Metformin may boost chemotherapy response in advanced breast cancer

Clinical takeaway: Metformin is inexpensive, widely available, and generally well tolerated. If its antitumor effects are confirmed in larger studies, it could become a practical adjunct to chemotherapy for patients with locally advanced or metastatic breast cancer.

Metformin could represent an accessible chemotherapy adjunct for selected patients with advanced breast cancer, although the evidence is not yet strong enough to support routine use.

Adding metformin to chemotherapy was associated with better tumor response among patients with locally advanced or metastatic breast cancer, according to a systematic review and meta-analysis of 8 randomized controlled trials involving 582 patients.

Compared with chemotherapy alone, the combination increased the likelihood of an overall pathologic response by approximately 30%. Although complete pathologic response was not significantly improved, patients receiving metformin were about 56% less likely to have no pathologic response.

The combination was also associated with a 52% greater likelihood of undergoing breast-conserving surgery. However, it did not significantly reduce the proportion of patients planned for mastectomy, suggesting that the surgical findings should be interpreted cautiously.

Overall adverse-event rates were similar between treatment groups. Grade 3 or 4 diarrhea occurred more frequently with metformin, but the estimate was imprecise, reflecting the small number of events and patients studied.

The authors concluded that adding metformin “improves response without increasing adverse events and may promote breast-conserving surgery,” while emphasizing that additional research is needed to evaluate survival outcomes.

The analysis was limited by the modest pooled sample, predominantly phase 2 and open-label trials, and variation in disease stage, tumor subtype, chemotherapy regimen, metformin dose, and metabolic status. The results therefore offer a promising efficacy signal—not evidence that metformin should yet become a standard oncology treatment.

Source: Kittleson MM, et al. (2026 July 17) Clin Breast Cancer. Metformin as a Neoadjuvant to Chemotherapy in Locally Advanced and Metastatic Breast Cancer: A Systematic Review and Meta-Analysis of Randomized Controlled Trials