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Thirty-five cases of adverse reactions to Mounjaro reported in first month of availability

Published: 10 Dec. 2025, 15:17

Boxes of Mounjaro, a tirzepatide injection drug used for treating type 2 diabetes [LILLY KOREA]

Thirty-five cases of adverse reactions or serious side effects were reported in Korea within the first month of the release of Mounjaro (tirzepatide), a weight loss drug developed by Eli Lilly.

According to data submitted by the Korea Institute of Drug Safety and Risk Management to Democratic Party Rep. Seo Mi-hwa, a member of the National Assembly’s Health and Welfare Committee, a total of 35 cases of adverse reactions were reported from August to September, the first month of the drug's availability in Korean pharmacies. Among the cases were four instances of diarrhea and one case of hypoglycemic shock, two reactions classified as serious adverse reactions.

Under Korea’s Regulations on the Safety of Pharmaceuticals and Related Products, serious adverse reactions include any medical occurrences that result in death, are life-threatening, require hospitalization or prolong existing hospitalization, cause permanent or significant disability or are otherwise considered medically significant and require treatment.

Other reported reactions included six cases of muscle pain, four of injection site bleeding, four of accidental underdosing, three of injection site pain, three of upper abdominal pain, three of indigestion, two of headaches, two of nausea, two of constipation and one of injection site erythema.

By gender, 15 cases were reported in women and four in men. The remaining 16 did not include gender data. By age, seven cases involved individuals aged 19 to under 65. The remaining 28 cases did not include age data.

According to medical sources, tirzepatide is frequently associated with gastrointestinal issues such as diarrhea, upper abdominal pain and indigestion, as well as musculoskeletal disorders like muscle pain, and administration site conditions including bleeding at the injection site.

A pedestrian walks about a pharmacy in Jongno District, central Seoul, on Feb. 9. [YONHAP]

Mounjaro is an innovative drug by Eli Lilly that shows efficacy in treating both obesity and diabetes. It is a single-molecule injectable that simultaneously activates two hormone receptors — GLP-1 and GIP — to promote insulin secretion, improve insulin sensitivity, reduce glucagon levels, delay gastric emptying and ultimately reduce food intake and body weight.

Along with Wegovy from Novo Nordisk, Mounjaro has secured a dominant position in Korea’s weight-loss drug market. However, concerns persist over adverse effects and misuse of such medications.

“There is rising demand for obesity drugs and growing concern about how they are prescribed,” Seo said. “The Ministry of Food and Drug Safety must strengthen crackdowns on deceptive advertising through its cyber monitoring unit, while the Korea Institute of Drug Safety and Risk Management should conduct thorough analyses of adverse reaction reports.”

The institute noted, however, that adverse reactions reported to the agency do not necessarily establish a causal relationship with the drug. Therefore, the reported data alone is insufficient to confirm that Mounjaro was directly responsible for the reactions.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.
BY KIM JI-HYE [[email protected]]