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 ).