Short answer: tirzepatide’s side effects are among the best-measured of any compound on this site, because it is an approved drug (Mounjaro and Zepbound) tested in tens of thousands of people. In SURMOUNT-1, the main obesity trial, nausea affected 25–33% of people vs 9.5% on placebo, diarrhea 19–23%, constipation 12–17% and vomiting 8–12%. Most of it happened while the dose was being increased. Fewer than 1 in 14 stopped because of side effects. The rarer warnings matter too: pancreatitis, gallbladder problems, and a US boxed warning about thyroid tumours seen in rodents.
Side effects in SURMOUNT-1, by dose
2,539 adults with obesity or overweight (without diabetes), 72 weeks. Average weight loss was 16.0%, 21.4% and 22.5% at 5, 10 and 15 mg, vs 2.4% on placebo (efficacy estimand).
| Side effect | 5 mg | 10 mg | 15 mg | Placebo |
|---|---|---|---|---|
| Nausea | 24.6% | 33.3% | 31.0% | 9.5% |
| Diarrhea | 18.7% | 21.2% | 23.0% | 7.3% |
| Constipation | 16.8% | 17.1% | 11.7% | 5.8% |
| Vomiting | 8.3% | 10.7% | 12.2% | 1.7% |
| Stopped due to side effects | 4.3% | 7.1% | 6.2% | 2.6% |
When side effects happen
Gastrointestinal effects cluster during dose escalation and usually fade over weeks. That is why the approved schedule starts at 2.5 mg and increases gradually. Slowing the stomach is part of how the drug reduces appetite, so some fullness and nausea come with the mechanism.
The serious warnings
- Thyroid C-cell tumours: US labels carry a boxed warning because these tumours occurred in rodents. Human relevance is unknown, but it is contraindicated in people with a personal or family history of medullary thyroid cancer or MEN 2.
- Pancreatitis: reported; severe upper abdominal pain needs urgent care.
- Gallbladder disease: more common with rapid weight loss.
- Low blood sugar: mainly when combined with insulin or sulfonylureas.
- Dehydration and kidney strain from vomiting or diarrhea.
Prescription vs research tirzepatide
Important difference: the numbers above come from pharmaceutical-grade tirzepatide in pens, prescribed and monitored. Research-grade powder is a different product; its purity, dose accuracy and sterility depend on the manufacturer. In Canada, tirzepatide is available by prescription as Mounjaro; tirzepatide sold online is an unauthorized drug.
See TrueLabs research tirzepatide and its COA →
Frequently asked questions
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What are the most common side effects of tirzepatide?
In SURMOUNT-1: nausea (25–33% vs 9.5% on placebo), diarrhea (19–23%), constipation (12–17%) and vomiting (8–12%), mostly during dose increases.
How many people stop tirzepatide because of side effects?
In SURMOUNT-1, 4.3% to 7.1% stopped due to adverse events depending on dose, vs 2.6% on placebo.
Does tirzepatide cause thyroid cancer?
Thyroid C-cell tumours occurred in rodents, so US labels carry a boxed warning. Whether this applies to humans is unknown, and it is contraindicated in people with a history of medullary thyroid cancer or MEN 2.
Do tirzepatide side effects go away?
Gastrointestinal side effects are most common during dose escalation and usually decrease over time.
Is tirzepatide approved in Canada?
Yes, by prescription as Mounjaro. Research-grade tirzepatide sold online is not an authorized product.
Related reading
- Tirzepatide explained: what SURMOUNT and SURPASS showed
- Retatrutide vs tirzepatide vs semaglutide
- Cagrilintide and CagriSema
References
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022;387:205–216. DOI
- Eli Lilly. SURMOUNT-1 results release. Lilly
All TrueLabs products are sold for in vitro and laboratory research use only. They are not intended for human or veterinary use, and nothing in this article is medical advice. For weight management, talk to a doctor about approved options.