The goal of this clinical trial is to learn if metformin can reduce the increase in endometrial thickness caused by tamoxifen in women with estrogen-positive breast cancer taking tamoxifen for more than 1 year. It will also learn about the safety of taking metformin with tamoxifen. The main questions it aims to answer are:

1. Does metformin decrease endometrial thickness in women receiving tamoxifen?
2. What medical problems do participants have when taking metformin with tamoxifen?

Researchers will compare tamoxifen plus metformin to tamoxifen plus placebo to see if metformin works to reduce endometrial thickness compared to placebo patients.

Participants will:

1. Take metformin daily along with tamoxifen, or take tamoxifen with placebo.
2. Acquire baseline assessment and then visit the clinic at 3 months, 6 months, 9 months and 12 months for checkups and trans abdominal ultrasound test for endometrial thickness, and to be monitored for any side effects or complications.

Inclusion Criteria:

• Postmenopausal Female patients (amenorrhea ≥ 12 months).
• Histologically proven (Estrogen-positive) breast cancer.
• Completed all required surgery and/or chemotherapy (if indicated)
• On adjuvant tamoxifen for at least 1 year.
• Accepts to enroll in the study

Exclusion Criteria:

• Refusing to enroll in the study
• Ongoing or History of: endometrial or ovarian malignancy
• Concurrent hormonal therapy that might affect endometrial thickness
• Concurrent Diabetes mellitus in which metformin is prescribed.
• Known hypersensitivity or severe intolerance to metformin

De-identified participant-level data, including endometrial thickness measurements, baseline demographics, and follow-up data relevant to primary outcomes.

Upon publication of the trial result on open access repository.

Access will be open to researchers and the public via a free, open-access data repository, with a DOI provided in the publication. Data will be de-identified to protect participant privacy.

This is a double-blind study. Neither the participants nor the ultrasound operator assessing the primary outcome (endometrial thickness) will be aware of the treatment allocation. Study medication and placebo will be identical in appearance and packaging. Randomization codes will be securely held and only revealed after completion of data analysis, unless required for patient safety.