Another approach to strengthen causal inference is mendelian randomization whereby genetic variants act as assumed unconfounded proxy measures for exposure status.34 One study reported weak evidence of association between a genetic variant within the CHRNA5-A3-B4 gene cluster and being prescribed antipsychotic medication.35 However, despite this association being stronger in smokers than nonsmokers (as would be expected if this was due to a causal role of smoking on psychosis), there was little statistical evidence for this (P = .60).35 The gene discussed is CHRNA5; the disease is psychotic disorder.