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So many people in western countries are turning to weight loss drugs such as Wegovy/Ozempic and Mounjaro that concerns have started to emerge about maintaining ready supplies. But with popularity comes scrutiny, and rising demand isn’t the only potential problem with weight loss jabs.

Gastrointestinal side-effects such as nausea, vomiting and constipation are common across these drugs, which are known as GLP-1 receptor agonists. Then there’s the now infamous “Ozempic face” – a gaunt, aged appearance that can result from rapid weight loss.

More serious concerns have also begun to surface, including possible links to eye disease, reduced libido and a potential increased risk of certain cancers. However, we’re still in the early days of understanding what the risks might be and the evidence for them is limited.

The most significant cancer-related worry is thyroid cancer. Studies in rodents found that high doses of GLP-1 drugs caused thyroid tumours, although this hasn’t been definitively proven in humans.

Still, a large-scale French study did find a potential link between GLP-1 use and thyroid cancer, especially in patients who used the drug for more than a year. As a precaution, these medications are not recommended for people with a personal or family history of thyroid cancer or specific genetic conditions that increase the risk of thyroid tumours.

There have also been concerns about pancreatic cancer, mostly because of early reports of pancreatitis: inflammation of the pancreas, which can, in some cases, be fatal. However, current studies have not confirmed a direct link between GLP-1 drugs and pancreatic cancer.

Concerns here are particularly relevant because of how the drugs work. Wegovy and Ozempic are brand names for a type of GLP-1 receptor agonist known as semaglutide. Originally developed to treat type 2 diabetes, it works by activating receptors in the pancreas to increase insulin release and reduce glucagon – a hormone that raises blood sugar.

Together, these effects help lower blood-sugar levels. The weight loss effects come from the drug’s ability to act on receptors in the brain as well as in gut and fat cells, to help to reduce appetite.

Mounjaro (the brand name for the compound tirzepatide) takes things a step further. It works not only on the GLP-1 receptor but also on a second one – the GIP (glucose-dependent insulinotropic polypeptide) receptor.

By stimulating both, tirzepatide boosts the pancreas’s ability to produce insulin and improves insulin sensitivity – meaning the body’s cells respond more effectively to insulin, helping regulate blood sugar more efficiently. This dual action results in even greater weight loss than semaglutide alone, making Mounjaro the latest star in the fight against obesity.

Tirzepatide has not been associated with increased cancer risk in clinical trials so far. However, like other GLP-1 drugs, it still carries the thyroid cancer warning based on earlier animal research. Interestingly, preliminary studies in animals suggest tirzepatide might even shrink certain tumours, including breast cancer, but these findings are still very early and not yet applicable to humans.

Obesity is also a cancer risk

It’s important to remember that obesity itself is a well established risk factor for several cancers, including breast, colon, liver and uterine cancers. By helping people lose significant amounts of weight and improve their metabolic health, GLP-1 drugs could indirectly reduce the risk of developing these conditions.

In fact, some population studies have found lower rates of obesity-related cancers in people using GLP-1 medications compared to those taking other treatments. However, it’s still unclear whether the reduced cancer risk comes from the drug’s action or from the weight loss itself. More research is needed to fully understand this connection.

So, the current picture is reassuring, yet tinged with uncertainty. The overall cancer risk associated with GLP-1 drugs and tirzepatide appears to be low.

But it’s important to emphasise that these medications are not recommended for people with a personal or family history of certain types of thyroid cancer, or also endocrine conditions such as multiple endocrine neoplasia syndrome, because these conditions may increase sensitivity to hormone-related tumour growth.

Weight-loss injections are not risk free, but they also hold enormous potential. It remains to be unravelled whether they’re miracle cures or just the latest chapter in the long saga of weight loss. One thing is certain: the conversation is far from over.

These drugs are rewriting the rules on how we think about weight, health, and risk. In the battle to outsmart the scales, they offer hope, science and a fair bit of gamble amidst the hype.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.