The active ingredient in Eli Lilly's Mounjaro and Zepbound leads to more weight loss than the active ingredient in Novo Nordisk's Ozempic and Wegovy, according to a study published Monday in JAMA Internal Medicine comparing the two popular drugs taken by injection.

The active ingredient in Eli Lilly's drug is tirzepatide, with the active ingredient in Novo Nordisk's being semaglutide. Both drugs are part of a new class of medications called GLP-1s.

The study looked at both drugs.

"We've tracked GLP-1 use over the last year and we've seen really dramatic increases, and yet, there's not a ton of information available on head-to-head comparisons," lead study author Tricia Rodriguez, a principal applied scientist at Truveta Research, a health care data and analytics company, told NBC News.

Analyzed were more than 41,000 adults who were overweight or obese, and who were prescribed one of the two drugs for the first time. A total of 18,836 adults were equally prescribed tirzepatide and semaglutide between May 2022 and September 2023. The analysis was completed on April 3.

Originally, there were 41,222 adults meeting the study criteria.

The racial makeup was 77.1% white, 11.8% Black, 1.9% Asian and the remainder segment unknown.

Participants with Type 2 diabetes weren't excluded and were just over 50%. These drugs were originally designed to treat the form of diabetes.

"In this population of adults with overweight or obesity, use of tirzepatide was associated with significantly greater weight loss than semaglutide," according to the study's authors. "Future study is needed to understand differences in other important outcomes."

Studied were how much weight the patients lost after three, six and 12 months.

Those taking semaglutide lost on average, 3.6% of their body weight after 3 months; 5.8% after 6 months; and 8.3% after 12 months.

Tirzepatide users lost a greater percentage of body weight each month: an average of 5.9% of their body weight after 3 months; 10.1% after 6 months; and 15.3% after 12 months.

"The majority of patients on both medications experienced clinically meaningful weight loss, but those patients that took tirzepatide experienced significantly more," Rodriguez said.

No significant difference was found in the risk of serious side effects, such as gastroparesis, also known as stomach paralysis, between the two drugs, Rodriguez said.

A randomized, controlled clinical trial has not been conducted. Last year, Lilly began a phase 3 trial of 700 patients comparing the two medications that is expected to be completed in November, according to ClinicalTrials.gov.

Clinical trials earlier found people who take the highest dose of Zepbound lost around 21% of their body weight over 72 weeks, compared with around 15% for patients on Wegovy after 68 weeks. LIlly's study had 2,539 participants over 72 weeks and Novo Nordisk 1,961 over 68 weeks. They were both published in the New England Journal of Medicine.

The new study had some "key limitations," according to a spokesperson for Novo Nordisk. That includes including people with Type 2 diabetes. Also sufficient information on the doses that patients started with and progressed was not listed.

"While lowering body weight is an important goal of obesity management, it is important to also consider other needs when choosing a treatment," the spokesperson told NBC News.

Eli Lilly did not immediately respond to a request for comment.

Semaglutide has been shown in trials to cut the risk of cardiovascular problems in people who are overweight or have obesity.

Tirzepatide has been shown to be a possible treatment for obstructive sleep apnea.

"That being said, if someone's main goal is weight loss, I'd probably go with tirzepatide," Dr. Susan Spratt, an endocrinologist and senior medical director for the Population Health Management Office at Duke Health in North Carolina, told NBC.

In December 2017, the Food and Drug Administration injectable version with the brand name Ozempic was approved for use by people with diabetes in the United States, and, in January 2018 in Canada.

In May 22, the FDA approved Zepbound in the United States.

According to a May 2024 Gallup poll, 6% of U.S. adults, or 15.5 million people, have used or are currently using the two new injectable diabetes medications. Of those, 3% are using it specifically for weight loss.

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