Historical concerns about GLP-1s and cancer
Early worries about GLP-1 medications and cancer began in 2009 when reports from the US Food and Drug Administration’s database suggested possible links between the first GLP-1 drugs and thyroid and pancreatic cancers.
These initial concerns weren’t based on controlled studies but on adverse event reports, which can’t prove cause and effect.
Still, they led to important safety requirements that remain today.
The main concerns focused on two specific cancer types:
- Thyroid cancer: Animal studies showed that very high doses of GLP-1 medications caused thyroid tumours in rodents. This led to warnings about medullary thyroid cancer, though no cases have been confirmed in human clinical trials.
- Pancreatic cancer: Some early reports suggested GLP-1 drugs might cause pancreatitis, which could theoretically increase pancreatic cancer risk. However, later studies found no clear connection.
These early safety signals prompted years of careful research into cancer risk with GLP-1 medications, including the newer dual-action drugs like Mounjaro.
The good news is that extensive human studies haven’t confirmed the concerns raised in animal studies, and researchers now have much clearer data on the actual cancer risk.
Current research evidence on Mounjaro and cancer
The most comprehensive analysis to date examined cancer rates in people taking Mounjaro across multiple clinical trials involving thousands of participants.
Published in Endocrinology and Metabolism in 2025, this systematic review and meta-analysis found no increased cancer risk with Mounjaro use.
Key findings include:
The study analysed data from randomised controlled trials lasting 26-72 weeks involving people with and without diabetes. Cancer rates were identical between the Mounjaro group and control groups (risk ratio 0.78, meaning if anything, slightly fewer cancers occurred with Mounjaro).
No difference in cancer risk appeared between different Mounjaro doses (5mg, 10mg, or 15mg weekly). The results were consistent whether people took Mounjaro, placebo, insulin, or other GLP-1 medications.
Importantly, no cases of medullary thyroid cancer were observed in any of the clinical trials, despite earlier concerns from animal studies.
Real-world safety data support these findings:
A 2024 analysis of post-marketing surveillance data from the US FDA found that Mounjaro was actually associated with lower risks of several cancers compared to other diabetes medications, including reduced thyroid mass risk.
However, researchers noted that there were very few cancer cases overall due to the relatively short follow-up periods in current studies.
Specific cancer types and Mounjaro
The research examined various cancer types individually to provide a complete picture of Mounjaro’s safety profile.
Thyroid cancer: Despite animal study concerns, no cases of medullary thyroid cancer appeared in human trials. Some participants had small increases in calcitonin levels (a thyroid hormone), but these weren’t clinically significant and didn’t lead to cancer diagnoses.
Pancreatic cancer: No increased risk was found, even though early GLP-1 medications raised theoretical concerns about pancreatitis potentially leading to pancreatic cancer.
Other cancer types: The analysis found no increased risk for breast, colorectal, kidney, liver, or other common cancer types that people with obesity face higher risks of developing.
In the UK, Mounjaro received approval from the Medicines and Healthcare products Regulatory Agency in November 2023, following thorough safety reviews that included a cancer risk assessment.
The prescribing information includes the standard warning about medullary thyroid cancer based on animal studies, but this applies to a very small number of people with specific genetic conditions.
Most people considering Mounjaro don’t have these risk factors and can use the medication safely regarding cancer risk.
How Mounjaro might reduce cancer risk
Rather than increasing cancer risk, Mounjaro may actually lower your chances of developing certain cancers through its effects on weight and metabolism.
Weight loss benefits
Obesity increases the risk of 13 different cancer types, including breast, colorectal, endometrial, kidney, liver, and pancreatic cancers.
Mounjaro typically leads to 15-25% body weight reduction, which can significantly lower these obesity-related cancer risks.
Research shows that losing just 5-10% of body weight can reduce cancer risk, and Mounjaro often achieves much greater weight loss than this threshold.
Other health improvements
Beyond weight loss, Mounjaro improves several factors linked to cancer development:
Lower insulin levels reduce cancer-promoting growth signals that high insulin can trigger. Better blood sugar control decreases inflammation throughout the body, creating a less favourable environment for cancer development.
Reduced visceral fat (the dangerous fat around internal organs) lowers the production of inflammatory compounds that can promote cancer growth.
Specific cancer risk reductions
Research shows that sustained weight loss can reduce breast cancer risk by up to 32% in postmenopausal women and may help reduce colorectal cancer risk, as obesity increases this cancer risk by 30% in men.
For people with diabetes, better glucose control from Mounjaro may reduce liver and pancreatic cancer risks, as high blood sugar levels can promote cancer development in these organs.
Study limitations and what we still need to learn
While current evidence is reassuring, important limitations exist in our understanding of Mounjaro’s long-term cancer effects.
Short follow-up periods: Most studies followed participants for 26-72 weeks, but cancer typically takes years or decades to develop. We need longer-term data to be completely certain about cancer risk.
Small numbers of cancer cases: Because cancer is relatively rare and studies were short, very few cancer cases occurred overall, making it harder to detect small increases in risk if they exist.
Limited real-world data: Mounjaro has only been available since 2022, so we don’t yet have extensive data on cancer rates in people who’ve used it for several years.
Ongoing research: Several long-term studies are following people taking Mounjaro for up to 5 years to gather more comprehensive safety data, including cancer outcomes. The SURPASS-CVOT trial, which is examining cardiovascular effects, will also provide valuable data on cancer safety as it continues.
What this means for you: Current evidence strongly suggests Mounjaro doesn’t increase cancer risk in the short to medium term, and it may reduce risk through weight loss and metabolic benefits.
The benefits of significant weight loss for cancer prevention likely outweigh any theoretical long-term risks that haven’t been detected in current studies.
Practical guidance for people considering Mounjaro
Based on current evidence, most people can use Mounjaro without concern for increased cancer risk; however, certain considerations apply.
Who shouldn’t take Mounjaro
Don’t use Mounjaro if you have a personal or family history of medullary thyroid cancer or Multiple Endocrine Neoplasia syndrome type 2.
These conditions affect fewer than 1% of the population, but the contraindication is absolute.
If you have a family history of other cancer types, this doesn’t prevent Mounjaro use, though you should discuss your individual risk factors with your healthcare provider.
Cancer screening recommendations
Continue all routine cancer screenings as recommended for your age and risk factors.
Mounjaro use doesn’t change standard screening guidelines for breast, cervical, colorectal, or other cancers.
Some doctors recommend baseline thyroid function tests before starting Mounjaro, particularly checking calcitonin levels if you have thyroid concerns.
Monitoring during treatment
Report any unusual symptoms to your healthcare provider, including persistent abdominal pain, unusual lumps, or significant changes in bowel habits.
Your GP should be aware you’re taking Mounjaro during routine health checks and cancer screenings.
Long-term perspective
Focus on the proven benefits of weight loss for cancer prevention rather than theoretical long-term risks that current evidence doesn’t support.
The cancer risk reduction from achieving and maintaining significant weight loss likely provides much greater benefit than any undetected risk from the medication itself.
Take home message
Current research provides reassuring evidence that Mounjaro doesn’t increase cancer risk and may actually reduce your chances of developing obesity-related cancers through significant weight loss and metabolic improvements.
The comprehensive 2024 analysis found identical cancer rates between people taking Mounjaro and those using other treatments, with no cases of the thyroid cancers that concerned researchers in early animal studies.
While we need longer-term data for complete certainty, the proven benefits of major weight loss for cancer prevention likely outweigh any theoretical risks that haven’t appeared in current studies.
Continue routine cancer screenings, avoid Mounjaro if you have specific thyroid cancer risk factors, and focus on the substantial health benefits that significant weight loss provides.
Work with your healthcare provider to make the best decision based on your individual health profile and cancer risk factors.
Second Nature’s medication programme
Second Nature provides Mounjaro as part of our Mounjaro weight-loss programme.
Why choose Second Nature over other medication providers, assuming you’re eligible?
Because peace of mind matters.
We’ve had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes.
We’re here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we’re the UK’s most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind and the support you need to take that first step.
Please note: We generally recommend speaking with your GP/doctor before you start taking any new medication like Mounjaro. If you do sign up for Second Nature's Mounjaro programme, then we'll also send your GP a letter to let them know you've started a medication-supported weight-loss journey.
