Genetic studies of nicotine addiction have identified an inverse relationship between the risk for nicotine addiction and the risk for cocaine addiction. For instance, the minor allele of rs16969968, a missense variant in CHRNA5, increases risk for the development of nicotine dependence, and independently decreases risk for cocaine dependence [15]. One hypothesis would be that the true underlying selective pressure is on cocaine related phenotypes or a characteristic that affects cocaine related reward pathways in the brain and that the alleles’ effect on nicotine dependence is merely an accidental consequence. However, caution must be used when interpreting this information, given that all drugs of addiction similarly affect the dopaminergic reward pathways.
