Background Clinical trials have examined the efficacy of drugs to prevent relapse in patients with bipolar disorder, however their design often limits generalization to routine clinical practice. Aims To estimate the effectiveness of drugs used for maintenance treatment in bipolar disorder. Methods We used national registers to identify 35,022 individuals diagnosed with bipolar disorder and information on lithium, valproate, carbamazepine, lamotrigine, quetiapine, and olanzapine treatment from 2006-2009. The main outcome was psychiatric hospitalizations. We used stratified cox regression to compare periods on and off medication within the same individual. Results Medication with lithium, valproate, lamotrigine, olanzapine, and quetiapine was associated with reduced the rates of hospitalization. Lithium was more effective than quetiapine and olanzapine. The effects of specific drugs depended on the polarity of the mood episode. Conclusions Our findings complement results from RCTs, but suggest that lithium is more effective than both quetiapine and olanzapine in routine clinical practice. </p
