Abstract— Bipolar disorder is a common chronic psychiatric condition, with treatment aimed at remission of symptoms and prevention of mood episodes. The pathophysiology of bipolar disorder I involves over expression of protein kinase C, which is thus considered a therapeutic target. Endoxifen, the metabolite of tamoxifen, has enhanced inhibitory action against protein kinase C, with a good safety profile. Endoxifen has shown promise in phase II and III clinical trials, with notable reduction in several symptom scale scores. This report describes the case of a woman with bipolar disorder diagnosed ten years prior, who was experiencing relapses. A change in medication to include endoxifen was effective. This is the first report of endoxifen use for duration of one year.