Objective: In a nation-wide population-based longitudinal register linkage study for the first time 1) to investigate long-term response to lithium in patients with bipolar disorder with and without comorbid epilepsy, and 2) within patients with bipolar disorder and comorbid epilepsy to compare differences in responses between lithium, valproate and lamotrigine. Methods: We used Danish nation-wide population-based longitudinal register linkage to identify 154 patients with bipolar disorder and comorbid epilepsy and 8381 patients with bipolar disorder without comorbid epilepsy during a study period from 1995 to 2017. Response was defined as continuous monotherapy with lithium, valproate or lamotrigine without switch to or add-on of an antipsychotic drug or an antidepressant or hospitalization during an up to ten-year follow-up period. We calculated standardized absolute risks and differences thereof with respect to age, gender, socioeconomic status and comorbidity with other physical disorders than epilepsy. Results: Response to lithium was decreased in patients with bipolar disorder with versus without comorbid epilepsy during the ten-year follow-up period and the difference remained after standardization for comorbidity with other physical disorders than epilepsy. Within patients with bipolar disorder and comorbid epilepsy, response to lithium was decreased compared with responses to valproate and lamotrigine. Conclusions: The findings suggest that valproate and lamotrigine should be given priority in patients with comorbid bipolar disorder and epilepsy. The study highlights the need for closely clinical monitoring and psychological support for patients with bipolar disorder and comorbid epilepsy and emphasize the need for further long-term studies of effect of interventions.