Background: Smoking is a serious health concern both globally and in the U.S. Because drinking amplifies the negative health effects of smoking, the high association of these behaviors is an additional source of population morbidity. However, very little is known about trends in the co-use of smoking and drinking over time.

Objective: To describe trends in tobacco use, alcohol use, and their co-use among U.S. youth, and to separate out trends in the association of smoking and drinking from trends in their marginal distributions.

Methods: We use data on the smoking and drinking behaviors of 12th graders from 1976 to 2010 in the Monitoring the Future study to examine trends in smoking, drinking, and co-use separately by gender. In each year we estimate the degree of co-use attributable to tobacco and alcohol use probabilities as well as the association between tobacco and alcohol use.

Results: Although the prevalence of tobacco and alcohol co-use has declined over time, the association of the two has increased. This association accounts for an increasing proportion of the co-use of tobacco and alcohol.

Conclusions: We conclude that co-users of tobacco and alcohol are an increasingly select subpopulation. This suggests that continued decreases in the contribution of substance use to population health and mortality may not continue apace.