The relationship between altered pain thresholds and analgesic responses to opioid administration for the A118G SNP has been well characterized. In a variety of populations, the G118 allele has been associated with elevated pain responses and decreased pain thresholds (Sia et al., 2008; Tan et al., 2009) and a reduced response to morphine or other opioids for patients receiving treatment for post-operative or chronic pain (Campa et al., 2008; Chou et al., 2006a; Chou et al., 2006b; Coulbault et al., 2006; Fillingim et al., 2005; Hayashida et al., 2008; Janicki et al., 2006; Klepstad et al., 2004; Landau et al., 2008; Oertel et al., 2006; Reyes-Gibby et al., 2007; Sia et al., 2008; Tan et al., 2009) (Table 2). Additionally, in healthy volunteers carrying the G118 allele, higher concentrations of alfentanil, an opioid analgesic, were required for pain relief following electrical pain stimulation (Oertel et al., 2006); of interest, this dose did not increase respiratory depression, suggesting it may be safe to give a higher dose of opioid analgesics to patients that carry the G118 allele. Other studies, however, have
