Similarity between spouses may result from prior similarity (selection) or interpersonal influence (causation) or both. We investigate spouses' mutual influences on marijuana use in a two-wave longitudinal cohort of 490 married pairs, using data obtained twice from each spouse over a 5½ year interval. To estimate processes during marriage free of sample selection bias, we also include marriages that dissolved during the interval, and we analyze the impact of divorce on the drug use of the spouse who was reinterviewed. We test hypotheses to disentangle causation effects of spouse (or event) from selection effects involved in assortative mating (or divorce), using models with and without controls for latent individual propensities to use marijuana. We find that marital selection effects predominate over causation effects and that divorce affects spouses' continued marijuana use. We discuss the implications of the findings for understanding the persistence of drug use in adulthood, gender differences in the relationship of substance use with marriage and divorce, and the study of interpersonal influences.