Abstract: Six randomised controlled trials (overall 876 participants) lasting 6 to 24 months were included. Two studies (overall 312 participants) compared valproate with placebo, four studies (overall 618 participants) valproate with lithium, one study (overall 23 participants) valproate with olanzapine and one study (overall 220 participants) valproate with the combination of valproate plus lithium. In terms of study quality, most studies reported the methods used to generate random sequence; however, only one study reported enough details on allocation concealment. Four of six included studies described their design as “double blind”, but only two trials reported full details about blinding. Valproate was more effective than placebo in preventing study withdrawal due to any mood episode (RR 0.68, 95% CI 0.49 to 0.93; NNTB 8), but no difference in efficacy was found between valproate and lithium (RR 1.02, 95% CI 0.87 to 1.20). Valproate was associated with fewer participants dropping out of treatment for any cause when compared with placebo or lithium (RR 0.82, 95% CI 0.71 to 0.95 and RR 0.87, 95% CI 0.77 to 0.98, respectively). However, combination therapy with lithium plus valproate was more likely to prevent relapse than was monotherapy with valproate (RR 0.78, 95% CI 0.63 to 0.96). Significant differences in adverse event frequencies were found, and lithium was associated with more frequent diarrhoea, polyuria, increased thirst and enuresis, whereas valproate was associated with increased sedation and infection. Limited evidence supports the efficacy of valproate in the long-term treatment of bipolar disorder. Clinicians and patients should consider acceptability and tolerability profile when choosing between lithium and valproate—their combination or other agents—as long-term treatment for bipolar disorder.

Plain-language summary: We searched, in a wide-ranging way, for all the useful studies (randomised controlled trials, or RCTs) we could find on long-term treatment of people with bipolar disorder using valproate or any other mood stabiliser, or antipsychotic drugs, or placebo. Three of us looked at RCTs to make sure they were fair experiments. We extracted data from the studies, put all of the evidence together and carried out a statistical analysis to look for significant results. We conducted these searches to 11 January 2013 and found six studies, including a total of 876 participants. The quality of the studies in terms of design was not good, which means that the effects of some drugs might have been overestimated. All of the trials taken together suggest that valproate might help to prevent relapse in bipolar disorder, especially depressive episodes. However, because of limited available evidence, conclusions on valproate compared with placebo and lithium (or other active drugs) cannot be made with any reliable degree of confidence. Lithium is an important drug to compare with valproate because lithium is already known to be effective in preventing relapses of bipolar disorder. When we combined the findings of all studies comparing valproate with lithium, the evidence did not favour valproate or lithium in terms of efficacy. People taking valproate over a long time were more likely than patients given lithium to keep taking their allocated medication. Clinicians and patients should consider the side effects of valproate, including alopecia, tremor and weight gain. We also found a study that compared valproate taken alone with valproate combination therapy (two drugs taken at the same time). This study compared people who took lithium only or valproate only with people who took valproate and lithium together. No evidence showed that use of valproate and lithium compared with lithium alone helped to ensure that patients kept taking their allocated medication.