Overweight and obese adults using popular diabetes medications to suppress their appetites are “significantly more likely” to shed pounds with Eli Lilly’s Mounjaro than with Novo Nordisk’s Ozempic, a new study has found, a result that could have major implications for one of the hottest new markets in the pharmaceutical field.

Writing Monday in JAMA Internal Medicine, seven medical experts compared the health records of 18,386 adults with excessive body mass indexes who used one of the two Type 2 diabetes drugs in clinical trials between May 2022 and September 2023.

While most patients lost at least 5% of their body weight within a year, the study found that those receiving Mounjaro were over two times more likely than Ozempic patients to experience 10% weight loss and three times more likely to trim 15%.

Researchers also found that patients without Type 2 diabetes lost more weight than those with it, but Mounjaro was more effective than Ozempic in both groups.

The study offers the first real-world comparison of the two appetite-suppressing drugs, which surged in popularity among diabetics and non-diabetics last year after dramatic reports of weight loss in patients.

Tricia Rodriguez, the study’s lead author, said its findings confirm earlier placebo-controlled trials that found tirzepatide, the primary active ingredient in Mounjaro, is more effective for weight loss than Ozempic’s key ingredient, semaglutide. She said further research is needed to explain why.

“While underlying mechanisms of action weren’t a focus of this study, tirzepatide has dual mechanisms, while semaglutide has a single mechanism, which may be the reason for differences in effectiveness,” Ms. Rodriguez, an applied scientist at Truveta Research in Bellevue, Washington, told The Washington Times.

While tirzepatide regulates two metabolic hormones affecting blood glucose levels, semaglutide regulates only one.

However, the study found no difference between the two drugs in the number of patients reporting moderate to severe gastrointestinal side effects. Side effects such as bowel obstruction, gastroparesis and pancreatitis become more common after sustained use.

With that in mind, the researchers noted a “high rate of discontinuation” from those taking the drugs, with 55.9% of Mounjaro patients and 52.5% of Ozempic patients quitting within a year.

The findings come as GLP-1s, the class of diabetes medication that includes Mounjaro and Ozempic, have fueled billions of dollars in pharmaceutical sales.

The Food and Drug Administration approved Ozempic as an insulin-boosting treatment for Type 2 diabetes in 2017. In 2021, the agency added Wegovy, a version approved for weight loss treatment, without obliging insurance companies to cover it.

Novo Nordisk, the Danish company with exclusive rights to sell Ozempic in the U.S. through 2031, has spent millions on advertising and lobbying to expand its market. Its patent monopoly forces patients to pay a monthly premium of $300 to $1,300 out-of-pocket for the drug, which costs about $35 to manufacture.

The FDA approved Mounjaro for Type 2 diabetes in 2022. After Indianapolis-based pharmacy giant Eli Lilly & Co. reported an explosion in sales, the FDA approved a version called Zepbound for weight loss in November.

Neither Eli Lilly nor Novo Nordisk responded Monday to emails seeking comment on the study.

In recent months, some pharmacies and clinics have started producing compounded versions of Ozempic and Mounjaro to capitalize on the absence of affordable generic versions.

With over 70% of U.S. adults estimated to be overweight or obese, some industry insiders have compared GLP-1s to Viagra as a “game-changer” that could reshape consumption habits nationwide.

Others view Mounjaro and Ozempic more cautiously. They point out that the costly drugs are difficult to tolerate, do nothing for the heart problems of patients who do not exercise, and require lifelong use to maintain an ideal weight, which increases the risk of severe side effects such as stomach paralysis. 

“These drugs are injectable anorexia, a crash diet,” said Calley Means, co-founder of TrueMed, an Austin, Texas-based company that prescribes diet and exercise plans for weight loss. “Long-term, the drugs are highly problematic with significant reports of [gastro-intestinal] dysfunction, a black box warning for thyroid cancer, and even suicidal ideation.”

Mr. Means, a former lobbyist for the food and pharmacy industries, has argued that the drugs offer companies a way of profiting from obesity for years without curing it.

As Mounjaro and Ozempic suppress the appetite, they cause users to feel fuller and lose weight in the short term. However, the weight often returns after patients stop their prescriptions.

According to Ms. Rodriguez, the study published Monday did not track patients after they quit using the drugs.

“Our study only included patients while they stayed on the medication, which may not be representative of all patients who start these medications,” she said.

Some retail leaders doubt the drugs will ever see the years of prescription sales gains they need to make restaurants, weight loss companies and gyms break a sweat.

Branden Muhl, CEO of Mahaska Bottling Co., which operates automated convenience stores at industrial manufacturing plants, said even the most “bullish estimates” show just 7% of Americans will use the drugs by 2035, putting any visible impact on his business in the “distant future.”

In the meantime, he pointed out that Monday’s study found that more than half of Mounjaro and Ozempic patients stopped using the drugs.

“No drug is ultimately effective in this category if patients cannot stay on it long-term,” Mr. Muhl said, citing recent data from the American Diabetes Association.