Key points
- The most common side effects of Wegovy and Mounjaro are nausea, diarrhea, vomiting and constipation, mostly while the dose is being increased, and they usually ease with time.
- Less common but potentially urgent: gallstones or gallbladder inflammation, and acute pancreatitis. Knowing the warning signs matters.
- The 2026 Korean audit figures count people treated after a prescription; they do not show that the drugs caused those conditions.
What did Korea's 2026 parliamentary audit find?
Data released ahead of the October 2026 National Assembly audit by Rep. Nam In-soon of the Health and Welfare Committee showed how many people were treated for side-effect conditions flagged by the Ministry of Food and Drug Safety (MFDS) after their first Wegovy or Mounjaro prescription. Of these, 378 went through an emergency room.
| Condition | People treated after a prescription |
|---|---|
| Gallstones | 5,973 |
| Gallbladder inflammation (cholecystitis) | 1,393 |
| Acute pancreatitis | 1,264 |
| Acute kidney injury | 511 |
| Low blood sugar | 335 |
| Total | 9,476 |
Eye conditions were also raised: 104 people (67 on Wegovy, 37 on Mounjaro) were treated for non-arteritic anterior ischemic optic neuropathy (NAION) after a prescription. Adverse event reports to MFDS totaled 2,705 through June 2026 (1,444 for Wegovy, 1,261 for Mounjaro).
Read these numbers with care. Obesity itself raises the risk of gallstones and pancreatitis, and prescriptions have grown sharply, so treatment counts alone can't measure the drugs' risk. The lawmaker who released the data also noted that causation has not been established. Monthly prescriptions rose from 21,457 in December 2024 to 337,026 in May 2026.
The "prone to misuse" designation
Since summer 2026, MFDS has been working to designate Wegovy and Mounjaro as medicines prone to misuse. That would add warnings to the packaging and leaflet and remove them from an exception that lets pharmacists dispense directly in some remote areas. The designation missed its September target and is under regulatory review; at the audit, MFDS said it would also look at prescribing controls with the Ministry of Health and Welfare. As of early November, please check MFDS announcements for the final decision.
How common were side effects in clinical trials?
These are the gastrointestinal side-effect rates in the main trials. Most were mild to moderate and occurred during dose escalation.
| Wegovy (semaglutide 2.4 mg) STEP 1, 68 weeks | Mounjaro (tirzepatide 5–15 mg) SURMOUNT-1, 72 weeks | |
|---|---|---|
| Average weight loss | 14.9% (placebo 2.4%) | 15.0–20.9% (placebo 3.1%) |
| Nausea | 44.2% (placebo 17.4%) | 24.6–33.3% (placebo 9.5%) |
| Diarrhea | 31.5% (placebo 15.9%) | 18.7–23.0% (placebo 7.3%) |
| Vomiting | 24.8% (placebo 6.6%) | 8.3–12.2% (placebo 1.7%) |
| Constipation | 23.4% (placebo 9.5%) | 11.7–17.1% (placebo 5.8%) |
| Gallbladder disorders | 2.6% (placebo 1.2%) | Reported rarely |
One reason for more gallstones is rapid weight loss itself. Losing weight quickly, by any method, raises cholesterol concentration in bile and makes stones more likely.
Which symptoms need urgent care?
Stop the medicine and seek care right away if you have
- Possible acute pancreatitis: sudden, severe pain in the upper abdomen that spreads to your back and lasts for hours, sometimes with vomiting
- Possible gallstones or cholecystitis: cramping pain in the upper right abdomen after a fatty meal, sometimes spreading to the right shoulder; fever; or yellowing of the eyes or skin
- Possible dehydration or kidney injury: vomiting or diarrhea so bad you can't keep fluids down, with much less urine
- Vision changes: sudden blurring or loss of part of the vision in one eye
- Low blood sugar: cold sweat, shakiness, intense hunger or dizziness (watch especially if you also use insulin or a sulfonylurea)
In June 2025, the European Medicines Agency's safety committee concluded that NAION is a very rare side effect of semaglutide (up to 1 in 10,000 people) and recommended adding it to the product information. Anyone whose vision suddenly worsens during treatment should tell their doctor without delay.
Tell your doctor before starting if
- You or a family member have had medullary thyroid cancer or multiple endocrine neoplasia type 2 (MEN2): thyroid C-cell tumors were seen in animal studies, so the drugs are not used in this case.
- You have had pancreatitis or gallstones
- You take diabetes medicines, especially insulin or a sulfonylurea: doses may need adjusting because of low blood sugar risk.
- You have surgery or an endoscopy planned: slowed stomach emptying can raise the risk of food entering the airway under anesthesia, so tell the care team in advance.
- You are planning a pregnancy: the drugs are not used during pregnancy.
In Korea, both are prescribed for weight management to adults with a body mass index (BMI) of 30 or more, or 27 or more with a weight-related condition such as high blood pressure or prediabetes. Buying them online or through social media is illegal in Korea, and products of unknown storage history can be dangerous.
Habits that reduce side effects
- Eat small portions slowly: stop as soon as you feel full, and cut back on greasy, spicy or rich food.
- Drink enough fluids: dehydration from diarrhea or vomiting can lead to kidney injury.
- Increase the dose only as prescribed: rushing the schedule makes stomach side effects worse.
- Get enough protein and do strength training: muscle can be lost along with fat, so strength training is especially recommended after 50. See exercise to prevent muscle loss on GLP-1 drugs.
Myths: true or false?
| Myth | What the evidence says |
|---|---|
| The audit shows nearly 10,000 people had side effects | It counts people treated for those conditions after a prescription; causation was not established. Still, gallbladder problems were also more common than placebo in trials, so knowing the signs is worthwhile. |
| The weight stays off after stopping | In the STEP 1 extension, people who stopped semaglutide regained about two-thirds of the weight they had lost within a year. Changing eating and exercise habits matters. |
| Nausea means the drug is working | Nausea and weight loss are not the same thing. If side effects are severe, talk to your doctor about the dose rather than putting up with them. |
| It's an injection, so it's as casual as a supplement | These are prescription medicines with defined eligibility and contraindications. Sharing pens is prohibited because of infection risk. |
Please keep in mind
- This article is based on information checked as of the date shown above. New research or changes in official guidance may change the content, so please use it as a reference rather than as definitive advice.
- For an accurate diagnosis and advice about your own health, please see a doctor in person.