The opioid system plays an important role in various biological functions including analgesia, drug response and pain reduction. Menon et al. [12] studied the influence of polymorphism in gene coding for the μ-opioid receptor highlighting the association between the OPRM1 A118G SNP and head pain severity in a clinical cohort of 153 female migraineurs with aura. In particular, G118 allele carriers were more likely to be high pain sufferers compared to homozygous carriers of the A118 allele.