Ozempic, GLP-1 medicines and thyroid cancer: what does the boxed warning mean?

What does the boxed warning say?

The US labels say these medicines caused thyroid C-cell tumours in rodents, and that nobody yet knows whether they do this in people.^1,2

The warning sits in a box at the top of the US label for Ozempic.¹ The same box is on the US labels for Wegovy, Rybelsus and Mounjaro.^2,3,4 Ozempic, Wegovy and Rybelsus all contain semaglutide, as our answer on whether semaglutide is the same as Ozempic explains. The box makes three points.

• Rodents got tumours. Semaglutide caused C-cell tumours in mice and rats, with more tumours at higher doses and with longer use.¹
• People are an open question. Whether it causes these tumours in humans "has not been determined".¹
• Some people must not use it. The US label rules it out for people with medullary thyroid cancer (MTC) in their own or family history. It also rules it out for people with MEN 2.¹

A box can rest on animal data. US rules let the FDA require a boxed warning based on "serious animal toxicity" when human data are missing.⁶ So this box records an animal finding, with no proof of harm in people.

The label also asks doctors to explain the possible MTC risk and its symptoms to each patient.¹

What happened in the animal studies?

In 2-year studies, mice and rats given semaglutide grew more thyroid C-cell tumours at exposures similar to human doses. Non-cancerous growths (adenomas) rose in both; cancers rose clearly in male rats.¹

In rats, the non-cancerous growths rose at every dose tested.¹ Tirzepatide, the medicine in Mounjaro, did the same in a 2-year rat study.² In standard lab tests, semaglutide did not damage genes.¹

The EU label for Ozempic says rodents are "particularly sensitive" to this effect.⁵ It calls the relevance for humans "low, but cannot be completely excluded".⁵

In December 2025, 3 UK thyroid and hormone societies explained why. They said the finding has not been repeated in monkeys or people.⁸ Human C cells have "minimal" activity at the docking points these medicines act on (GLP-1 receptors).⁸ Our GLP-1 full form answer explains the hormone these medicines copy.

What do "C-cell" and "medullary" mean?

The thyroid has two main kinds of cell, and medullary thyroid cancer (MTC) is the cancer that starts in the C cells.^9,10

Follicular cells use iodine from the blood to make thyroid hormone.¹⁰ C cells make calcitonin, a hormone that helps keep blood calcium at a healthy level.⁹

MTC makes up less than 5% of all thyroid cancers.¹⁰ About 1 in 4 cases runs in families, and about 3 in 4 do not.¹⁰

An inherited change in a gene called RET can cause MTC.⁹ MEN 2 (multiple endocrine neoplasia type 2) is an inherited condition that raises the risk of MTC.⁹ The US National Cancer Institute also lists being Asian among the risk factors for thyroid cancer in general.⁹

So the US label's ban covers a small group. It means people with MTC in their own or family history, and people with MEN 2.¹

What did Europe's medicines regulator conclude?

In October 2023, Europe's medicines regulator found that "the available evidence does not support a causal association" with thyroid cancer.⁷

Its safety committee, called PRAC, read published studies plus trial and safety-report data from the makers.⁷ It covered exenatide, liraglutide, dulaglutide, semaglutide and lixisenatide.⁷ Tirzepatide was not on that list. The committee said no label change was needed, and told makers to keep watching.⁷

The EU and US labels differ here. The EU Ozempic label lists only allergy to the medicine as a reason not to use it.⁵ Its thyroid finding sits in the animal-study section.⁵ The US label bans use with MTC or MEN 2.¹

The UK societies take a middle line. They see no sign of a clinically meaningful short-term rise in risk.⁸ They still advise avoiding these medicines after MTC, with a known RET gene change, or with MEN 2 in the family.⁸

What do studies in people show?

Most large studies find no clear rise in thyroid cancer. One French study found a higher risk, and some pooled trial data show a small, uncertain signal.^{11,12,13,14,15,16,17}

Two kinds of number appear below. Relative risk compares two groups ("58% higher"). Absolute risk counts how many people it happened to ("1 in 10,000 a year").

"Not significant" means the result could easily be chance. The 2026 trial pool gave no rate per person, so we show its pooled result. Checked 5 Oct 2026.

Two rates that can't be compared. The international study counted all thyroid tumours, including non-cancerous ones, and only its US data passed the authors' checks.¹³ So its 9 to 10 per 10,000 can't be set beside the Scandinavian cancer rate.

What 1 in 10,000 looks like. In the Scandinavian study, about 1 in every 10,000 users got thyroid cancer each year.¹² That was close to the rate on the comparison medicine.¹²

The French 58% sounds large, but it applies to a rare event. If it were applied to the Scandinavian comparison rate of 1.46 per 10,000 a year, it would mean about 2.3. That is our illustration, mixing two different groups of people, not a figure from either paper.^11,12

The 2024 trial signal was fragile. Its authors gave it a "fragility index" of 1, meaning one case counted differently would have erased the result.¹⁴

Why don't the studies agree?

Thyroid cancer is rare, people on a new medicine get checked more often, and most studies follow users for only a few years.^8,12,15

The Scandinavian authors put it plainly: their study "cannot exclude a small increase in risk".¹² The 2026 trial review says the same about a clinically relevant rise.¹⁶

What does the Indian label say?

We could not find the Indian package insert for Ozempic, Wegovy, Rybelsus or Mounjaro online, so we can't confirm whether it carries the US warning.

Indian inserts do exist. In August 2025, CDSCO's expert committee recommended approval of an updated Rybelsus insert, aligned with the EU version.²¹ The news report did not publish the insert's text.²¹ The EU label has no MTC ban, so the Indian wording can't be guessed from the US one.⁵

India's regulator has asked about MTC in trials. On 20 Aug 2025, its expert committee reviewed trials of a generic semaglutide for diabetes and for weight.¹⁸ It asked for an ultrasound check for MTC before people join, and an MTC caution in the consent form.¹⁸

That was a condition for joining a trial. For patients, the US label calls routine calcitonin tests or thyroid ultrasound of "uncertain value". They can lead to unnecessary procedures.¹

A government backgrounder of 1 Apr 2026 lists medullary thyroid cancer among the side effects of GLP-1 medicines.¹⁹ The US labels word it more carefully, saying it is unknown whether these medicines cause MTC in people.¹ Our answer on whether Ozempic is available in India covers its approval here.

Which symptoms does the label list?

The US Ozempic label lists a lump or swelling in the neck, hoarseness, trouble swallowing and shortness of breath as possible signs of thyroid cancer.¹

This is the label's own list, and it tells patients to report these to their healthcare provider.¹ Other conditions can cause the same signs, the US National Cancer Institute notes.⁹

The label also says a thyroid lump found on an exam or scan should be looked at further. The same goes for a high calcitonin result.¹

Routine checks are a different matter. The US label calls routine calcitonin tests or ultrasound of uncertain value.¹ The UK societies do not recommend them for people with no particular reason.⁸ Guide 4, Should I consider a GLP-1?, covers the other reasons these medicines may not suit someone.

Our posts on Ozempic side effects and Mounjaro side effects, and the "I'm already on it" path cover the other warning signs.

Questions people ask

How we researched this

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