Weight loss drugs are effective at helping people lose weight in the short term, but whether they reduce cancer risk remains unanswered. While obesity is the second-leading preventable cause of cancer in the UK after smoking, researchers have not yet proven that GLP-1 agonists like semaglutide (Wegovy) and tirzepatide (Mounjaro) lower cancer risk, even though they produce significant weight loss .

Why Don't We Know If Weight Loss Drugs Prevent Cancer?

The logic seems straightforward: obesity increases cancer risk, weight loss drugs help people lose weight, therefore weight loss drugs should reduce cancer risk. But the reality is more complicated. The problem isn't just about losing weight; it's about how that weight is lost and what happens to the body's fat tissue in the process .

Richard Martin, Professor of Clinical Epidemiology at Bristol Medical School, explained the gap in our understanding. "It's actually not been explicitly proven that these drugs reduce cancer risk," Martin stated. "It's assumed because of the extent of weight loss many people experience using these drugs." The key issue is that general weight loss doesn't necessarily mean efficient fat loss, which may be what actually matters for cancer prevention .

Richard Martin, Professor of Clinical Epidemiology at Bristol Medical School

"Fat tissue takes on specific properties and has localised effects, inflammatory effects, for example. 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," explained Richard Martin.

Richard Martin, Professor of Clinical Epidemiology at Bristol Medical School

What Do We Know About How These Drugs Work?

GLP-1 agonists are medications that mimic a hormone your body naturally produces called glucagon-like peptide-1 (GLP-1). This hormone helps control blood sugar and reduce appetite. The drugs work by increasing insulin production after eating and creating a feeling of fullness, which leads to reduced calorie intake .

The two most commonly prescribed weight loss drugs in the UK are semaglutide (Wegovy) and tirzepatide (Mounjaro), both given as weekly injections. Clinical trials show that people using semaglutide can expect to lose around 15% of their body weight depending on the dose, while those using tirzepatide can expect to lose around 20% .

However, there's a significant catch: most people who stop using these drugs regain the weight they lost. This weight cycling, or losing and regaining weight in a repeated pattern, raises questions about the long-term health benefits of these medications .

How to Understand the Cancer Risk Connection

• Obesity and Cancer Link: Overweight and obesity is the biggest cause of cancer in the UK after smoking, and cancer risk depends on both how overweight someone is and how long they remain overweight.
• Fat Distribution Matters: Different types of fat tissue, including where fat is stored in the body and organ-specific fat, may have different effects on cancer risk, but researchers don't fully understand these relationships yet.
• Inflammation as a Factor: Fat tissue produces inflammatory effects that may be carcinogenic, but scientists haven't identified the exact mechanisms or the best ways to target these effects to reduce cancer risk.

What Research Is Underway to Answer These Questions?

Recognizing the gap in knowledge, researchers are now conducting studies to compare different weight loss methods and their effects on cancer risk. Martin and his team are comparing three approaches: reducing calories, gastric bypass surgery, and GLP-1 weight loss injections. The goal is to determine whether different methods affect how fat is lost and whether this impacts cancer risk differently .

"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," Martin noted .

What About Side Effects and Safety Concerns?

While weight loss drugs are generally well-tolerated, they do carry risks. Common side effects include digestive issues like nausea, diarrhea, or constipation, as well as dizziness, tiredness, and headaches. More serious but less common side effects include pancreatitis (inflammation of the pancreas), which is estimated to affect up to 1 in 100 people taking these injections .

Wegovy carries a formal warning in the United States about a potential risk of thyroid tumors, based partly on early studies in rodents. However, a recent analysis of data from 48 different trials showed no evidence of an increased risk of thyroid cancer .

The Bottom Line: What Should You Know?

Weight loss drugs are effective tools for short-term weight loss when used alongside support for physical activity and a healthy diet. Around 1.6 million adults in the UK have reported using weight loss drugs, with an additional 3.3 million saying they would be interested in using them .

However, the question of whether these drugs reduce cancer risk remains open. The assumption that they do is logical but not yet proven by research. If you're considering using weight loss drugs, it's important to speak with your GP first. They can provide advice tailored to your specific circumstances and health situation, and may refer you to specialist weight management services if appropriate .

As researchers continue to investigate the relationship between different types of weight loss and cancer risk, we may gain a clearer picture of how these increasingly popular medications fit into cancer prevention strategies.