BACKGROUND: Prior studies suggest anticonvulsants purchasers may be at greater risk of suicide than lithium purchasers. METHODS: Longitudinal, retrospective cohort study of all individuals in Denmark purchasing anticonvulsants (valproic acid, carbamazepine, oxcarbazepine or lamotrigine) (n=9952) or lithium (n=6693) from 1995-2001 who also purchased antipsychotics at least once (to select out nonpsychiatric anticonvulsant use). Poisson regression of suicides by medication purchased (anticonvulsants or lithium) was conducted, controlling for age, sex, and calendar year. Confounding by indication was addressed by restricting the comparison to individuals prescribed the same medication: individuals with minimal medication exposure (e.g., who purchased only a single prescription of anticonvulsants) were compared to those individuals with more consistent medication exposure (i.e., purchasing > or = 6 prescriptions of anticonvulsants). RESULTS: Demographics and frequency of anticonvulsant, lithium, or antipsychotic use were similar between lithium and anticonvulsant purchasers. Among patients who also purchased antipsychotic at least once during the study period, purchasing anticonvulsants more consistently (> or = 6 prescriptions) was associated with a substantial reduction in the risk of suicide (RR=0.22, 95% CI=0.11-0.42, p<0.0001), similar to patients consistently purchasing lithium (RR=0.27, 95% CI=0.12-0.62, p=0.006). Absolute suicide risks of consistent anticonvulsant and consistent lithium purchasers were similar. LIMITATIONS: Lack of information about diagnoses and potential confounders, as well as other covariates that may differ between minimal and consistent medication purchasers, are limitations to this study. CONCLUSIONS: In this longitudinal study of anticonvulsant purchasers likely to have psychiatric disorders, consistent anticonvulsant treatment was associated with decreased risk of completed suicide.