Morphine-6β-glucuronide (M6G) is an active metabolite of morphine that has a greater analgesic potency but a reduced potency in affecting respiratory depression (Mantione et al., 2005). In G118 allele-carrying subjects, there was a reduced potency of M6G in eliciting an analgesic response, though there was no difference in M6G-induced respiratory depression (Romberg et al., 2005). A reduction in M6G-induced miosis was also found in G118 allele-carriers (Lotsch et al., 2002).
