Retrospective study links GLP-1 drugs to lower breast cancer mortality and recurrence, but experts question data quality

A retrospective study published in May 2026 in JAMA Network Open analyzed breast cancer outcomes in more than 800,000 U.S. women with obesity or type 2 diabetes. The researchers found that use of GLP-1 receptor agonists was associated with lower all-cause mortality and recurrence over 10 years, although experts questioned the quality of the data, according to the study authors.

The study, titled “Survival and Recurrence With GLP-1 Receptor Agonists in Breast Cancer,” was published in the May 2026 issue of JAMA Network Open (PMID 42113513). Researchers conducted a retrospective analysis of breast cancer outcomes in 841,831 U.S. women, primarily White (72%) with a mean age of 69 years, who had either obesity or type 2 diabetes (T2D). The investigators used propensity score matching to balance measured covariates and examined associations between GLP-1 receptor agonist (GLP-1 RA) use and two primary outcomes: all-cause mortality and recurrence-free survival (RFS) over a 10-year follow-up period.

Among patients with breast cancer and obesity, GLP-1 RA use was associated with a significantly lower risk of all-cause mortality, with a hazard ratio of 0.35 (95% CI, 0.21–0.58; P < .001).

Among patients with breast cancer and obesity, GLP-1 RA use was associated with a significantly lower risk of all-cause mortality. The hazard ratio (HR) for mortality was 0.35, with a 95% confidence interval (CI) of 0.21 to 0.58, and a P value of less than .001, according to the study authors. The analysis also found a lower risk of cancer recurrence in this group, with an HR for RFS of 0.44 (95% CI, 0.30–0.64; P < .001). These results were based on Kaplan-Meier estimators calculating 5- and 10-year survival and recurrence probabilities.

In the cohort of patients with T2D, GLP-1 RA use was compared with insulin or metformin treatment. The study reported an even stronger association with lower all-cause mortality, showing an HR of 0.09 (95% CI, 0.06–0.15; P < .001). GLP-1 RA users also had a lower risk of recurrence, with an HR for RFS of 0.33 (95% CI, 0.21–0.50; P < .001). However, when GLP-1 RAs were compared with sodium-glucose cotransporter-2 (SGLT2) inhibitors in patients with T2D, no significant differences in mortality or recurrence were observed.

The study included prespecified subgroup analyses focusing on postmenopausal patients and conducted landmark analyses at six and 12 months to assess potential time-dependent effects. The authors noted that 10-year RFS estimates were less precise due to fewer patients remaining at risk beyond five years. They emphasized that findings were derived from observational data rather than a randomized controlled trial, limiting causal inferences.

Expert reactions published by the Science Media Centre raised concerns regarding the study’s methodology and interpretation. Experts highlighted that recurrence-free survival is a “softer” endpoint susceptible to ascertainment bias, as patients receiving newer medications like GLP-1 RAs may be monitored more closely, potentially increasing detection of relapses. Commentators also pointed to the unusually large effect sizes reported, describing them as a red flag for residual confounding or bias. One expert cited by the Science Media Centre said the magnitude of the reported reductions in mortality and recurrence was unlikely to be explained solely by the drug’s effect.

The study contributes to a growing body of literature exploring potential benefits of GLP-1 RAs beyond their established roles in glucose control and weight management. A 2025 review published in PMC noted preliminary data suggesting GLP-1 RA use does not increase breast cancer recurrence risk but underscored the need for prospective studies to confirm any protective effects. The authors and external experts alike stressed that the current evidence shows association rather than causation and that further research, including randomized trials, is required to clarify the relationship between GLP-1 RA use and breast cancer outcomes.