The rapid rise of new-generation weight-loss drugs, including Ozempic, Wegovy, and Mounjaro, has brought with it significant benefits for obesity treatment and the reduction of risks for conditions like heart attacks and kidney failure. However, concerns are surfacing regarding a potential link to thyroid cancer, stemming from early animal studies and reported patient experiences.
Initial safety studies on rodents indicated that several GLP-1 receptor agonists, the class of drugs that includes Ozempic and Wegovy (semaglutide) and Mounjaro (tirzepatide), caused thyroid tumors. While subsequent human clinical trials involving tens of thousands of patients did not reveal the same cancer signal, these animal findings are noted in the official safety information for the drugs. For semaglutide, the prescribing information states that the relevance of rodent findings to humans, while considered low, cannot be entirely ruled out. Similarly, tirzepatide caused thyroid tumors in rats, with its relevance to humans listed as unknown.
While researchers have not definitively established a causal link between these drugs and thyroid cancer in humans in the short to medium term, a review of online patient accounts has uncovered individuals questioning a connection between their cancer diagnoses and the use of these medications. These claims have not been independently verified, and there is no evidence presented to confirm the drugs caused these cancers. Nevertheless, these anecdotal reports raise questions about potential risks, particularly for patients who may require long-term use of the drugs.
Some patients have shared their experiences on social media forums. One user reported being diagnosed with thyroid cancer after taking a semaglutide-containing GLP-1 drug, with their doctor suggesting a possible link due to the timing of the diagnosis after starting the medication. Another individual stated they were diagnosed with thyroid cancer six months after starting a GLP-1 drug, underwent surgery, and later required radiation and chemotherapy for recurring cancer after resuming the medication. These accounts, while unverified, contribute to concerns about possible thyroid complications.
In light of these concerns, medical experts are advising caution. The British Thyroid Foundation recommends that individuals with a personal or family history of medullary thyroid cancer (MTC) should avoid certain GLP-1 drugs. MTC is a rare form of thyroid cancer that originates in C-cells, the same cells where GLP-1 drugs induced tumors in rodent studies. Individuals with multiple endocrine neoplasia type 2 (MEN2), an inherited condition that increases MTC risk, are also advised to avoid these medications. Those with a history of other types of thyroid cancer are encouraged to discuss the potential risks with their doctor.
Professor Penny Ward, a pharmaceutical expert at King's College London, noted that while an increase in risk has not been widely observed in patients treated for diabetes with GLP-1s, the doses used for obesity are higher. This difference in dosage means there is less long-term evidence in humans taking the drugs at higher concentrations for weight loss. Scientists acknowledge that cancer can develop over years, and the possibility of a risk emerging after prolonged use of higher doses cannot yet be completely excluded.
Representatives from Novo Nordisk, the maker of Ozempic and Wegovy, stated that patient safety is a priority and encouraged those with concerns to speak with a healthcare professional. Eli Lilly, the manufacturer of Mounjaro, affirmed that patient safety is paramount and that their safety data has not demonstrated a causal association between their GLP-1 medications and thyroid cancer in humans. They also emphasized that prescribing decisions should be made between a patient and their healthcare provider after a thorough assessment of medical history. In Britain, over 150,000 reports of suspected adverse reactions linked to these drugs have been documented in the past month, with common side effects including nausea and diarrhea, and rarer complications like pancreatitis.