It’s a disturbing question that has emerged alongside the extraordinary rise of the new generation of weight-loss drugs. Can cause them cancer?
Drugs like Ozempic, Wegovy and Mounjaro have transformed the treatment of obesity, helping patients lose significant amounts of weight and, as an emerging side effect, protecting against a host of serious conditions, including reducing the risk of heart attacks, strokes and kidney failure.
But with their extraordinary benefits comes a slew of negative side effects, some of which are serious.
Just last month, it emerged that patients in Britain had made more than 150,000 reports of suspected side effects linked to the drugs.
Collectively known as GLP-1 receptor agonists or GLP-1s, they commonly cause nausea, vomiting and diarrhea, while rarer complications can include inflammation of the pancreas, known as pancreatitis.
But it’s the possibility of thyroid cancer that has proven to be one of the most alarming concerns.
The fear stems from early safety studies in rodents, which showed that several drugs caused thyroid tumors.
Although subsequent clinical trials involving tens of thousands of patients did not reveal the same cancer signal in humans, the findings in animals were serious enough to be highlighted in the official safety information for the drugs.
Tirzepatide, the active ingredient in Mounjaro, caused thyroid tumors in rats, although the prescribing information states that its relevance to humans is unknown.
For semaglutide, the active ingredient in Ozempic and Wegovy, the prescribing information states that while the relevance of the findings in rodents to humans is considered low, it ‘cannot be completely excluded’.
Similar findings have been observed with other weight loss medications.
Tirzepatide, the active ingredient in Mounjaro, also caused thyroid tumors in rats, although the prescribing information states that its relevance to humans is unknown.
So far, researchers have not conclusively determined that the drugs cause thyroid cancer in humans – at least in the short to medium term.
But a Mail on Sunday analysis has revealed a string of patients are wondering online whether their own cancer diagnosis could be linked to the drugs.
There is no evidence that the drugs caused the cancers described in these reports, and the claims cannot be independently verified. Nevertheless, the stories are likely to be worrying for patients – especially as many may need to continue taking the weight-loss drugs for the rest of their lives.
One user on the social media forum Reddit described being diagnosed with thyroid cancer after taking a GLP-1 drug containing semaglutide. For legal reasons, The Mail on Sunday does not mention the brands in the messages. The patient said they stopped taking the drug for six months before returning to their doctor in hopes of restarting treatment. Instead, they said the appointment took a terrifying turn.
“He said my thyroid looked a little off, so he decided to do an ultrasound, which led to a biopsy within minutes,” the patient said. ‘It turns out I have thyroid cancer and I need to undergo radiation and have my thyroid removed as soon as possible.
‘My doctor feels it has something to do with it [the GLP-1] because I never had this before the medication and he no longer recommends that I take it again.’
Another Reddit poster wrote, “I was busy [a GLP-1] six months later she was diagnosed with thyroid cancer. I had a total thyroidectomy which was surgically removed.
‘I moved on again [a GLP-1] and four months later I had radiation – chemotherapy to treat the cancer that had grown.
“I know it causes thyroid complications. I wonder if anyone else has experienced this too.’
Again, there is no way to determine from these reports whether the drugs played a role in patients’ cancers at all. But they are among dozens of similar stories, which begs the question: Could this be a serious, if unusual, risk?
While scientists are now trying to figure this out, doctors are already urging caution.
Experts from the British Thyroid Foundation say that people with a personal or family history of medullary thyroid cancer (MTC) should not take certain GLP-1 drugs.
Known as MTC, it is a rare form of thyroid cancer that develops in specialized cells in the thyroid gland known as C cells – the same cells in which GLP-1 drugs caused tumors in rodents.
People who suffer from the hereditary condition multiple endocrine neoplasia type 2, or MEN2, are also advised to avoid certain GLP-1 drugs, as the condition already puts them at high risk for developing MTC.
Those who have had other forms of thyroid cancer are advised to discuss the risks of GLP-1s with their doctor.
So what should someone taking the drugs make of the warnings?
One question continues to perplex scientists: Why did the weight-loss drugs cause thyroid tumors in rodents?
There is one reassuring clue. Despite their recent explosion in popularity, GLP-1s themselves are not new. Doctors have been prescribing them for more than a decade to people with diabetes – the condition for which they were originally developed.
Professor Penny Ward, a pharmaceutical expert at King’s College London, said: ‘We haven’t really seen an increase in risk.’ But there is an important caveat: “The doses used in obesity are higher than the doses used in diabetics.”
Professor Penny Ward, a pharmaceutical expert at King’s College London, said: ‘We haven’t really seen an increase in risk.’
That leaves scientists with less long-term evidence about people taking the drugs at the higher doses used for obesity.
Cancer can also develop years before a tumor becomes visible.
Scientists cannot therefore completely rule out the possibility that a risk arises after patients have taken higher doses of the medicines for a longer period of time.
A spokesperson for Novo Nordisk, which makes Ozempic and Wegovy, said: ‘We attach great importance to patient safety.
‘Anyone with concerns about side effects, or whether a medicine is suitable for them, should contact their doctor, nurse or pharmacist.’
A spokesperson for Eli Lilly, maker of Mounjaro, said: ‘Patient safety is our top priority, and we actively monitor, evaluate and report safety information for all of our medicines.
‘The safety data collected in Lilly’s large clinical trial programs and post-marketing surveillance have not established a causal relationship between Mounjaro (tirzepatide), Foundayo (orforglipron) or Trulicity (dulaglutide) and thyroid cancer in humans.
‘Prescribing decisions should always be made between a patient and their healthcare provider after reviewing the patient’s medical history. If anyone experiences side effects while taking a Lilly medicine, they should discuss them with their doctor or other healthcare professional.”
