Dozens report side effects in first month of Mounjaro’s rollout in Korea

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At least 35 cases of side effects, including two serious ones, were reported within a month of weight loss drug Mounjaro’s release in Korea, raising safety concerns amid surging demand.

According to data obtained by Rep. Seo Mi-hwa of the ruling Democratic Party of Korea from the Korea Institute of Drug Safety and Risk Management, the cases were reported between August and September, following the local rollout of Mounjaro, whose active ingredient is tirzepatide.

Health officials said two of the 35 reported cases met the threshold for a serious reaction, with symptoms such as diarrhea and hypoglycemic shock. Under Korean drug safety regulations, serious adverse events are defined as those that lead to death, are life-threatening, require hospitalization or produce permanent impairment.

Fifteen cases involved women and four involved men, while the gender of the remaining 16 patients was not reported. Seven cases were reported among people between the ages of 19 and 65, with no age data given for the others. Common side effects linked to tirzepatide include gastrointestinal distress, abdominal pain, indigestion, muscle pain and injection site bleeding.

Mounjaro, developed by Eli Lilly, targets both GLP-1 and GIP receptors to enhance insulin secretion, improve blood sugar control and reduce appetite. Despite its effectiveness for obesity and diabetes, safety concerns and potential misuse continue to draw scrutiny as use of weight loss drugs soars nationwide.

Seo urged regulators to tighten monitoring of misleading advertisements and to closely analyze reported side effects. The state-run drug safety agency noted that reported cases do not necessarily prove a causal relationship with the drug.

Eli Lilly Korea said it has "faithfully reported all safety information" and will "continue to cooperate with authorities to ensure the medicine’s safe and appropriate use."

The report comes amid Korea’s fast-growing weight loss drug market and lingering concerns over long-term safety.

According to the Health Insurance Review & Assessment Service, there were more than 1.14 million weight loss drug prescriptions in the first half of this year — roughly one for every 50 Koreans — with the annual figure expected to surpass 2 million, up from 1.8 million last year.