For decades, we've been wrong about one of the world's most prescribed medications.

Metformin—taken by hundreds of millions of people worldwide to manage Type 2 diabetes—works primarily in the gut, not the liver as researchers have believed since the drug entered widespread use. That's the conclusion of a study published in Nature Metabolism by researchers at Northwestern University.

This matters because understanding how a drug works opens doors to making it work better—or to developing entirely new treatments based on the same mechanism.

Here's what we thought: Metformin travels to the liver and suppresses glucose production, thereby lowering blood sugar. Simple, elegant, and—as it turns out—incomplete.

Here's what actually happens: Metformin slows down energy production in intestinal cells by inhibiting their mitochondria (the cellular power plants). With their normal energy production throttled, gut cells are forced to consume more glucose from the bloodstream to keep functioning. As Dr. Navdeep Chandel, professor of biochemistry at Northwestern, put it: "Metformin essentially helps the intestine suck the glucose out of the bloodstream."

The researchers proved this by creating genetically engineered mice with a clever trick. They gave the mice's intestinal cells a yeast enzyme called NDI1 that's resistant to metformin's effects. When the gut cells could still produce energy normally despite metformin, the drug's blood-sugar-lowering power vanished.

That's definitive proof that the gut—not the liver—is where the action happens.

This discovery also explains several quirks about metformin that never quite made sense under the old model. Why do metformin users have lower blood sugar spikes after meals? Because their intestines are actively pulling glucose out of circulation. Why does the drug promote modest weight loss and reduced appetite? Because the metabolic stress in gut cells triggers release of a hormone called GDF15 that affects appetite regulation.

Now, before you rush to your doctor demanding a different diabetes medication, understand that metformin still works. We were just wrong about the mechanism. It's a bit like discovering that your car engine works differently than the manual says—the car still gets you where you need to go.