Retatrutide is an investigational drug that combines the activity of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon receptors.
Because it is in the same family as the widely used GLP-1 receptor agonists, many people wonder whether it might cause cancer, as GLP-1 drugs carry FDA warnings about thyroid C-cell tumors, including medullary thyroid carcinoma (MTC).
This article reviews the latest research on cancer risks linked to retatrutide and similar drugs, based on findings from clinical trials and other studies.
Why is cancer risk a concern with GLP-1-based drugs?
When the first GLP-1 receptor agonist (exenatide) was developed, regulators required animal studies to look for potential toxicity. High doses given to rodents led to hyperplasia (an increase in the number of cells) and tumors in the C-cells of the thyroid.
C-cells produce the hormone calcitonin, and their tumors are known as medullary thyroid carcinoma. These findings prompted the FDA to require a boxed warning on GLP-1 receptor agonists advising against use in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 (MEN2).
The FDA caution applies specifically to medullary thyroid carcinoma, which represents only 3-4% of all thyroid cancers.
Detection bias in early observational studies
Another reason for concern came from observational studies that appeared to show an increased risk of thyroid cancer in people using GLP-1 drugs. A case-control study in France found that people who used GLP-1 RAs for one to three years had about a 58% higher rate of thyroid cancer overall and a 78% higher rate of medullary thyroid cancer compared to non-users.
However, in an international multisite cohort, researchers found no increase in thyroid cancer risk among GLP-1 users and argued that elevated early diagnoses are more likely due to detection bias rather than the drug.
Another study initially suggested an increased risk, but showed that the higher number of thyroid cancer cases mainly happened within the first year of treatment. During that first year, people were about 85% more likely to be diagnosed, but after that, the risk went back to normal and wasn’t higher in later years.
This pattern is important because cancers usually take years to develop, not just months. So instead of the drug causing new cancers, it’s more likely that existing cancers were simply detected earlier once patients started treatment and were monitored more closely.
Dr. David Toro Tobon, an endocrinologist at Mayo Clinic and study author, said that it’s unlikely that GLP-1 drugs would cause noticeable tumors to develop within just a few months of starting treatment. He added that it’s more likely that the FDA warning has made both doctors and patients more alert, resulting in more frequent thyroid checks when these medications are started.
Obesity, diabetes, and baseline cancer risk
People who qualify for retatrutide and similar medications often already have an elevated risk of cancer. Obesity and type 2 diabetes increase the likelihood of developing multiple malignancies, including:
-
- liver
- biliary tract
- pancreas
- stomach
- colorectal
- kidney
- bladder
- breast
- endometrial cancers
Obesity alone is associated with an increased risk of at least 13 cancers. This means that participants in trials of weight-loss medications start from a higher baseline risk. If researchers observe more cancers among these patients over time, the cause may be the underlying metabolic condition rather than the drug.
Type 2 diabetes and excess body fat create a pro-inflammatory environment and insulin resistance. Elevated insulin and insulin-like growth factors can promote the growth of abnormal cells. Chronic low-grade inflammation and oxidative stress also contribute to DNA damage.
Can retatrutide cause cancer?
At present, there is no evidence that retatrutide causes tumors. While rodent studies initially raised alarms about thyroid tumors from GLP-1 stimulation, recent animal research suggests that incretin triple agonists like retatrutide may help slow the progression of cancers linked to obesity.
In a study published in Nature, retatrutide reduced pancreatic tumor size by 14-fold, compared to a 4-fold reduction with semaglutide. It also delayed tumor onset and slowed progression. In a lung cancer model, retatrutide led to 50% fewer tumors forming and a 17-fold reduction in tumor size compared to controls.
These benefits persisted even after some weight regain. Researchers also observed immune changes that may help the body fight cancer, but more studies in humans are needed to confirm these findings.
Safety data of retatrutide from clinical trials
Because retatrutide is still in development, only a few clinical trials have reported safety outcomes. In its phase 2 trial, adverse events were primarily gastrointestinal, such as nausea, diarrhea, and vomiting, and most were mild or moderate in severity. There were no reports of medullary thyroid carcinoma, C-cell hyperplasia, or other cancers during the trial period.
Another placebo-controlled study examined retatrutide in adults with type 2 diabetes. This trial primarily assessed glucose control and weight loss, and it also found gastrointestinal side effects similar to those of the obesity trial. The study did not report any cases of thyroid cancer or other malignancies during its follow-up period.
If retatrutide is approved, long-term post-marketing surveillance will be necessary to assess cancer risk conclusively.
GLP-1 receptor agonists and cancer risk
Across multiple studies, GLP-1 therapies are generally linked to no increased cancer risk or even a slight reduction. When small increases appear, they are usually not statistically significant and may be explained by detection bias or underlying health conditions.
Thyroid cancer
Large studies and meta-analyses show no meaningful increase in thyroid cancer risk with GLP-1 drugs. In one analysis of nearly 100,000 GLP-1 users vs 2.49 million comparators, no higher risk was seen over ~2-3 years.
A US study of 351,913 patients found 0.17% of users developing thyroid cancer. Although diagnoses were higher in the first year (~85% increase), this was likely due to increased monitoring, as risk returned to normal.
Meta-analyses support these findings. One found a 37% higher relative risk (not statistically significant), while another showed a 52% increase, but the results are inconclusive. Overall, experts agree that the absolute risk is very low, and current evidence does not show a clear causal link.
Pancreatic cancer
Initial reports raised concerns that GLP-1 drugs might promote pancreatitis and pancreatic cancer, leading to lawsuits and media coverage. Later studies have mostly eased these concerns.
In a study of 89,596 patients with chronic pancreatitis, GLP-1 users had a 51% lower risk of pancreatic cancer, with a similar 47% reduction seen in those with type 2 diabetes. A meta-analysis also found a 22% lower risk, while another review showed little or no effect on pancreatic cancer risk.
Collectively, current evidence suggests GLP-1 drugs do not raise pancreatic cancer risk and may even reduce it in high-risk groups.
Colorectal and other gastrointestinal cancers
Colorectal cancer is a major health concern, and some hypothesize that GLP-1 drugs might influence risk because they slow gastric emptying and alter gut hormones.
The latest findings suggest GLP-1 drugs may have a neutral or even protective effect. One meta-analysis found a 19% lower risk of colorectal cancer, along with reduced risks for liver cancer (26% lower) and a possible reduction for gastric cancer. Benefits appeared stronger in older adults and those with obesity.
Breast, ovarian, and endometrial cancers
Evidence regarding breast cancer risk has been mixed, but no strong signal suggests harm. A 2025 analysis found that GLP-1 RAs have little to no effect on breast cancer risk. The data shows about 10 fewer to 12 more cases per 10,000 people, showing that any difference is very small.
Another large study found GLP-1 users had a 17% lower overall cancer risk, with notable reductions in endometrial (25% lower) and ovarian cancer (47% lower).
Kidney and other cancers
Most studies do not show a meaningful increase in kidney cancer risk with GLP-1 drugs, although one study noted a slight, non-significant trend upward. Researchers believe this may be due to other factors, like obesity or high blood pressure, rather than the drugs themselves. For other cancers, such as gallbladder, bladder, and esophageal cancer, there is limited data, but overall findings suggest little to no effect.
Bottom line
At present, there is no convincing evidence that retatrutide causes cancer. Animal studies suggest that retatrutide may even slow the growth of certain tumors, and early human trials have not identified any cancer signals. If you’re considering retatrutide, discuss your personal risk factors with a healthcare professional to balance potential risks against the health benefits of weight loss.
Frequently Asked Questions
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