Mounjaro vs Wegovy
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A head-to-head trial win, record sales and a falling U.S. obesity rate show how fast these injections have moved into everyday life, along with several caveats

Few medicines have changed the conversation around weight as quickly as the weekly injections known as GLP-1 drugs. The numbers from 2025 are striking, and a few of them need careful reading.

A head-to-head win for tirzepatide

The comparison drawing the most attention is the SURMOUNT-5 trial, published in the New England Journal of Medicine. It randomly assigned 751 adults with obesity but without diabetes to tirzepatide or semaglutide at the maximum tolerated dose. At 72 weeks, average weight change was -20.2% on tirzepatide against -13.7% on semaglutide, or about 22.8 kg against 15.0 kg. More tirzepatide patients also hit each weight-loss threshold, with 31.6% losing at least 25% of their body weight versus 16.1% on semaglutide.

The "Mounjaro vs. Wegovy" shorthand needs a clarification. The trial tested tirzepatide under its obesity brand name, Zepbound, against semaglutide, sold as Wegovy. Mounjaro is the same drug as Zepbound, marketed for type 2 diabetes. Two further caveats apply. The study was open-label and funded by Eli Lilly, the maker of tirzepatide. Independent follow-up research will matter.

The world's best-selling drug

The commercial results followed. In the third quarter of 2025, Mounjaro and Zepbound together brought in $10.1 billion, passing Merck's cancer drug Keytruda to become the world's top-selling drug. For the full year, the two generated $36.5 billion, about 56% of Lilly's total revenue.

A falling U.S. obesity rate, with a caveat

The most talked-about public health figure comes from Gallup. Its survey found the U.S. adult obesity rate fell to 37% in 2025 from a record 39.9% in 2022, an estimated 7.6 million fewer adults with obesity. Gallup also found 12.4% of adults taking these drugs for weight loss, up from 5.8% in February 2024. The biggest declines were among adults aged 40 to 64, who also used the drugs most.

The timing fits, but it doesn't prove cause. Gallup relies on self-reported height and weight, though it has used the same method throughout. The same survey found diagnosed diabetes at an all-time high of 13.8%, and Gallup said the drugs should not be viewed as a cure-all for overall health.

One in 20 in the UK

Britain has seen a sharp rise. About 2.49 million packs of Mounjaro and Wegovy were sold in July 2025, against 493,000 in July 2024, roughly sevenfold growth. Mounjaro also became available through NHS GP practices in June 2025. One estimate put 2.5 million people, close to one in 20 adults, on the drugs, though Eli Lilly raised the private price of Mounjaro by 170% to £330 a month for the top dose.

Estimates differ, because sales packs are not the same as people. A UCL analysis estimated around 1.6 million adults in Great Britain used weight-loss drugs between early 2024 and early 2025, and found that four in five of those taking medication solely for weight loss were on tirzepatide. The one-in-20 figure is best read as a recent, approximate estimate.

Changes users report

The infographic cites two lifestyle figures: 72% of users went down a clothing size and 63% eat fewer sweet snacks. I couldn't trace those numbers to a published survey, so treat them as unverified user-reported figures. Related research points the same way. Kantar found that four in five users it surveyed planned to eat fewer chocolates and crisps, and the firm noted that four in 100 British households had at least one GLP-1 user. Self-reported surveys capture intentions and perceptions, not measured outcomes.

Beyond diabetes

These drugs began as diabetes treatments and are now widely used for weight management. Gallup's figures show the shift in the U.S., and the UCL work shows it in Britain, where most weight-loss use is private. The change brings issues of cost, access and long-term use, and some people obtain the drugs outside regulated channels. Anyone considering a GLP-1 drug should talk to a doctor or pharmacist about benefits, side effects such as nausea and other digestive problems, and what happens if treatment stops.