Effect of Metformin on Endometrial Thickness in Postmenopausal Breast Cancer Patients Receiving Tamoxifen

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.

Completed

Looking for future studies?

Notify Me

Key information

Conditions

Breast Cancer Breast Diseases Breast Neoplasms Endometrial Hyperplasia Endometrial Hyperplasia and Endometrial Cancers Endometrial Neoplasms Female Urogenital Diseases Female Urogenital Diseases and Pregnancy Complications Genital Diseases Genital Diseases, Female Genital Neoplasms, Female Neoplasms Neoplasms by Site Skin Diseases Skin and Connective Tissue Diseases Urogenital Diseases Urogenital Neoplasms Uterine Diseases Uterine Neoplasms

Sex eligibility

Female

Study type

Interventional

Phase

Not applicable

Primary location

Baghdad Medical City / Department of Oncology

Baghdad, Baghdad Governorate, 10045, Iraq

Who can participate

Healthy volunteers accepted: No

Only the study team can determine whether someone qualifies for participation.

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

Treatment and study plan

Metfomin

Drug

Metformin Tab. 500mg twice daily for 12 months (titrated to tolerance if needed).

Tamoxifen

Drug

Standard prescribed oncological dose.

Placebo matching Metformin

Drug

An inert, film-coated tablet identical in appearance and packaging to metformin 500 mg (same size, shape, color, coating, weight, and dosing schedule), manufactured without active ingredient using standard excipients (e.g., microcrystalline cellulose, starch/croscarmellose, povidone, magnesium stearate) and identical film coat (e.g., hypromellose, polyethylene glycol, titanium dioxide). Packaged in sealed blisters to minimize odor/taste cues. Dispensed by the investigational pharmacy per randomization; to be taken twice daily"

Primary outcomes

Change in endometrial thickness

Time frame: Baseline, 3 months, 6 months, 9 months, and 12 months from enrollment

Endometrial thickness will be measured using transabdominal ultrasound (in millimeters) by a trained radiologist. The primary outcome is the difference in endometrial thickness between the metformin plus tamoxifen group and the placebo plus tamoxifen control group.

Sponsors and collaborators

Lead sponsor

Uruk University

Other

Registry information

Important dates

Study start: 2024
Primary completion: 2025
Study completion: 2025
First posted: Aug 28, 2025
Registry last updated: Sep 12, 2025

OpenTrials presents study information sourced from ClinicalTrials.gov. The official registry record should be consulted for the latest information.

View the official ClinicalTrials.gov record (opens in a new tab)

This listing is for discovery and informational purposes only. It is not medical advice, does not guarantee that a study is recruiting, and does not determine eligibility. Contact the study team and a qualified healthcare professional when considering participation.

Published trials that share one or more normalized conditions with this study.

Completed