Fig. 6. Steady state response of the morphine-6-glucuronide (M6G) concentration versus pain tolerance ( A ) and the acute hypoxic ventilatory response ( B ). For pain tolerance but not for respiration, a significant difference was observed between the OPRM1:c.118AA and OPRM1:c.118GA genotype with lesser M6G efficacy in OPRM1:c.118GA heterozygotes . For pain tolerance (but not for respiration) we performed a post hoc analysis including six OPRM1:c.118GA heterozygotes from a previous study (Romberg et al. 14 ).