The authors say the drug "seems to reduce the risk of death and suicide by more than 60% compared with placebo" and suggest this review "reinforces lithium as an effective agent to reduce the risk of suicide in people with mood disorders."

Mood disorders are a leading cause of global disability. The two main types are unipolar disorder (often called clinical depression) and bipolar disorder (often called manic depression). Both are serious, long term conditions involving extreme mood swings, but people with bipolar depression also experience episodes of mania or hypomania.

People with a mood disorder have a 30 times greater risk of suicide than the general population. Treatment with mood stabilising drugs like lithium, anticonvulsants or antipsychotics can help keep mood within normal limits, but their role in suicide prevention is still uncertain.

So a team of researchers from the universities of Oxford, UK and Verona, Italy set out to assess whether lithium has a specific preventive effect for suicide and self harm in people with unipolar and bipolar mood disorders.

They reviewed and analysed the results of 48 randomised controlled trials involving 6,674 participants. The trials compared lithium with either placebo or active drugs in long term treatment for mood disorders.

Lithium was more effective than placebo in reducing the number of suicides and deaths from any cause, but no clear benefits were seen for lithium compared with placebo in preventing deliberate self harm.

When lithium was compared with each active individual treatment, a statistically significant difference was found only with carbamazepine for deliberate self harm. Overall, lithium tended to be generally better than the other active treatments, with small statistical variation between the results.

"This updated systematic review reinforces lithium as an effective agent to reduce the risk of suicide in people with mood disorders," say the authors.

They suggest that lithium may exert its anti-suicidal effects "by reducing relapse of mood disorder," but add "there is some evidence that lithium decreases aggression and possibly impulsivity, which might be another mechanism mediating the anti-suicidal effect."

They acknowledge that lithium has several side effects, but say clinicians "need to take a balanced view of the likely benefits and harm of lithium in the individual patient." And they conclude: "Understanding the mechanism by which lithium acts to decrease suicidal behaviour could lead to a better understanding of the neurobiology of suicide."