Breast Cancer Patients on Endocrine Therapy Lose Less Weight on GLP-1 Drugs

Patients with breast cancer who take GLP-1 receptor agonists while undergoing endocrine therapy lose substantially less weight than the reductions reported in the drugs’ landmark trials, according to a retrospective cohort analysis published in Breast Cancer Research and Treatment. Among 269 patients at a single academic center who received a GLP-1 receptor agonist alongside adjuvant endocrine therapy for at least one year, mean weight loss at 12 months was 4.5% of body weight, or 3.9 kilograms (95% CI: −6.0% to −3.0%; −4.96 to −2.93 kg). That figure falls far short of the 10% to 20% reductions observed in postmenopausal subgroups of GLP-1 randomized trials, a gap that was statistically significant across comparisons (all p < 0.001). Only 21.3% of patients achieved a weight loss of 10% or more, and 10.4% reached 15% or greater loss.

The attenuated response may be rooted in the metabolic biology of hormone receptor–positive breast cancer treatment itself. Endocrine therapy is associated with adverse metabolic effects, including increased fat mass, decreased lean muscle mass, and insulin resistance, changes that the authors note may attenuate the efficacy of GLP-1 receptor agonists. The study was designed specifically to evaluate weight loss outcomes in patients receiving the two therapies concurrently. The primary outcome was percentage weight change from GLP-1 initiation to 12 months, with a tolerance window of plus or minus two months, and medication data were drawn from electronic medication reconciliation lists and pharmacy dispense reports rather than self-report.

Not all GLP-1 agents performed equally. Weight loss differed significantly by drug (p = 0.003), and the pattern was striking: patients on semaglutide lost 5.8% on average and those on tirzepatide lost 4.7%, while patients on dulaglutide, which is approved only for type 2 diabetes and not for weight management, essentially did not lose weight at all, averaging a gain of 0.1%. The authors conclude that GLP-1 agent selection meaningfully influences outcomes in this population.

Several patient-level factors shaped the results. Higher baseline weight was associated with greater absolute weight loss, with a modest correlation coefficient of −0.19 (p = 0.002), although percentage weight loss did not differ significantly across baseline BMI categories (p = 0.303). Patients whose tumors were progesterone receptor–negative lost more weight than those with progesterone receptor–positive disease, 8.2% versus 4.2%, a statistically significant difference with a moderate effect size (d = 0.46). The biological basis for this association is not established by the study, but it raises the possibility that tumor hormone receptor status and metabolic response to these therapies are linked in ways that merit further investigation.

Type 2 diabetes was common in the cohort, present in 70.6% of patients, which means most participants were taking GLP-1 receptor agonists in a context that includes glycemic control. Diabetes status itself was not significantly associated with weight loss on t-test (p = 0.288), although the numbers trended in one direction: patients without type 2 diabetes were numerically more likely to achieve 15% or greater weight loss, at 21.5% versus 5.8% among those with diabetes. The authors report this difference without overinterpreting it.

The findings arrive amid growing interest in whether GLP-1 receptor agonists might do more than trim weight in cancer patients. The authors note emerging evidence of oncologic benefit from these agents, and they call for prospective trials to test outcomes in this setting. Endocrine therapy’s tendency to promote fat gain and reduce lean mass has been a persistent problem for patients with hormone receptor–positive disease, making effective weight management strategies during treatment an active area of investigation.

As with any retrospective, single-center cohort, the analysis has constraints that temper its conclusions. Patients were required to have initiated GLP-1 therapy during the study period and to have maintained concurrent GLP-1 receptor agonist and endocrine therapy for at least one year, which introduces the possibility that individuals who tolerated the combination poorly or stopped early are underrepresented. Weight change was measured over a one-year window with a two-month tolerance, and the comparison against randomized trial subgroups is cross-study rather than head-to-head, meaning differences in population characteristics, dosing, and adherence could contribute to the observed attenuation.

Still, the central result is clear: statistically significant but substantially smaller weight loss than the trials reported, with drug choice emerging as one factor that differed across outcomes. For breast cancer patients prescribed these medications alongside endocrine therapy, the study suggests that weight loss expectations should account for the concurrent treatment context, and the authors argue that prospective trials are needed given the emerging evidence of oncologic benefit.

Subject of Research: Weight loss outcomes with GLP-1 receptor agonists in breast cancer patients receiving concurrent adjuvant endocrine therapy

Article Title: GLP-1 receptor agonist–associated weight loss in breast cancer patients receiving concurrent endocrine therapy: a retrospective cohort analysis

Article References: Cronin, K. M., & Tiersten, A. (2026). GLP-1 receptor agonist–associated weight loss in breast cancer patients receiving concurrent endocrine therapy: a retrospective cohort analysis. Breast Cancer Research and Treatment, 220(1), Article 17. https://doi.org/10.1007/s10549-026-08096-z

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08096-z

Keywords: GLP-1 receptor agonists, breast cancer, endocrine therapy, semaglutide, tirzepatide, dulaglutide, obesity, weight loss, retrospective cohort, hormone receptor-positive, type 2 diabetes, oncology