it is possible that some of our results were diluted because the interview instrument does not distinguish the use of methamphetamine from several other stimulant drugs. Finally, the significant enrichment for nicotinic acetylcholine receptor genes in the pathway analysis may be the result of either comorbidity and/or pleiotropy with nicotine dependence. To explore this possibility, we conducted a secondary association analysis for the top-ranked results using models that included a covariate for nicotinic dependence severity measured by the number of DSM-IV criteria endorsed. The results were not meaningfully different from those of the primary analyses.
