Obesity treatments such as Wegovy and Mounjaro, dubbed "miracle weight-loss drugs" amid a global craze, may cause rapid muscle loss, researchers have warned. While the drugs deliver powerful weight-reduction effects, muscle mass can also decline sharply during the process, according to the analysis.
According to The Wall Street Journal, the American Diabetes Association recently disclosed findings that GLP-1 (glucagon-like peptide-1) class obesity treatments can trigger reductions in muscle mass as well as body fat.
GLP-1 drugs were originally developed to treat type 2 diabetes, but they emerged as a "game changer" in the obesity treatment market after their potent weight-loss effects were confirmed. Representative products include Wegovy from Danish drugmaker Novo Nordisk and Mounjaro and Zepbound from U.S.-based Eli Lilly. The drugs have spread rapidly in Korea as well, with users saying "everyone around me is taking them," driving explosive popularity.
Both Fat and Muscle Disappear: Up to 10% Muscle Loss Possible
The medical community is increasingly concerned that rapid weight loss may bring an equally heightened risk of muscle loss. According to the analysis, some patients can experience muscle mass reductions of up to 10%. Researchers described this as "comparable to the muscle loss that may appear after more than 10 years of aging."
Muscle loss can occur during ordinary dieting, but the problem grows when it progresses sharply over a short period. It can lead to lethargy, weakness, impaired balance and reduced exercise capacity, and may also cause the "yo-yo effect," in which weight rebounds as basal metabolic rate falls.
Daniel Green, a researcher who participated in the analysis, noted that "in the process of treating obesity, we are actually increasing frailty."
A separate study found the risk of muscle-loss side effects is higher in women. Researchers at a university hospital in Turin, Italy, analyzed that muscle loss following GLP-1 drug use may occur more frequently in female patients.
Experts stress that drug use must be combined with strength training and protein intake. Eli Lilly, which developed Mounjaro and Zepbound, also said "the treatments should be used alongside increased physical activity."
'Mounjaro Surge' Overtakes Keytruda; Korean Prescriptions Jump Fivefold
According to first-quarter earnings figures released by Bloomberg, Eli Lilly's obesity and diabetes drug Mounjaro generated sales of approximately $8.7 billion (about 12.6 trillion won). The figure surpassed Merck's immuno-oncology drug Keytruda (about $7.9 billion), which had long held the top spot in global drug sales. It marked the first time in three years that Keytruda surrendered its position as the world's best-selling drug.
Adding Zepbound, an obesity treatment based on the same ingredient (tirzepatide), the growth trajectory becomes even steeper. The industry already expects annual sales of tirzepatide-based drugs to far exceed those of Keytruda.
"The shift in the market's center of gravity from Keytruda to tirzepatide is a natural flow," said Evan Seigerman, managing director at BMO Capital Markets. "Considering the therapeutic effects and safety profile, the results are not surprising."
The Mounjaro wave has also surged in the Korean market. According to the Health Insurance Review and Assessment Service, Mounjaro prescriptions reached approximately 97,000 in November last year. The figure represents more than a fivefold increase compared with the drug's first month on the market.
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