Does Ozempic Cause Thyroid Cancer? What the Evidence Says
Ozempic thyroid cancer risk explained: the boxed warning, why rodent tumours started it, what large human studies found, MTC and MEN 2, and warning signs.
Anonymous user report. Not medical advice. Peptides discussed are not FDA-approved for the uses described. Editorial process →
Short answer
Ozempic (semaglutide) carries a boxed warning for thyroid C-cell tumours because semaglutide caused them in mice and rats. Whether it does the same in people is unknown. The largest human studies so far, including a 2024 Scandinavian cohort of about 145,000 GLP-1 users and a 2025 six-database study that included Ontario, found no clear increase in thyroid cancer over a few years. One French study and a pooled analysis of trials did find a higher risk, so the question is not closed, especially for long-term use. The label says not to use it with a personal or family history of medullary thyroid cancer (MTC) or MEN 2. Not medical advice. Last reviewed October 21, 2026.
What the boxed warning says
The US Ozempic prescribing information (DailyMed, June 2026) and the Canadian Ozempic product monograph (July 2026) say much the same thing:
- —In mice and rats, semaglutide caused thyroid C-cell tumours (benign and cancerous) that increased with dose and length of exposure, at exposures similar to those in patients.
- —It is unknown whether Ozempic causes C-cell tumours, including MTC, in humans. The Canadian monograph adds that no MTC cases were seen in its clinical trials.
- —It must not be used by anyone with a personal or family history of MTC or with MEN 2 (multiple endocrine neoplasia type 2, an inherited syndrome that causes MTC).
- —Routine calcitonin blood tests or thyroid ultrasounds are of uncertain value for catching MTC early and may lead to unnecessary procedures, because thyroid lumps are common and the calcitonin test is not very specific.
- —Patients should know the warning signs of a thyroid tumour: a lump or swelling in the neck, trouble swallowing, shortness of breath, or hoarseness that does not go away.
Why the warning exists: rodents vs people
C-cells are a small group of cells in the thyroid that make the hormone calcitonin; MTC is the cancer that starts in them. It is different from papillary thyroid cancer, the most common type, which starts in other thyroid cells. Rats and mice have many GLP-1 receptors on their C-cells, and GLP-1 drugs push those cells to release calcitonin and multiply. A Novo Nordisk study found human and monkey C-cells have few of these receptors, that monkeys given liraglutide for 20 months at far higher exposures did not develop C-cell overgrowth, and that calcitonin stayed in the normal range in patients treated for two years (Bjerre Knudsen 2010). The authors still said the long-term effect on the human thyroid is unknown, which is why the warning stays.
What human studies found
These are the main studies people cite. Most looked at GLP-1 drugs as a group, mostly in type 2 diabetes, not Ozempic alone.
| Study | Who | Finding |
|---|---|---|
| Bezin 2023 (Diabetes Care) | French national insurance data: 2,562 people with thyroid cancer vs 45,184 matched controls, all with type 2 diabetes | Higher risk with 1 to 3 years of GLP-1 use: all thyroid cancer HR 1.58, MTC HR 1.78. This is the study that set off the 2023 headlines |
| Silverii 2024 (meta-analysis of trials) | 64 randomised trials of at least a year | Thyroid cancer odds 1.52 times higher on GLP-1 drugs, but rare: about 1 extra case per 1,349 people treated for 5 years. No significant rise for papillary cancer or MTC on their own, and the result was fragile |
| Pasternak 2024 (BMJ) | 145,410 GLP-1 users vs 291,667 DPP-4 inhibitor users in Denmark, Norway and Sweden, about 4 years | No increase: HR 0.93 (95% CI 0.66 to 1.31). Rates were 1.33 vs 1.46 cases per 10,000 person-years |
| Baxter 2025 (Thyroid) | 98,147 GLP-1 users vs about 2.5 million DPP-4 inhibitor users in six databases, including Ontario, Canada | No increase: pooled HR 0.81 (0.59 to 1.12), and no rise with higher cumulative use. Follow-up was only 1.8 to 3 years, too short to rule out long-term risk |
| Pollack 2025 | 89,646 adults with type 2 diabetes who stayed on a GLP-1 drug for at least a year, vs other diabetes drugs | No increased thyroid cancer risk over a median 4.5 years |
What regulators concluded
After the French study, the European Medicines Agency's safety committee reviewed it with the other evidence and concluded in October 2023 that the data do not support a causal link between GLP-1 drugs, including semaglutide, and thyroid cancer; it made no change to product information and asked manufacturers to keep monitoring (EMA PRAC, October 2023). The US and Canadian labels kept their warnings, which are based on the rodent findings rather than on human cases.
Why the studies disagree
- —Observational studies can show links that are not caused by the drug. People who start a new medicine often see doctors more and get more scans and tests, which can turn up small thyroid cancers that were already there.
- —Comparison groups differ. Bezin compared users with matched controls; Pasternak and Baxter compared new GLP-1 users with people starting another diabetes drug, which is designed to reduce that bias.
- —Follow-up is short. Most studies cover about 2 to 5 years. Cancers can take longer to appear, so long-term risk is not settled.
- —Small numbers. Thyroid cancer, and MTC especially, is rare, so a few cases can move the results.
- —Most data come from diabetes doses and diabetes patients. Fewer data exist for the higher semaglutide doses used for weight loss (Wegovy).
Wegovy, Mounjaro and Zepbound
Wegovy is semaglutide too, and its US label has the same boxed warning. Mounjaro and Zepbound (tirzepatide) carry the same warning based on C-cell tumours in rats, with the same MTC and MEN 2 restriction. Tirzepatide is newer, so there is even less long-term human data. See Mounjaro side effects and Zepbound side effects. Unapproved "research" semaglutide or tirzepatide sold online has none of the quality controls of the approved drugs.
In Canada
Health Canada's approved product monograph for Ozempic puts the thyroid C-cell warning in its Serious Warnings and Precautions box, with the same MTC and MEN 2 contraindication as the US label. Ontario's health records were one of the six data sources in Baxter 2025, which found no increase. For what provincial drug plans pay for, see Ozempic coverage in Canada, and for side effects in a Canadian context, Ozempic side effects in Canada.
What users report here
None of the 408 confessions on our semaglutide, tirzepatide, liraglutide and retatrutide hubs mentions the thyroid (checked October 21, 2026). The closest is #146, a tirzepatide user of two years, who reacts to a wave of GLP-1 cancer headlines by pointing out that they came from observational data and that healthier people may be the ones taking the drugs. That is one anonymous opinion, not evidence. Every report is on the semaglutide hub.
Related topics
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More confessions from the archive
Community Q&A
- Does Ozempic cause thyroid cancer?
- It is not known. Semaglutide caused thyroid C-cell tumours in rodents. Large human studies from Scandinavia, Canada and other countries found no clear increase over a few years, but one French study and a trial meta-analysis found a higher risk, and long-term data are limited.
- Why does Ozempic have a black box warning?
- Because semaglutide caused thyroid C-cell tumours in mice and rats at exposures similar to those in patients. The warning says it is unknown whether this happens in humans.
- Who should not take Ozempic because of thyroid cancer risk?
- The label says anyone with a personal or family history of medullary thyroid carcinoma or with multiple endocrine neoplasia type 2 (MEN 2) should not use it.
- What are the warning signs of thyroid cancer on Ozempic?
- The label lists a lump or swelling in the neck, trouble swallowing, shortness of breath and hoarseness that does not go away. Anyone with these should see a clinician.
- Can you take Ozempic if you have hypothyroidism or had thyroid cancer?
- The label contraindication is specific to medullary thyroid cancer and MEN 2. Other thyroid conditions, including other thyroid cancer types, are a question for the prescriber, who can weigh the full history.
- Do I need a thyroid ultrasound or calcitonin test before Ozempic?
- The label says routine calcitonin testing or ultrasound is of uncertain value and can lead to unnecessary procedures. If a test is done and is abnormal, or a nodule is found, the person should be evaluated further.
Sources
Anonymous first-hand reports plus sourced context — educational only, not medical advice. Editorial policy & corrections.