For more than 60 years, metformin has been the most prescribed drug for type 2 diabetes. Affordable, well-tolerated, and remarkably effective, it has helped hundreds of millions of people manage their blood sugar. But here’s the thing — until very recently, scientists didn’t fully understand why it worked so well.
A landmark study published in Science Advances in 2025 may finally have the answer. And it comes from a place no one expected: the brain.
A Drug We Thought We Knew
The standard explanation for metformin has always pointed to the liver and the gut. The dominant view held that metformin acted mainly through peripheral organs — especially the liver and the gastrointestinal tract — where it dampens glucose output and alters gut-derived signals.
That explanation is not wrong. But according to this new research, it is incomplete.
The Brain Has Been Involved All Along
Scientists at Baylor College of Medicine and international collaborators have discovered a previously unrecognized new player mediating clinically relevant effects of metformin: the brain. By uncovering a brain pathway involved in metformin’s anti-diabetic action, researchers have discovered new possibilities for treating diabetes more effectively and precisely.
The key lies in a small protein called Rap1, sitting inside a region of the brain known as the ventromedial hypothalamus (VMH). This area is a critical control center for whole-body glucose metabolism, regulating appetite, body temperature, and overall energy balance.
What the research team found was striking: metformin’s ability to lower blood sugar at clinically relevant doses depends on turning off Rap1 in this brain region.
What the Experiments Showed
The researchers, led by pathophysiologist Makoto Fukuda at Baylor College of Medicine, tested their hypothesis with a series of carefully designed experiments on mice.
They used genetically modified mice that lacked Rap1 in their VMH. These mice were fed a high-fat diet to mimic type 2 diabetes. When given low doses of metformin, the drug failed to lower their blood sugar. However, other diabetes medications like insulin and GLP-1 agonists still worked.
This was a critical finding — it wasn’t that the mice were resistant to all treatment. Metformin specifically, without Rap1, had no effect.
The team then pushed further. To further show that the brain is a key player, the researchers injected tiny amounts of metformin directly into the brains of diabetic mice. The result was a significant drop in blood sugar, even with doses thousands of times smaller than what’s typically given by mouth.
They also zoomed in on exactly which brain cells were responding. “We found that SF1 neurons are activated when metformin is introduced into the brain, suggesting they’re directly involved in the drug’s action,” Fukuda said. Using brain slices, the scientists recorded the electrical activity of these neurons. Metformin made most of them more active, but only if Rap1 was present.
Why This Matters for Patients
“This discovery changes how we think about metformin,” Fukuda said. “It’s not just working in the liver or the gut, it’s also acting in the brain. We found that while the liver and intestines need high concentrations of the drug to respond, the brain reacts to much lower levels.”
This is more than academic. If the brain is a primary site of metformin’s action, it reframes decades of research and opens the door to far more targeted approaches to diabetes treatment — ones that work at lower doses with potentially fewer side effects.
Beyond Diabetes: The Anti-Aging Connection
The implications stretch even further. Metformin has long been observed to have benefits beyond blood sugar control — including what some researchers describe as “gerotherapeutic” effects, meaning it appears to slow certain aging processes. Evidence suggests metformin can limit DNA damage, promote gene activity associated with longevity, and even slow brain aging.
Now, with the Rap1 pathway identified, researchers have a concrete mechanism to investigate. “These findings open the door to developing new diabetes treatments that directly target this pathway in the brain,” Fukuda said. “In addition, metformin is known for other health benefits, such as slowing brain aging. We plan to investigate whether this same brain Rap1 signaling is responsible for other well-documented effects of the drug on the brain.”
An Important Caveat
It is worth being clear: the current evidence comes from animal models, and the authors note that confirmation in human studies is required before clinical translation. This research is a breakthrough in understanding, not yet a change in prescribing practice. Human trials will be the next critical step.
What This Means for the Future of Diabetes Care
We are entering an era where diabetes treatment is becoming increasingly personalized and precise. Understanding how medications work at the neurological level is not just a scientific curiosity — it directly informs better treatment protocols, more effective combinations of therapies, and ultimately, better outcomes for patients.
The research agenda that follows will determine whether the decades-long story of metformin becomes one of expanded neural-targeted therapies or a refined appreciation of multi-organ mechanisms for the same drug. Either way, the brain is now firmly part of the conversation.
