Tirzepatide's side effects are gastrointestinal, concentrated during dose escalation, and dose-dependent. But if you look the numbers up you will find two different sets for the same molecule: nausea at 18% in one label and 28% in the other, at the identical 15 mg dose. Both are correct. The difference is not the drug, and understanding why is the most useful thing on this page — because it tells you which number applies to you.
The same molecule, two labels
Tirzepatide is sold as Mounjaro for type 2 diabetes and Zepbound for weight management. Identical molecule, identical doses, separate trial programmes, separate labels. Here is what each reports at the maximum dose, against its own placebo group.
| Adverse reaction | Mounjaro 15 mg (type 2 diabetes) |
Zepbound 15 mg (obesity) |
|---|---|---|
| Nausea | 18% | 28% |
| Diarrhoea | 17% | 23% |
| Vomiting | 9% | 13% |
| Constipation | 7% | 11% |
| Abdominal pain | 5% | 10% |
| Dyspepsia | 5% | 10% |
| Placebo, nausea | 4% | 8% |
Look at the last row. Placebo nausea doubles between the two programmes as well — 4% against 8% — and placebo contains no tirzepatide at all. Whatever inflates the Zepbound column inflates the sugar-water column with it.
What differs is who was in the trials and what was asked of them. Mounjaro's pool was adults with type 2 diabetes; Zepbound's was adults with obesity, most without diabetes, on a reduced-calorie diet. Different populations report symptoms at different rates, and obesity trials collect adverse events more aggressively. The useful reading is not "Zepbound is harsher" but "these are two measurements of the same drug taken with different instruments."
Which column applies to you depends on which trial population you resemble, not which brand is on the pen.
What only the Zepbound label reports
The obesity trials recorded reactions the diabetes trials did not surface at the 5% threshold, so these appear in one label and not the other. They are not new risks of the weight brand; they are the same molecule observed more closely.
- Injection site reactions — 8% at 15 mg against 2% on placebo
- Fatigue — 7% against 3%
- Hair loss — 5% against 1%
- Gastro-oesophageal reflux — 5% against 2%
- Hypersensitivity reactions — 5% against 3%
- Dizziness — 4% against 2%
- Hypotension — 2% against 0%
Hair loss is the one people are usually surprised to find on a label at all. At 5% against 1% on placebo it is real but uncommon, and it is a recognised consequence of rapid weight loss rather than something specific to this molecule.
How many people actually stop
Incidence tells you how many noticed something. Discontinuation tells you how many could not live with it, which is the more decision-relevant number.
On Mounjaro, gastrointestinal reactions caused 3.0% of patients at 5 mg, 5.4% at 10 mg and 6.6% at 15 mg to stop, against 0.4% on placebo. Overall gastrointestinal adverse reactions ran 37.1%, 39.6% and 43.6% by dose, against 20.4% on placebo.
On Zepbound, severe gastrointestinal reactions were 1.7%, 2.5% and 3.1% by dose against 1% on placebo. In the trial with a lifestyle lead-in, 10% of patients stopped for any adverse reaction against 2% on placebo.
For context on how that compares with the alternative molecule: in SURMOUNT-5, the head-to-head against semaglutide, 6.1% of tirzepatide patients discontinued for adverse events against 8.0% on semaglutide, and for gastrointestinal reasons specifically 2.7% against 5.6%. The stronger drug was the better tolerated one. That comparison, and what it means for choosing, is in the best GLP-1 for weight loss.
Compounded tirzepatide is a third case
Neither table above describes compounded tirzepatide, and a good share of the people looking up this molecule are on exactly that — a compounded preparation from a telehealth clinic, with no brand name to search.
The molecule is the same, so the profile should be the same. What is not the same is everything around it. Compounded products are not FDA-approved, are not manufactured to the same standardisation, and are frequently dosed in units or fractions of a vial rather than on the label's escalation schedule. The dose-response above is the clearest thing in both labels — every reaction rises step by step with dose — so a preparation whose delivered dose is less certain has a correspondingly less predictable side effect profile. Faster-than-label escalation is the specific risk, because the labels are unanimous that reactions cluster during escalation.
None of that makes compounded tirzepatide unusable, and cost pushes many people toward it. It does mean the percentages on this page are the best available estimate rather than a measurement of what you are taking. How the compounded market works, and what to check, is in compounded GLP-1 online.
The serious risks, which are the same for both
These come from the warnings section rather than the incidence tables, and they do not differ by brand because they do not differ by molecule.
- Thyroid C-cell tumours — a boxed warning on both, from rodent studies. Contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2.
- Acute pancreatitis — including fatal cases with GLP-1 receptor agonists. Persistent severe abdominal pain, sometimes radiating to the back, means stop and seek care.
- Gallbladder disease — in the Zepbound trials, cholelithiasis 1.1% against 1% on placebo, cholecystitis 0.7% against 0.2%, cholecystectomy 0.2% against none. The label ties these to the weight loss itself. More in GLP-1 and the gallbladder.
- Acute kidney injury from dehydration — reported postmarketing, mostly in people who became dehydrated through vomiting or diarrhoea. This is the mechanism by which a tolerable side effect becomes a serious one.
- Hypoglycaemia — low risk from tirzepatide alone, materially higher combined with insulin or a sulfonylurea, which is why it matters more on the diabetes side.
- Severe gastroparesis — tirzepatide is not recommended.
When they happen, and what helps
Both labels say the same thing about timing: the majority of nausea, vomiting and diarrhoea occurs during dose escalation and decreases afterwards. That is the single most useful fact for anyone starting, because it means the worst weeks are usually the early ones rather than a permanent state.
It also means the lever that works is the escalation schedule. Every rate above climbs with dose, so holding a dose longer before stepping up is the standard response to poor tolerance — not pushing through. The week-by-week pattern is mapped in the GLP-1 side effects timeline, and the practical measures in managing nausea on GLP-1.
For the brand-specific detail, including dose-by-dose breakdowns, see Mounjaro side effects and Zepbound side effects. Why the molecule does what it does is in how tirzepatide works.
Scientific References
3 sources- 1
Eli Lilly and Company
MOUNJARO (tirzepatide) injection — Prescribing Information, §5 Warnings and Precautions, §6.1 Table 1
DailyMed, U.S. National Library of Medicine · 2026
NIH - 2
Eli Lilly and Company
ZEPBOUND (tirzepatide) injection — Prescribing Information, §5.2 and §5.4, §6.1 adverse reaction table
DailyMed, U.S. National Library of Medicine · 2026
NIH - 3
Aronne LJ, Horn DB, le Roux CW, et al.
Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5)
New England Journal of Medicine · 393(1) · 2025PMID: 40353578
NEJM
References open in a new tab. Content is reviewed against peer-reviewed literature as part of our editorial policy.
About the author
Editor, Modern Weight Science
Claudiu Gheorghe is the editor of Modern Weight Science. He is not a physician. His role is to synthesize peer-reviewed studies, clinical-trial data, and FDA prescribing information into clear, plain-language explanations, and to make sure every factual claim on the site traces back to a cited source. Any decision about starting, changing, or stopping a medication belongs with a licensed clinician who knows your history.
Every claim is checked against peer-reviewed research through our review process and fact-checking policy.
Frequently Asked Questions
What are the most common side effects of tirzepatide?
Gastrointestinal: nausea, diarrhoea, vomiting, constipation, abdominal pain and dyspepsia. At the 15 mg dose the Mounjaro label reports nausea in 18% of patients and the Zepbound label 28% — same molecule, same dose, different trial populations. All of them rise with dose and most occur during escalation rather than at a steady maintenance dose.
Why do Mounjaro and Zepbound list different side effect rates if they are the same drug?
Because they were tested in different people. Mounjaro's trials enrolled adults with type 2 diabetes; Zepbound's enrolled adults with obesity, mostly without diabetes, on a reduced-calorie diet. The clearest evidence that this is the explanation rather than a difference in the drug is the placebo groups: placebo nausea is 4% in the Mounjaro pool and 8% in the Zepbound pool, and placebo contains no tirzepatide at all.
Does tirzepatide cause hair loss?
It appears in the Zepbound label at 5% against 1% on placebo, so it is real but uncommon, and it does not appear in the Mounjaro table at all. Hair shedding is a recognised consequence of rapid weight loss generally rather than something specific to this molecule, which fits it showing up in the obesity trials and not the diabetes ones.
How many people stop taking tirzepatide because of side effects?
On Mounjaro, gastrointestinal reactions caused 3.0% of patients at 5 mg, 5.4% at 10 mg and 6.6% at 15 mg to discontinue, against 0.4% on placebo. On Zepbound, 10% stopped for any adverse reaction in the trial with a lifestyle lead-in, against 2% on placebo. In the head-to-head against semaglutide, tirzepatide was the better tolerated of the two at 6.1% against 8.0%.
How long do tirzepatide side effects last?
Both labels state that the majority of nausea, vomiting and diarrhoea occurs during dose escalation and decreases with continued treatment. Since every reported rate climbs with dose, the usual response to poor tolerance is holding the current dose longer before stepping up rather than pushing through — a decision for your prescriber.
Are compounded tirzepatide side effects the same?
The molecule is the same, so the profile should be. The predictability is not. Compounded preparations are not FDA-approved, are not standardised the same way, and are often dosed in units or fractions of a vial rather than on the label's escalation schedule. Because every side effect rate in both labels rises step by step with dose, a less certain delivered dose means a less certain side effect profile, and escalating faster than the label is the specific risk.
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Not medical advice. This guide is for general education only. GLP-1 medications, dosing, and treatment suitability are decisions for you and a licensed clinician who knows your full medical history.