Reversibility of the analgesic effect of meptazinol in volunteers.

A double-blind randomized placebo-controlled study was carried out in 8 healthy volunteers to determine whether the analgesic and other actions of meptazinol could be reversed by naloxone or hyoscine. Heart rate, blood pressure, respiratory rate, pupil diameter, forearm ischaemic exercise tolerance and dental pain threshold were measured before and up to 45 minutes after intramuscular meptazinol 100 mg or placebo, when 'reversal' was attempted with intravenous naloxone 2 mg, hyoscine 1.6 mg or placebo. Meptazinol caused mild pupillary constriction which is unlikely to be clinically significant. Forearm ischaemic exercise tolerance failed to show any analgesic effect of meptazinol. Measurement of dental pain threshold strongly suggested reversal of meptazinol analgesia by naloxone but not by hyoscine, but inter-subject variation in analgesic response was great and these results require confirmation in a group pre-selected for consistent response.