The GLP-1 thyroid cancer warning: what it says and what studies show

What does the GLP-1 boxed warning say?

The boxed warning on Ozempic, Wegovy, Mounjaro and Zepbound is titled "Risk of thyroid C-cell tumors". The Ozempic version reads¹:

"In rodents, semaglutide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. It is unknown whether OZEMPIC causes thyroid C-cell tumors, including medullary thyroid carcinoma (MTC), in humans as human relevance of semaglutide-induced rodent thyroid C-cell tumors has not been determined."

The tirzepatide warning is the same in shape, but the tumors were seen in rats only^3–4. In a 6-month study in a type of mouse, tirzepatide didn't cause tumors³.

Who can't take a GLP-1 because of thyroid cancer risk?

The US labels say these medicines are "contraindicated" (must not be used) in people with a personal or family history of medullary thyroid carcinoma, or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)^1–4. The Medication Guide describes MTC as "a type of thyroid cancer" and MEN 2 as "an endocrine system condition"³.

The labels don't list other thyroid cancers, nodules or an underactive thyroid as reasons not to use them. If you have a thyroid condition, your prescriber is the one to talk it through with.

The labels also say routine calcitonin blood tests or thyroid ultrasounds are "of uncertain value" for catching MTC early, and may lead to unnecessary procedures^1,3. If calcitonin is measured and high, or a nodule is found, the labels say it should be looked into further^1,3.

Is there a thyroid cancer warning on GLP-1s in the UK?

No boxed warning, and no MTC or MEN 2 contraindication. The only contraindication in the UK SmPCs for Wegovy, Ozempic and Mounjaro is allergy to the medicine or its ingredients^5–7.

The rodent findings sit in section 5.3, the animal safety data. For semaglutide it says the tumors come from a mechanism "to which rodents are particularly sensitive", and "the relevance for humans is considered to be low, but cannot be completely excluded"^5–6. For tirzepatide it says "the human relevance of these findings is unknown"⁷. The Mounjaro SmPC lists raised blood calcitonin as a side effect⁷.

Do GLP-1s cause thyroid cancer in humans?

It isn't known. Some studies in people have found a higher risk and others haven't. Thyroid cancer is uncommon, which makes it hard to study. The labels note that MTC has been reported in people on liraglutide, an older GLP-1, but say those reports can't "establish or exclude" a cause^1–2.

Reading it both ways: the French study found a raised risk, which is why the question is still open⁸. The largest cohorts and trial pools haven't confirmed it, but their ranges still allow a small increase^9,11–12. The Scandinavian authors say their data rule out more than a 31% rise in relative risk over about 4 years⁹. The trials in the meta-analyses weren't designed to look for cancer and had short follow-up^11–12.

Thyroid symptoms to watch for on a GLP-1

The US labels list a lump in the neck, trouble swallowing, shortness of breath and hoarseness that won't go away as possible signs of a thyroid tumor^1,3. The Zepbound Medication Guide says to tell your healthcare provider if you get any of these³.

For other warnings on these labels, see GLP-1 side effects and pancreatitis and gallbladder problems. Questions about your own thyroid history are for your prescriber or pharmacist.