Do Ozempic and other GLP-1 drugs cause thyroid cancer?

Ozempic (active ingredient semaglutide) and Mounjaro (active ingredient tirzepatide) are once-weekly injections for type 2 diabetes, and many people know them as "weight-loss jabs". The official prescribing information for both drugs carries a special warning about a "risk of thyroid tumours" ^{[1, 2]}. This warning comes from studies in mice and rats: in animals given these drugs throughout their lives, tumours were more common in a group of thyroid cells called C cells ^{[1, 2]}.

C cells make a hormone called calcitonin, and a rare type of thyroid cancer called medullary thyroid cancer starts in these cells ^{[12, 13]}. The most common types of thyroid cancer start in other cells ^[13]; for the different types, see thyroid cancer.

Researchers found that C cells in rodents have many receptors for these drugs, but in humans and monkeys these receptors are very scarce. In monkeys given very high doses of a similar drug, liraglutide, for nearly 2 years, these cells did not increase ^[3]. So it is still not known whether the findings in rodents apply to humans, and the warning remains in the prescribing information ^{[1, 3]}.

• A study in France: the records of more than 2,500 patients with diabetes who had thyroid cancer were compared with those of more than 45,000 patients with diabetes who did not. People who had used these drugs for 1 to 3 years had about one and a half times as much thyroid cancer ^[4].
• A study in Denmark, Norway and Sweden: more than 145,000 patients with diabetes who started these drugs were compared with nearly 300,000 patients who started a different diabetes medicine. Over nearly 4 years, thyroid cancer was equally rare in both groups: fewer than 2 people in every 10,000 each year ^[5].
• A study in six countries (Canada, Denmark, Norway, South Korea, Sweden and Taiwan) of more than 98,000 users also showed no higher risk; the researchers wrote that because patients were followed for only about 2 to 3 years, an effect of long-term use cannot yet be ruled out ^[6].
• Pooled results of drug studies: in one review, thyroid cancer was more common in the groups given these drugs, but cases were very few and the actual increase was very small: roughly one extra case for every 1,350 people taking the drug for 5 years ^[8]. A newer review in 2026 found no definite increase and wrote that, because there were so few cases, no firm conclusion can be drawn ^[9].

In 2023, after reviewing the evidence available at the time (including the French study), the European Medicines Agency stated that the available evidence does not show that these drugs cause thyroid cancer, and asked the manufacturers to keep monitoring the question ^[10].

In a study in the United States of more than 350,000 patients with diabetes, more thyroid cancer was diagnosed only in the first year after starting these drugs ^[7]. The researchers wrote that the likely reason is that these patients were examined and tested more, so small cancers that were already there were found earlier, rather than the drug causing new cancers ^[7].

To understand why finding a small thyroid cancer does not always mean danger, see overdiagnosis of thyroid cancer.

It is also worth knowing that most of these studies were in patients with diabetes and followed people for a few years ^{[4, 5, 6, 7]}; so research on longer use continues ^[6].

If you have no symptoms, usually not. According to the official prescribing information, regular calcitonin blood tests or thyroid ultrasound scans in everyone taking the drug are of no clear value for finding medullary thyroid cancer early ^[1]. Because thyroid conditions are very common and the calcitonin test can be high even without cancer, this testing may lead to unnecessary biopsies ^[1].

But if your doctor finds a thyroid nodule on examination or imaging, or your blood calcitonin is high, it should be investigated further ^[1]. See Thyroid nodules and TI-RADS: when are they dangerous?