What the boxed warning says
In rodents, semaglutide and tirzepatide caused thyroid C-cell tumours at exposures relevant to human doses. The labels say it is unknown whether they cause these tumours, including medullary thyroid carcinoma (MTC), in humans, because the human relevance of the rodent finding has not been determined.[1][2] C-cells are a specific cell type in the thyroid; MTC is a rare cancer that starts in them and is different from the common thyroid cancers.
Who should not take them
Both labels say the drugs are contraindicated (must not be used) in people with a personal or family history of MTC, or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), an inherited condition that raises MTC risk.[1][2] If anyone in your family has had thyroid cancer, it is worth finding out which type, because that changes the answer.
What human studies show
The largest study, from Denmark, Norway and Sweden, compared 145,410 people on GLP-1 drugs with 291,667 on another diabetes drug class over a mean of 3.9 years. Thyroid cancer rates were 1.33 versus 1.46 per 10,000 person-years, a hazard ratio of 0.93, meaning no substantial increase.[3] A French national study reported a 58% higher risk with one to three years of use, which is one reason the question stays open.[4] A 2025 multinational study of people with type 2 diabetes, using health records from several countries, again found no increased risk of thyroid tumours compared with two other diabetes drug classes.[4]
Honest summary: the rodent signal is real; the larger human studies over about four years do not show a meaningful increase, though one French study did; a small or slow-developing risk cannot be excluded yet. The labels also say routine blood calcitonin tests or thyroid ultrasounds are of uncertain value for catching MTC early.[1][2]
Symptoms to report
- A lump or swelling in the neck
- Trouble swallowing
- Shortness of breath
- Hoarseness that does not go away
These are the symptoms the labels tell prescribers to counsel patients about.[1][2]
Heard vs evidence
The short version
Short on the way. A 35-second captioned version of this page is in production for the Fewer Bites channel. It will be embedded here when it is live.
Questions to bring to your prescriber
Print these or screenshot them. They are conversation starters, not a checklist of things to do.
- Someone in my family had thyroid cancer: do we know which type, and does it matter for me?
- Is there any reason for me to have thyroid checks while on this medicine?
- Which neck or swallowing symptoms should I report straight away?
Education, not advice. This page explains what trials, labels and guidelines report. It does not tell anyone to start, change or stop a medicine. Your prescriber knows your history; this page does not.
Sources
- Novo Nordisk / FDA. Wegovy (semaglutide) injection and tablets, prescribing information, revised 06/2026. label
- Eli Lilly / FDA. Zepbound (tirzepatide) prescribing information, revised 08/2026. label
- Pasternak B et al. Glucagon-like peptide 1 receptor agonist use and risk of thyroid cancer: Scandinavian cohort study. BMJ 2024;385:e078225. abstract only · cohort
- Morales DR et al. Risk of thyroid tumors with GLP-1 receptor agonists: a retrospective cohort study. Diabetes Care 2025 (also summarises Bezin et al. 2023, French study). abstract only · cohort; Bezin figure secondary