This double-blind, placebo-controlled RCT was designed to evaluate the efficacy of the herbal medicine Free and Easy Wanderer Plus (FEWP) as an adjunct to carbamazepine (CBZ) in patients with bipolar disorders. One hundred and twenty-four bipolar depressed and 111 manic patients were randomised to treatment with CBZ alone, CBZ plus FEWP or equivalent placebo for 12 weeks. CBZ was initiated at 300 mg/day and FEWP was given at a fixed dose of 36 g/day. Efficacy measures included the Hamilton Rating Scale for Depression, the Montgomery–Asberg Depression Rating Scale, the Young Mania Rating Scale, the Bech–Rafaelsen Mania Scale and Clinical Global Impression–Severity (CGI–S). CBZ monotherapy produced significantly greater improvement on manic measures at week 2 through endpoint, and CGI–S of depression at endpoint compared to placebo. CBZ monotherapy also yielded significantly higher clinical response rates than placebo on bipolar depression (63.8 vs. 34.8%, P=0.044) and mania (87.8 vs. 57.1%). Compared to CBZ monotherapy, adjunctive FEWP with CBZ resulted in significantly better outcomes on the three measures of depression at week 4 and week 8, and significantly greater clinical response rate in depressed subjects (84.8 vs. 63.8%), but failed to produce significantly greater improvement in manic measures and the response rate in manic subjects.