Background: Recent reports have linked cognitive ability (IQ) with alcohol dependency, but the relationship with illegal drug use is not well understood. Methods: Participants were 14,362 male US Vietnam veterans with IQ test results at entry into military service in 1965-1971 (mean age 22.58) who participated in a telephone interview in 1985-1986. A structured diagnostic telephone interview was used to ascertain habitual drug use during military service (for once a week, ≥3 months) and in civilian life (in the past 12 months, ≥once a week), combat exposure, and post-traumatic stress disorder according to established Diagnostic and Statistical Manual of Mental disorders criteria (version III). Results: In unadjusted analysis, men with high IQ scores were less likely to be habitual users of cannabis (OR = 0.89, 95% CI = 0.86, 0.93), cocaine (OR = 0.69, 95% CI = 0.61, 0.78), heroin (OR = 0.80, 95% CI = 0.73, 0.88), amphetamines (OR = 0.90, 95% CI = 0.83, 0.98), barbiturates (OR = 0.79, 95% CI = 0.72, 0.86) and LSD (OR = 0.91, 95% CI = 0.82, 0.99) during military service and civilian life. These associations were markedly attenuated after adjustment for socioeconomic status in early and later civilian life. Conclusion: In this cohort, socioeconomic position might lie on the pathway linking earlier IQ and later habitual drug use but might also act as a surrogate for IQ. This suggests interventions to prevent drug use could attempt to improve early life IQ and opportunities for employment. © 2012 Elsevier Ireland Ltd.
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