The effect sizes of individual risk alleles underlying substance use are small, with most genotype relative risks in the range of 1.1–2.0. The joint effect of all measured DNA variants explained 19–28% of the variance in smoking initiation, 24–44% in current smoking (30) and 6% in cannabis use (26). These findings suggest that individuals may be at risk for substance use through multiple genetic variants each with a small contribution.
