The hidden cost of fast weight loss on drugs like Mounjaro and Ozempic

Both tirzepatide and semaglutide, the active ingredients behind blockbuster weight-loss drugs including Mounjaro, Zepbound, Ozempic and Wegovy, came with a hidden cost, according to a new study published in The BMJ: a harmful loss of lean mass, meaning everything in the body that isn’t fat.

Losing too much lean mass is dangerous, researchers note, as it is tied to a higher risk of falls, bone fractures and early death. The finding emerged from an analysis of 262 clinical trials involving roughly 100,000 participants and 19 different medications, which found a clear trade-off: the more weight a drug helped someone lose, the more likely that person was to suffer side effects or quit the treatment altogether.

Tirzepatide and the still-unapproved CagriSema topped the charts for the biggest weight drops among the medications studied, while subcutaneous semaglutide showed the strongest evidence for reducing death rates and major heart issues. Yet despite these gains, the drug groups showed no clinically meaningful improvement in quality-of-life scores when compared with people making lifestyle changes alone.

“Most agents do not improve quality of life meaningfully and few show cardiovascular benefits,” the study authors wrote, underscoring that treating obesity involves trade-offs that go beyond the number on a scale.