Study Finds No Link Between Side Effects of GLP-1s and Thyroid Cancer

Researchers say thyroid screening for most users of GLP-1 medications is unnecessary, unless there is a prior diagnosis of certain rare forms of cancer.

A number of unexpected side effects have been linked to popular drugs like Ozempic and Mounjaro, including stomach paralysis and vision loss, yet a new study suggests concerns over an increased risk of thyroid cancer may be unjustified.

A report (PDF) released this month by the Clayman Thyroid Center in Tampa, Florida indicates there is no sign the use of injectable glucagon-like peptide-1 (GLP-1) receptor agonists raise the risk of a thyroid cancer diagnosis, nor do they accelerate the growth of existing thyroid cancer cells.

GLP-1 medications include both diabetes treatments like Ozempic, Mounjaro, Trulicity and Rybelsus, as well as weight loss drugs like Wegovy and Zepbound. The drugs have become popular over the last couple years for their weight loss effects.

GLP-1 Side Effects

As use of the drugs has become more widespread, concerns have emerged about GLP-1 side effects and whether manufacturers fully researched all of the potential health risks. There are also questions as to whether patients and the medical community were fully warned about what those health risks were.

Along with the blockbuster sales have come reports of serious health risks that were not fully described on the initial drug labels, including gastrointestinal problems like stomach paralysis and non-arteritic anterior ischemic optic neuropathy (NAION), a rare but serious form of optic-nerve damage that can cause permanent blindness.

The manufacturers Novo Nordisk and Eli Lilly now face a growing number of Ozempic lawsuits, Mounjaro lawsuits and Wegovy lawsuits, each raising similar allegations that a desire for profits led to a lack of adequate testing and safety warnings.

GLP-1 drugs currently carry a boxed warning based on animal studies that suggested a potential risk of thyroid disease. In preclinical testing, rodents developed C-cell hyperplasia and experienced accelerated tumor growth after repeated exposure to the medications.

The report, led by Drs. Gary L. Clayman and Rashmi Roy, looked at clinical trial data on humans, adverse event reports and postmarketing surveillance studies. However, the researchers say none of that data contained any indication that drugs like Ozempic and Mounjaro increased the risk of thyroid cancer.  

The researchers explained previous signs as the result of rat and mice cells responding more intensely to GLP-1 exposure than the same cells in humans. In addition, they noted that some previous human studies suggesting increased thyroid cancer risks appear to be the result of detection bias, with patients taking the drugs being screened more regularly than those who are not.

“The best available human evidence does not show that GLP-1 receptor agonists cause common thyroid cancers or even has an effect if you have developed a thyroid cancer. The FDA warning specifically addresses a rare type called medullary thyroid carcinoma (MTC) and is based primarily on rodent studies, not human data.”

– Clayman Thyroid Center Hospital for Endocrine Surgery researchers

The report concludes that patients with a history of MTC or multiple endocrine neoplasia, type 2 (MEN2) should still avoid taking GLP-1 medications. However, those with more common forms of thyroid cancer do not have to worry about GLP-1 medications impacting the disease, the researchers noted.

The report states the use of drugs like Ozempic, Mounjaro and Wegovy do not require additional thyroid monitoring.

Data in the report backs up similar findings published in January 2025 by researchers from the Mayo Clinic and Yale School of Medicine, who found that increases in thyroid diagnosis detected in other studies disappeared after the first year of treatment.

GLP-1 Injury Lawsuits

The report was released amid increased scrutiny of GLP-1 medications, which have led to more than 3,000 product liability lawsuits. Most of the claims focus on gastrointestinal issues like stomach paralysis but also include claims of vision loss.

Both the GLP-1 stomach injury lawsuits and vision loss lawsuits have been consolidated into two separate MDLs, or multidistrict litigations, before U.S. District Judge Karen Marston in the Eastern District of Pennsylvania.

Judge Marston is leading both sets of litigation through coordinated discovery and pretrial proceedings, and indicates she plans to select a small group of representative “bellwether” cases to serve as early test trials for both types of claims. These will allow the parties to see how an actual jury responds to evidence and testimony that would probably be repeated throughout thousands of similar claims.

However, before those trials can move forward, Judge Marston has ordered both sides to resolve several issues that could have broad implications for the litigation, including preemption and general causation. Once those matters are addressed, the MDLs are expected to proceed with preparing the first sets of cases for jury consideration. While the verdicts will not directly determine the outcome of other GLP-1 claims, they are likely to shape settlement negotiations.

If the parties do not reach GLP-1 settlement agreements after the bellwether trials, Judge Marston is likely to begin remanding the cases back to their original courts for individual trial dates.

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