Antisuicidal effect of lithum in bipolar patients: Is there an age-specific effect?

Our group reported previously a comparable overall antisuicidal effect of lithium and valproate in bipolar patients. We investigated factors associated with higher antisuicidal efficacy of lithium in older individuals. The age-related antisuicidal effect of lithium and valproate was compared in ninety-four (n = 94) high-risk bipolar suicide attempters who participated in a 2.5-year randomized, double-blind trial. Age significantly moderated the effect of lithium vs. valproate on the risk of suicide event during the study (z = −1.98, p = 0.049). We found that those who were 42 years or older (above the 75th percentile), and on lithium had significantly lower risk of suicidal behavior than older patients on valproate (>42y) or younger (<42 y) patients on either medication (interaction HR = 0.09, 95%CI: 0.01–0.89, z = −2.07, p = 0.039). This difference in risk differences was not explained away by age-related differences in the proportion of participants with bipolar II disorder (Fisher's test p = 0.020) or higher lethality of past suicide attempts in younger participants (Wilcoxon test p = 0.024); neither was there any correlation with age in the longitudinally measured blood lithium levels (t = 1.04, df = 36, p = 0.307) or valproate levels (t = −0.50, df = 41, p = 0.621). Besides the fact that this is a secondary analysis, a limitation is that the study is not powered to detect suicide deaths or suicide attempts. Bipolar patients randomized to lithium and older than 42 years had less suicidal behavior compared to same aged patients on valproate or younger patients (<42 y) on either medication. This effect was independent of clinical and sociodemographic characteristics.