GLP-1s and cancer: what the boxed warning is based on
The warning on the box is about one tumor type, found in rodents. Whether it happens in people is what the labels say is unknown — and the trials have been pooled twice, with answers that differ at the margin.
Do GLP-1 drugs cause cancer?
The labels say these drugs cause thyroid C-cell tumors in rodents and that it is unknown whether they do in humans. All nine GLP-1 labels on this site carry that boxed warning, and each contraindicates the drug for anyone with a personal or family history of medullary thyroid carcinoma or with Multiple Endocrine Neoplasia syndrome type 2. The rodent finding is the basis; human cases have not established it.
Pooled trials point both ways on thyroid cancer and nowhere on the rest. A 2026 meta-analysis in Annals of Internal Medicine, 48 placebo-controlled trials and 94,245 people, found GLP-1 drugs probably have little or no effect on thyroid cancer (odds ratio 1.37, 95% CI 0.82 to 2.31) or on pancreatic, breast or kidney cancer. A 2024 analysis of 64 trials found a 52% higher relative risk of thyroid cancer (odds ratio 1.52, 1.01 to 2.29) — about one extra case for every 1,349 people treated for five years. Neither had long follow-up; the trials were not designed to find cancer.
Last updated 2026-09-02 — the newest FDA label version quoted below. Read and dated by Alex Seot.
The boxed warning, label by label
The first sentence of each boxed warning names the species the tumors were found in. Every one of these labels then says human relevance has not been determined, and contraindicates the drug with a personal or family history of medullary thyroid carcinoma or with MEN 2.
Rats; human relevance not determined
“In rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. It is unknown whether ZEPBOUND causes thyroid C-cell tumors, including medullary thyroid carcinoma (MTC), in humans as human relevance of tirzepatide-induced rodent thyroid C-cell tumors has not been determined.”
Rats
“In both male and female rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures.”
Rodents
“In rodents, semaglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures.”
Rodents
“In rodents, semaglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures.”
Rodents
“In rodents, semaglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures.”
Rats and mice
“Liraglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures in both genders of rats and mice.”
Rats and mice
“Liraglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures in both genders of rats and mice.”
Rats, after lifetime exposure
“In male and female rats, dulaglutide causes a dose-related and treatment-duration-dependent increase in the incidence of thyroid C-cell tumors (adenomas and carcinomas) after lifetime exposure.”
Stated for the drug class, not from orforglipron's own rodent data
“In products with glucagon-like peptide-1 (GLP-1) receptor agonist activity that are pharmacologically active in rats and mice, rodent thyroid C-cell tumors (adenomas and carcinomas) have been observed”
Labels that say nothing on it
No thyroid warning on these — none of them is a GLP-1:
What the pooled trials found
48 placebo-controlled trials, 94,245 participants. Probably little or no effect on thyroid (OR 1.37, 0.82 to 2.31), pancreatic (0.84, 0.53 to 1.35), breast (0.95, 0.60 to 1.49) or kidney cancer (1.12, 0.78 to 1.60), moderate certainty; may have little or no effect on colorectal, esophageal, liver, gallbladder, ovarian or endometrial cancer, low certainty. Consistent for semaglutide and tirzepatide. The trials were short and not designed for cancer.
64 randomised trials, 26 with at least one thyroid cancer. Overall thyroid cancer odds ratio 1.52 (1.01 to 2.29), a five-year number needed to harm of 1,349, with a fragility index of 1. No significant association for papillary (1.54) or medullary thyroid cancer (1.44) separately.
GLP-1s and cancer, answered
- What cancer is linked to GLP-1s?
- The labels name one: medullary thyroid carcinoma, the tumor type seen in rodents, which is why the drugs are contraindicated for people with a personal or family history of it or with MEN 2. Pooled trials have not linked them to pancreatic, breast, kidney or colorectal cancer; the largest analysis, in 2026, found little or no effect on each.
- Should I avoid a GLP-1 if thyroid cancer runs in my family?
- If the family history is medullary thyroid carcinoma, or MEN 2, every GLP-1 label on this site says not to take one. The contraindication names that type specifically; papillary thyroid cancer, the common form, is not named.
- Do GLP-1s lower cancer risk?
- Some observational studies report lower rates of obesity-related cancers in people taking them. The randomised evidence does not yet show it: the 2026 meta-analysis found little or no effect in either direction and says longer studies are needed. Headlines that GLP-1s cut cancer risk are reporting the observational data.
- What symptoms do the labels tell patients to report?
- A mass in the neck, trouble swallowing, shortness of breath, or persistent hoarseness — listed in the labels as possible signs of a thyroid tumor.