How to read these numbers
Each percentage is the share of trial participants who reported that side effect at least once, during trials lasting more than a year. The placebo column is people who got a drug-free injection and still reported the same thing, because headaches, tiredness and upset stomachs happen anyway. The part the drug is responsible for is the gap between the two columns.[1][2]
The common ones
| Side effect | Wegovy 2.4 mg | Placebo | Zepbound 5 / 10 / 15 mg | Placebo |
|---|---|---|---|---|
| Nausea | 44% | 16% | 25 / 29 / 28% | 8% |
| Diarrhea | 30% | 16% | 19 / 21 / 23% | 8% |
| Vomiting | 24% | 6% | 8 / 11 / 13% | 2% |
| Constipation | 24% | 11% | 17 / 14 / 11% | 5% |
| Abdominal pain | 20% | 10% | 9 / 9 / 10% | 5% |
| Headache | 14% | 10% | under 5%, not tabled | |
| Fatigue | 11% | 5% | 5 / 6 / 7% | 3% |
| Indigestion (dyspepsia) | 9% | 3% | 9 / 9 / 10% | 4% |
| Dizziness | 8% | 4% | 4 / 5 / 4% | 2% |
| Burping (eructation) | 7% | under 1% | 4 / 5 / 5% | 1% |
| Reflux (GERD) | 5% | 3% | 4 / 4 / 5% | 2% |
| Hair loss | 3% | 1% | 5 / 4 / 5% | 1% |
| Injection-site reactions | 1.4% | 1% | 6 / 8 / 8% | 2% |
The higher 7.2 mg Wegovy dose has its own table: nausea 39% versus 35% on 2.4 mg and 13% on placebo, and a skin sensation called dysesthesia (burning, tingling or sensitive skin) in 22% versus 6% on 2.4 mg.[1] The diabetes versions, Ozempic and Mounjaro, use lower doses and report lower gut rates; nausea was 15.8 to 20.3% on Ozempic and 12 to 18% on Mounjaro.[3][4]
How many people stopped
Feeling sick and stopping are very different numbers. In the pooled semaglutide trials, 4.3% stopped because of gut side effects, against 0.7% on placebo; 98.1% of gut side effects were rated mild or moderate.[5] For tirzepatide, 1.9%, 3.3% and 4.3% stopped for gut reasons at 5, 10 and 15 mg, against 0.5% on placebo.[2]
The serious warnings
Both labels carry a boxed warning about thyroid C-cell tumours seen in rodents. The warnings section also covers acute pancreatitis, gallbladder disease (gallstones 1.6% versus 0.7% on Wegovy), kidney injury from dehydration, severe gut reactions, serious allergic reactions, low blood sugar in people taking insulin or sulfonylureas, diabetic eye complications, a raised heart rate, and food staying in the stomach during anaesthesia.[1][2] After the drugs reached the market, reports also came in of bowel obstruction, ileus (a stalled bowel) and severe constipation with fecal impaction; reports like these cannot give a rate.[1][2]
- Severe stomach pain that does not go away, sometimes felt through to the back, with or without vomiting: the label lists this as a sign of pancreatitis.[1]
- Pain in the upper stomach, fever, yellow skin or eyes, clay-coloured stools: possible gallbladder problems.[1]
- Vomiting or diarrhea you cannot keep up with, little urine, dizziness on standing: dehydration, which can injure the kidneys.[1][2]
- Swelling of the face, lips, tongue or throat, trouble breathing or swallowing: a serious allergic reaction; emergency care.[1]
Each side effect, in depth
Each row in the table links to its own page, with when it starts, how long it usually lasts, what has been tested and what has not. The how long side effects last page puts the timelines side by side.
The short version
Short on the way. A 35-second captioned version of this page is in production for the Fewer Bites channel. It will be embedded here when it is live.
Questions to bring to your prescriber
Print these or screenshot them. They are conversation starters, not a checklist of things to do.
- Which of these side effects would you want me to call about, and which can wait for my next visit?
- Do any of my other medicines (blood pressure, diabetes, water pills) change my risk of dizziness, low blood sugar or dehydration?
- Do I have a history (gallstones, pancreatitis, kidney disease, eye disease) that changes how closely you want to watch me?
Education, not advice. This page explains what trials, labels and guidelines report. It does not tell anyone to start, change or stop a medicine. Your prescriber knows your history; this page does not.
Sources
- Novo Nordisk / FDA. Wegovy (semaglutide) injection and tablets, prescribing information, revised 06/2026. label
- Eli Lilly / FDA. Zepbound (tirzepatide) prescribing information, revised 08/2026. label
- Novo Nordisk / FDA. Ozempic (semaglutide) prescribing information, revised 05/2026. label
- Eli Lilly / FDA. Mounjaro (tirzepatide) prescribing information, revised 08/2026. label
- Wharton S et al. Gastrointestinal tolerability of once-weekly semaglutide 2.4 mg: post hoc analysis of STEP 1–3. Diabetes Obes Metab 2022;24:94–105. primary read · post hoc · industry-funded