The single nucleotide polymorphism (SNP) in the human gene coding for the μ-opioid
receptor 1, the OPRM1 A118G (rs1799971), may affect nociceptive signalling. For example,
carriers of the G-allele seem to have less pronounced effects of opioids [11-15], increased
pressure pain thresholds [16] and reduced cortical responses to nociceptive olfactory CO2
stimulation, indicating decreased nociceptive activation [17]. However, the G-allele has been
associated with higher pain ratings and amount of self-administered intravenous morphine in
women after caesarean section