GLP-1 agonist drugs like Zepbound and Mounjaro from drugmaker Eli Lilly (LLY 0.84%) continue to rise in popularity as people are hopeful about the weight loss they can achieve with their help. But a growing number of studies suggest that they could be useful in indications beyond diabetes and weight loss.

One particularly intriguing study involves cancer, and it reveals a potential role for GLP-1 agonists in that area of healthcare. If GLP-1 drugs end up obtaining approval as treatments related to oncology, that could unlock a massive new sales growth opportunity for them. That could make a stock like Eli Lilly an even better buy than it is today.

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Study finds tumors shrank in mice with the help of tirzepatide

Previous studies have shown that when people shed weight, they reduce their risks of developing many illnesses, among them, cancer. But a recent study presented at the Endocrine Society's annual meeting found that GLP-1 agonists could have a more direct impact: They may also reduce the size of breast cancer tumors.

The study was modest in scope and not on human patients: Its subjects were 16 mice. Those mice that received injections of tirzepatide (the active ingredient in both Zepbound and Mounjaro) for 16 weeks lost around 20% of their weight, which was comparable to the weight loss that humans achieve while on the drug. And their tumors shrank by around that percentage as well. "Researchers found that tumor volume was significantly correlated with body weight," reported ScienceDaily.

But even though these results are promising, they are very preliminary. It would take years of studies on humans before a GLP-1 drug could conceivably obtain a label expansion for use in treating any type of cancer. However, there is hope that GLP-1 drugs can do more than just help with diabetes and weight reduction. Regulators did approve tirzepatide for the treatment of obstructive sleep apnea last year. And another study found that it can help reduce the risk of heart failure.

As more research is done on GLP-1 drugs, the number of indications they can treat may grow significantly. And if tirzepatide is able to help treat cancer, it could make this already massive drug a sales behemoth for Eli Lilly.

Business has been booming for Eli Lilly, and more growth is still ahead

Eli Lilly has already been performing incredibly well, thanks in large part to its highly successful GLP-1 drugs. In the first three months of 2025, its sales soared by 45% year over year to $12.7 billion. Zepbound and Mounjaro combined to make up $6.2 billion of that tally.

And what's exciting is that these drugs are still in their early growth stages. That's a big reason why Eli Lilly stock trades at a hefty premium of 65 times its trailing earnings. Its 5-year price-to-earnings growth (PEG) multiple of 1.2 indicates, however, that it may not be that expensive relative to its medium-term growth potential.

Eli Lilly stock is a no-brainer buy

Although the stock may not look cheap, Eli Lilly could be among the best long-term investments you can add to your portfolio today. If tirzepatide racks up more indications and Eli Lilly's already approved drugs reach greater numbers of people, its sales and profits are likely to rise in the years ahead.

This year, the stock is up a modest 4% as the hype around it appears to have cooled. But if you're in it for the long haul, it would be hard to go wrong with Eli Lilly. It may just be one of the best growth stocks in the healthcare sector.