A chart compares 2 popular weight-loss drugs, Ozempic and metformin — from cost to risks and benefits

• Ozempic, or semaglutide, is a new obesity drug, delivered in the form of a hormone injection.
• Metformin is a pill that was first developed for type 2 diabetes.

Some people have had success melting away extra pounds with a cheap, decades-old diabetes drug called metformin, while others have spent thousands out of pocket to inject a new hormonal treatment called Ozempic, also known as semaglutide, which changes the way a person's body reacts to food.

Both of these drugs may help people lose weight – but the ways they work on the body are different. Here's what you need to know.

Metformin has been used as a makeshift weight-loss drug for years

Metformin is a chemical compound that was originally derived from French lilacs over 100 years ago. It's been around as a once- or twice-daily pill for decades, and cheap generics are widely available for as little as $5 a month.

The US Food and Drug Administration approved metformin to treat Type 2 diabetes almost thirty years ago, but it's not officially considered a weight loss drug. It has been used off-label by doctors for many years to help some patients manage their weight. People who take metformin can lose, on average, anywhere from five to 15 pounds.

Metformin changes the way the body regulates blood sugar, and, as a result, it can decrease a person's appetite. It activates one of the body's master regulators of metabolism, an enzyme called AMP kinase, triggering some of the same benefits that fasting and exercise provide to our cells.

Common side effects of metformin include abdominal pain, nausea, vomiting and diarrhea.

Semaglutide is a newer drug, and it is FDA-approved for weight-loss

Semaglutide is a much newer injectable drug that mimics an important hunger-regulating hormone in our bodies, called GLP-1. It is FDA-approved to treat both type 2 diabetes and, more recently, obesity. It is sold under the brand name Ozempic when it's used for diabetes, and Wegovy when it's being taken for weight loss. It's the same drug, but Wegovy can be prescribed in larger doses.

Common side effects of semaglutide include upset stomach, vomiting, diarrhea, and constipation.

Metformin's weight loss results are more mixed than semaglutide

Dr. Caroline Apovian, an endocrinologist who co-directs the Center for Weight Management and Wellness at Brigham and Women's Hospital in Boston, says that she doesn't use metformin "too much" for non-diabetic patients seeking weight loss these days.

"Way back, when there really weren't too many other agents for obesity treatment," she'd prescribe more of it, she told Insider, but that was largely because "there wasn't anything else."

But metformin's effects on weight loss are "kind of mild," compared to newer drugs like semaglutide, Apovian said, and persistent side effects like stomach cramps and diarrhea are a common concern among patients taking metformin.

Dr. Carolyn Bramante, who works in the weight management clinic at the University of Minnesota says she still uses metformin "quite often" for patients with a host of weight-related issues, including type 2 diabetes, pre-diabetes, and obesity, too.

It works "pretty well in most people" for improving blood sugar, she said, but "for weight, there does seem to be a little bit more variation" in how well it helps people trim down.

Some patients take metformin and semaglutide together. Apovian said many of her patients have obesity and diabetes. Some trying semaglutide for weight loss may already be on metformin to manage their blood sugar.

Some people take metformin for other reasons besides weight loss

Because metformin is both cheap, and a relatively well-studied drug, it's often the first thing doctors reach for to treat type 2 diabetes. But it also has anti-viral effects on the body, and other unique properties that make it a popular off-label treatment for various diagnoses, including COVID.

Metformin is sometimes prescribed to women who have polycystic ovary syndrome because it helps regulate their hormones and improves their metabolism dysfunction. This often leads to weight loss for women with PCOS, a more consistent menstrual cycle, and a newfound ability to conceive children.

Some doctors harbor hopes that metformin can even help people age more gracefully, by delaying age-related eyesight issues, improving cellular health, and even, potentially, stalling cancer and dementia. But, the idea is still controversial, and a large, nationwide trial of older adults, slated to test the hypothesis, hasn't started yet.