CRD summary: This review evaluated the effect of lithium on the incidence of suicide and suicide attempts in patients with recurrent major depressive disorders. The authors concluded that treatment with lithium might reduce the rate of suicide and suicide attempts. These conclusions have to be considered with caution given the limitations of the review in terms of the literature search, potential for bias, and failure to assess study quality.

CRD commentary: This review addressed a well-defined question that was supported by specified inclusion criteria for the interventions, participants and outcomes. Since only one relevant database was searched and it was unclear how extensively the search for unpublished studies was conducted, publication bias cannot be excluded. Publication bias was not assessed. It was not stated whether any language restrictions had been applied, therefore language bias also cannot be ruled out. It was not stated whether the study selection and data extraction processes were carried out in duplicate, so reviewer error and bias might have been introduced. No quality assessment of the included studies was reported and the potential impact of methodological flaws in the primary studies upon the reliability of the review findings cannot, therefore, be assessed; this is of particular importance given the variety of study designs included. Statistical heterogeneity was not formally tested, which makes it unclear whether the decision to combine the studies in a meta-analysis was appropriate. The authors' conclusions have to be viewed with caution given the relatively low number of patients enrolled in the included studies, as well as the methodological and reporting weaknesses of the review.