Kessing LV, et al. - As part of the International Society for Bipolar Disorders (ISBD) Task Force on Lithium Treatment, researchers undertook the first systematic review of observational studies on the efficiency of lithium monotherapy in comparison with other maintenance mood stabilizers in monotherapy and in combination. Results from eight out of nine identified studies including a total of < 14,000 patients demonstrate that maintenance lithium monotherapy results in superior outcome compared with another mood stabilizer in monotherapy, including valproate, lamotrigine, olanzapine, quetiapine, unspecified anticonvulsants, carbamazepine/lamotrigine, unspecified atypical antipsychotics and unspecified antipsychotics. The four largest register-based studies largely addressed confounding, but, as ever, residual confounding cannot be excluded. Nonetheless, the observational findings essentially add to the findings from randomized controlled trials, whose designs often limit the validity of the comparison between medicines.