A comparison of the effects of codeine and tramadol on laryngeal reactivity

The effect of tramadol on laryngeal reflex activity was assessed in a double-blind cross-over study in six volunteers receiving single oral doses of either codeine 50 mg, tramadol 50 mg or tramadol 100 mg. Laryngeal reactivity was measured by the response to the inhalation of dilute ammonia vapour. The minimum ammonia concentration required to induce a glottic stop was recorded prior to drug administration, and at 15, 30, 45, 60, 90, 120, 150 and 180 min thereafter. Psychometric tests were performed at 0, 45 and 105 min to detect any relationship between central sedation and changes in laryngeal reflex activity. The concentration of ammonia required to induce a glottic stop increased in all treatment groups, but more so in the tramadol groups. The time course suggested that the codeine effect peaked early, and had returned to normal within 2 h. For tramadol 100 mg, laryngeal depression appeared to be still increasing at the end of the 3-h study period. No correlation was found between laryngeal and sedative effects. Tramadol is produced as a racemic mixture, in which one isomer acts through an opioid receptor pathway whilst the other affects noradrenergic and serotonergic mechanisms. Both of these routes of action may be involved in the suppression of the response to experimentally induced cough.