Metformin intervention against ovarian toxicity during chemotherapy for early breast cancer: study protocol for a randomized double-blind placebo-controlled trial

•This study can provide a potential alternative drug protecting female fertility for early breast cancer patients, which could be useful for clinical programs.•This study may provide a useful strategy that improves the life quality and long-term prognosis such as OS, ORR and DFS for early breast cancer patients.•This project can be of clinical and long-term strategic significance, which could alleviate economic pressure as the cancer patients are trending younger.