Though they are still primarily prescribed for weight loss and diabetes, researchers are looking at the effects of GLP-1s for a wide variety of conditions from substance abuse disorders to kidney disease. But could they also have an effect on cancer? At ASCO 2026, we saw mounting evidence that they could, in the form of retrospective trials that took advantage of the number of people already taking these drugs, including cancer patients.

“This is one of the fastest growing drug classes in history. I mean, there're estimates that one in eight Americans are on a GLP-1 right now,” Dr Jess Paulus, senior director of real-world research at Ontada and an author on one of the studies presented at the conference, told Deep Dive in a video interview. “In my humble opinion, this is the perfect place for real-world data to make an impact. We're in the phase of hypothesis generation, you know, and hypothesis testing, but not necessarily doing the definitive causal study.”

Paulus’ study used EHR data from more than 1,000 GLP-1 users and more than 400,000 non-GLP-1 users and matched them by cancer type and diagnosis year to compare outcomes. They saw statistically significant improvements in overall survival in breast cancer and prostate cancer.

Another retroactive matched cohort study looked at 94,000 women, including those who developed cancer and those who didn’t, as well as women who both did and did not use GLP-1 drugs. That study found that GLP-1 users had a lower chance of developing breast cancer in the first place. Another study found similar preventative results in colorectal cancer. And that’s just a handful of the many retrospective GLP-1 studies in ASCO posters and presentations.

“So many of us are asking this question with different datasets and different tumour types and coming to a pretty consistent answer,” Paulus said.

But why should GLP-1s affect either cancer incidence or treatment efficacy? Well, researchers aren’t sure, but there are a lot of plausible theories. One, Paulus said, is that improved glycaemic control gives cancer cells less energy to grow. Another is that the drugs themselves might limit cancer cell proliferation.

“There's multiple different potential pathways, including an intriguing one that I talked about yesterday with some oncologists about the reduction in the amount of adipose tissue,” Paulus said. “So, that's not even necessarily a direct effect of the drug on cancer, but more it could be allowing chemotherapy or other cancer treatments to work better through the mechanism of reducing adipose tissue.”

But there’s one thing the many researchers who presented similar research at ASCO agree on: retrospective studies will only take them so far. EHR data and even claims data are very limited when it comes to capturing GLP-1 use because so many people get the drugs outside the mainstream healthcare system. And in real-world studies it's harder to account for confounding variables that might make the effect seem larger than it is. To really understand the relationship between GLP-1s, cancer risk, and treatment outcomes, researchers will have to move from retrospective to prospective studies.

“We're about ready to motivate prospective studies,” Paulus said. “I think there's enough retrospective evidence that we need to take another step forward.”

You can watch the whole interview with Paulus, conducted on-site at ASCO 2026, below.