Bipolar disorder (BPD) is a long-term illness with big mood swings. Endoxifen is a special chemical made from the drug Tamoxifen. This study looks at how safe and helpful 8 mg of Endoxifen is for people with Bipolar I Disorder. Patients will receive Endoxifen or a placebo (a pill with no active drug) to see if Endoxifen works better than the placebo. Protein Kinase C (PKC) is involved in brain communication and sometimes gets overactive in people with bipolar disorder. Endoxifen helps by stopping too much PKC activity. The study focuses on reducing symptoms quickly and keeping patients feeling better over time.

• The study involves a 21-day in-patient treatment period.
• Participants must be hospitalized for at least 2 days before randomization.
• Both safety and efficacy of Endoxifen will be compared with a placebo.

People aged 18-65 with Bipolar I Disorder can join if they meet specific health criteria. Those with other major health issues or recent drug trial participation cannot join. Participants should avoid certain medications and agree to follow study rules.