- GLP-1 medicines have not been shown to cause thyroid cancer in humans: in the largest cohort study, 1.33 vs 1.46 cases per 10,000 person-years occurred on GLP-1s vs a comparison drug.4
- The boxed warning on semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro) comes from thyroid tumors in rodents. The labels say human relevance is unknown.1,2
- A French study found a 58% higher relative risk after 1 to 3 years of use, and pooled trials show a small signal.3,5 Part of that may come from people on these medicines simply getting checked more.8
- The labels list symptoms to report: a lump in the neck, trouble swallowing, shortness of breath or lasting hoarseness.1
Why do GLP-1 labels have a thyroid cancer warning?
The US labels for Wegovy, Ozempic and Zepbound open with a boxed warning, the FDA's most prominent kind. It says the drug caused dose-dependent thyroid C-cell tumors in mice and rats (rats only for tirzepatide) at exposures similar to those in people.1,2 C-cells make the hormone calcitonin. The cancer that can come from them is called medullary thyroid carcinoma (MTC), a rare type, different from the more common papillary thyroid cancer.
The labels say plainly that it is unknown whether this happens in humans. They also note that cases of MTC have been reported in people taking liraglutide, an older GLP-1 medicine, but that these reports are not enough to establish or rule out a link.1
| What the label says | Wegovy / Ozempic (semaglutide) | Zepbound (tirzepatide) |
|---|---|---|
| Animal finding | C-cell tumors in mice and rats | C-cell tumors in rats |
| Do not use if | Personal or family history of MTC, or multiple endocrine neoplasia type 2 (MEN 2) | |
| Symptoms to know | Mass in the neck, trouble swallowing, shortness of breath, persistent hoarseness | |
| Routine calcitonin or ultrasound checks | Of uncertain value; may lead to unnecessary procedures | |
What human studies found
Two large studies from Europe reached different conclusions. Both were observational, so neither can prove cause or rule it out.
France (2023): a raised risk. Researchers used the French national health insurance database to study people with type 2 diabetes. They compared 2,562 people with thyroid cancer with 45,184 matched people without it.3 Using a GLP-1 medicine for 1 to 3 years was linked to a higher risk of all thyroid cancer (hazard ratio 1.58) and of medullary thyroid cancer (1.78).
Denmark, Norway and Sweden (2024): no clear increase. This study used national registers to follow 145,410 adults aged 18 to 84 who started a GLP-1 medicine and compared them with 291,667 who started a DPP-4 inhibitor, another diabetes drug, between 2007 and 2021.4 Most GLP-1 users took liraglutide (57.3%) or semaglutide (32.9%).
The authors said the results rule out more than a 31% rise in relative risk, over an average of 3.9 years of follow-up. For medullary thyroid cancer the estimate was 1.19, but with so few cases the range was wide (0.37 to 3.86). People on semaglutide were followed for only 1.1 years on average.4
What randomized trials show
Trials of these medicines were not designed to study cancer, and thyroid cancer is rare, so each trial has only a handful of cases. Researchers pool them to get a clearer picture.
| Analysis | Trials | Thyroid cancer result |
|---|---|---|
| Silverii, 2024 | 64 | Odds ratio 1.52, just significant; about 1 extra case per 1,349 people treated for 5 years |
| Silverii, 2025 | 50 | Odds ratio 1.55, significant; overall cancer 1.05, not significant |
| Ko, 2026 (94,245 people) | 48 | Odds ratio 1.37, not significant; 1 fewer to 9 more cases per 10,000 treated |
The 2024 analysis found the result was fragile: moving a single case would change it. It found no significant increase for papillary or medullary thyroid cancer on their own.5 The 2026 analysis rated its evidence as moderate certainty and found the same pattern for semaglutide and tirzepatide trials.6
Could more testing explain part of the signal?
Possibly. The US labels note a "high background incidence of thyroid disease", so more checking tends to find more.1 People starting a GLP-1 medicine may get more blood tests and scans because of the warning. The Scandinavian authors suggested the French result could partly reflect this detection bias, and saw a small, non-significant rise only in the first year of treatment, when extra checks are most likely.4 A 2026 review of 29 studies found that thyroid screening in GLP-1 users varied widely and that only 11 of them discussed this detection bias, despite documented increases in imaging.8 Detection bias may explain part of the signal, but it has not been shown to explain all of it.
What about other cancers?
In trials, GLP-1 medicines have not changed overall cancer risk (odds ratio 1.05 across 50 trials).7 Observational data hint at possible benefits for some obesity-related cancers. A US health-record study of 1,651,452 people with type 2 diabetes found GLP-1 users had lower rates of 10 of 13 obesity-associated cancers than insulin users, but no lower rate of thyroid cancer.9 Compared with metformin, there was no lower risk of any cancer and a higher rate of kidney cancer (hazard ratio 1.54). This kind of study shows links, not causes. For the long-term picture on weight, see long-term GLP-1 results.
What this means in practice
- The absolute numbers are small. Thyroid cancer occurred in roughly 1 to 1.5 people per 10,000 per year in the Scandinavian study, on either drug.4
- The question is not closed. Follow-up in most studies is only a few years, and there is less long-term data on semaglutide and tirzepatide than on older GLP-1 drugs.
- Family history matters. The labels rule these medicines out for anyone with a personal or family history of MTC or MEN 2. Make sure your prescriber knows your family history.
- Report neck symptoms. A lump in the neck, trouble swallowing, shortness of breath or hoarseness that does not go away should be raised with your doctor.1
Related: semaglutide and vision loss (NAION) and common GLP-1 side effects.
Common questions
Does Ozempic cause thyroid cancer?
It has not been shown in humans. The boxed warning on semaglutide (Ozempic, Wegovy) is based on thyroid tumors in mice and rats. In a 2024 Scandinavian study of 145,410 adults starting a GLP-1 medicine, thyroid cancer occurred at 1.33 cases per 10,000 person-years vs 1.46 on a comparison drug, with no increase in risk.
Why does Zepbound have a thyroid cancer warning?
Because tirzepatide (Zepbound, Mounjaro) caused thyroid C-cell tumors in rats at exposures similar to those in people. The US label says it is unknown whether this happens in humans, and that Zepbound must not be used by people with a personal or family history of medullary thyroid carcinoma or with MEN 2.
Do GLP-1 medicines raise the risk of cancer overall?
Randomized trials have not shown that. A 2025 meta-analysis of 50 trials found no difference in overall cancer (odds ratio 1.05), and a 2026 analysis of 48 placebo-controlled trials with 94,245 people found little or no effect on thyroid, pancreatic, breast or kidney cancer. Trials were short and not designed to study cancer.
What did the French study on GLP-1s and thyroid cancer find?
A 2023 French nationwide study of people with type 2 diabetes compared 2,562 people with thyroid cancer with 45,184 people without it. Using a GLP-1 medicine for 1 to 3 years was linked to a higher risk of thyroid cancer (hazard ratio 1.58). The study was observational and could not prove cause.
What thyroid symptoms should be reported on a GLP-1 medicine?
The US labels for Wegovy, Ozempic and Zepbound list a lump or mass in the neck, trouble swallowing, shortness of breath and hoarseness that does not go away. Anyone with these symptoms should tell their doctor.
Sources
- Novo Nordisk. WEGOVY (semaglutide) prescribing information. Revised 06/2026. DailyMed. The Ozempic label carries the same boxed warning: DailyMed
- Eli Lilly. ZEPBOUND (tirzepatide) prescribing information. Revised 08/2026. DailyMed
- Bezin J, Gouverneur A, Pénichon M, et al. GLP-1 Receptor Agonists and the Risk of Thyroid Cancer. Diabetes Care. 2023;46(2):384-390. PubMed
- Pasternak B, Wintzell V, Hviid A, et al. Glucagon-like peptide 1 receptor agonist use and risk of thyroid cancer: Scandinavian cohort study. BMJ. 2024;385:e078225. PubMed · Full text (PMC)
- Silverii GA, Monami M, Gallo M, et al. Glucagon-like peptide-1 receptor agonists and risk of thyroid cancer: A systematic review and meta-analysis of randomized controlled trials. Diabetes Obes Metab. 2024;26(3):891-900. PubMed
- Ko A, Chang YC, Bahar F, et al. Risk for Cancer With Glucagon-Like Peptide-1 Receptor Agonists and Dual Agonists: A Systematic Review and Meta-analysis. Ann Intern Med. 2026;179(2):216-229. PubMed
- Silverii GA, Marinelli C, Bettarini C, et al. GLP-1 receptor agonists and the risk for cancer: A meta-analysis of randomized controlled trials. Diabetes Obes Metab. 2025;27(8):4454-4468. PubMed
- Al-Mashhadani M, Khalifa A, Al-Mashhadani Y, et al. Thyroid surveillance practices in GLP-1 receptor agonist use and its potential influence on thyroid cancer detection: A scoping review. Diabetes Res Clin Pract. 2026;242:113630. PubMed
- Wang L, Xu R, Kaelber DC, et al. Glucagon-Like Peptide 1 Receptor Agonists and 13 Obesity-Associated Cancers in Patients With Type 2 Diabetes. JAMA Netw Open. 2024;7(7):e2421305. PubMed · Full text (PMC)
