It’s hard to get away from news about weight loss drugs like Mounjaro and Wegovy. Known as GLP-1 agonists, they’ve recently moved out of the lab and into our lives, on red carpets, stock markets and online pharmacies. Now, they’re even featuring in the Government’s health plans.

There’s a lot of evidence that these drugs can help people reach a healthy weight, but there are also a lot of unanswered questions – especially when it comes to cancer. In this article, we break down what these drugs are and what they could mean for cancer.

What are weight loss drugs?

Weight loss drugs are medications used to help people manage their weight by supporting weight loss. The most commonly prescribed ones in the UK today are tirzepatide (Mounjaro) and semaglutide (Wegovy), which are both weekly injections. They’re licensed for general weight loss in people with weight related health conditions, who are over a certain BMI.

GLP-1 (Glucagon-Like Peptide-1) is a hormone produced in our bodies that helps to control blood sugar and reduce appetite. Weight loss injections are known as GLP-1 agonists because they mimic GLP-1, helping to increase insulin production after eating and creating a feeling of fullness.

The rise of weight loss injections

When thinking about weight loss injections, the first that might come to mind is Ozempic. It was one of the first GLP-1 agonists to be developed, by a Danish pharmaceutical company called Novo Nordisk. But in the UK Ozempic is specifically used as a treatment for people with type 2 diabetes, not for general weight loss.

Novo Nordisk’s main focus is on diabetes care, but it quickly became clear that their new drug could also help with weight loss, which led to people purchasing it without a prescription. Social media also helped Ozempic’s rapid rise in popularity. Towards the end of the pandemic, celebrities and influencers began sharing their GLP-1 agonist assisted weight loss journeys with their online followers, which gave the drug a huge amount of publicity.

In the following years, Wegovy and Mounjaro were licensed in the UK for weight loss.

Our research shows there’s a new demand for medication to help people lose weight. Around 1.6 million adults in the UK have reported using weight loss drugs in a recent survey, with an additional 3.3 million saying they would be interested to use them. And researchers believe this number could rise.

Do weight loss injections really help people lose weight?

Clinical studies show that weight loss injections can be an effective tool for weight loss when used alongside support to increase physical activity and have a healthy balanced diet.

Not everyone who uses weight loss injections will lose weight, but trials suggest that most people using semaglutide (Wegovy) can expect to lose around 15% of their body weight, depending on the dose they’re taking, while people using tirzepatide (Mounjaro) can expect to lose around 20%.

However, trials also show that most people who stop using these drugs regain the weight they’ve lost. This could limit the potential health benefits of GLP-1 agonists. It also raises questions around the physical and mental health impacts of weight cycling (losing and regaining weight in a repeated pattern).

Do weight loss injections have side effects?

Not everyone will experience side effects from weight loss injections, but like with any drug, there’s always a risk. People who take weight loss injections sometimes experience side effects related to the digestive system, like feeling sick or having diarrhoea or constipation. The injections can also cause people to feel dizzy, tired or experience headaches.

These medications also have less common but potentially more serious side effects, such as inflammation of the pancreas (also known as pancreatitis). It’s estimated that up to 1 in 100 people taking these injections may develop the condition, according to patient information leaflets.

Currently, Wegovy has a formal warning in the US citing a potential risk of thyroid tumours, partly based on early studies done in rodents. However, a recent study looked at data from 48 different trials and showed no evidence of an increased risk of thyroid cancer.

Could weight loss injections reduce weight-related cancer cases?

GLP-1 agonists are being used widely in the UK, but they’re relatively new as a weight management tool. This means we still need to answer a lot of questions about their long-term effects.

So far, we know these drugs are effective at reducing weight in the short term for many people. We also know that people who are overweight are more likely to develop cancer compared to people with a healthy weight. So, can weight loss injections help reduce cancer risk?

The answer isn’t so simple.

Overweight and obesity is the biggest cause of cancer in the UK after smoking. And we know that cancer risk depends on the extent to which someone is overweight, as well as how long someone is overweight for. So, logically, we might assume that losing weight with weight loss injections reduces the risk of cancer.

However, “it’s actually not been explicitly proven that these drugs reduce cancer risk,” Richard Martin, Professor of Clinical Epidemiology at Bristol Medical School explains. “It’s assumed because of the extent of weight loss many people experience using these drugs.” 

Martin explains that general weight loss doesn’t necessarily mean efficient fat loss, which may be the key to reducing cancer risk.

“Fat tissue takes on specific properties and has localised effects – inflammatory effects, for example,” says Martin. “It may be those inflammatory effects which are carcinogenic, but we just don’t know for certain the specific mechanisms and the best ways to intervene on those mechanisms to reduce cancer risk. We also don’t fully know the relationship between general fat tissue, the distribution of fat, organ specific fat and cancer.”

With our funding, Martin and his team are now aiming to uncover the relationship between fat loss and cancer risk. They’ll be comparing different weight-loss methods like reducing calories, gastric bypass surgery and GLP-1 weight loss injections to see if the different methods affect exactly how fat is lost and how this impacts cancer risk.

“We don’t know the exact mechanism by which these drugs would reverse someone’s cancer risk,” says Martin. “It’s important to have a biological understanding of what you’re doing to someone in order to reduce their risk, but also to potentially help guide us to develop new and better targeted weight loss tools, especially when it comes to cancer prevention.”

What questions do we still have about weight loss injections?

The accessibility of these drugs is becoming an important topic of discussion within the UK Government to help address obesity. In July 2025, Wes Streeting, the health secretary, pledged to widen public access to weight loss injections in England. The UK Government wants to reduce the inequalities gap between those who are having to wait for these drugs on the NHS versus those who can afford to access them privately.

But there are some big underlying questions about these medications, and Martin is one of the researchers prioritising which ones need to be answered first.

“For me, it’s the duration,” he says. “Can the drugs be taken for a short period to maintain the level of obesity reduction that’s required to reduce cancer risk or do you need to take those drugs longer term, and what then are the wider benefits versus harm implications of longer-term use for cancer prevention?”

Additionally, many people who have come off these weight loss injections report putting the weight back on. Sustainable weight loss will need to be a key focus for the UK government.

“While these drugs can aid weight loss, they’re not a silver bullet,” says Jo Harby, our director of health information.

“The world around us shapes our health, and the UK Government must do more to create healthier environments for all.”

Weight loss drugs can’t be used in isolation to make a sustainable impact; they must be part of a wider plan that also prioritises preventing people from becoming overweight and obese in the first place. This includes initiatives like new promotion and advertising restrictions, which help take unhealthy food out of the spotlight and create healthier shopping journeys.

The weight loss drug industry isn’t slowing down. Pill versions of these drugs are already approved in the US, making them easier to manufacture and for people to consume. And as more people are able to access these drugs, it will be important to understand who benefits most from using them, how long they should be used for and what extra medical support would be needed to ensure people can keep the weight off.

When it comes to cancer, it’s simply too early to truly understand the potential impact of GLP-1 agonists. But research is moving quickly to help us uncover the answers we need.

You can listen to more about weight loss drugs on our podcast, That Cancer Conversation.