the tests done 1.5 and 2.5 h after diazepam. No major interactions were found between opiates and diazepam in objective tests with the exception that nystagmus was stronger after the combined treatment than after diazepam alone. Codeine reduced the absorption of diazepam. Subjectively codeine and pentazocine counteracted the effects of diazepam. The subjects overestimated their performance after opiate + diazepam when compared to diazepam alone.(ABSTRACT TRUNCATED AT 250 WORDS)