How common are Mounjaro side effects?
Mounjaro is a dual GIP and GLP-1 receptor agonist made by Eli Lilly.[7] Its side effects are largely the ones you would expect from this class of medicine: predominantly gastrointestinal, mostly mild to moderate, dose-related, and most noticeable when the dose is first increased.[3] They are common — most people notice at least some — but for most they are a nuisance to manage rather than a reason to stop. The clearest picture comes from SURMOUNT-1, the 72-week trial in adults with obesity that supported the weight-management licence; the frequencies below are from it.
Common gastrointestinal side effects (with real frequencies)
| Side effect | 5 mg | 10 mg | 15 mg | Placebo |
|---|---|---|---|---|
| Nausea | 24.6% | 33.3% | 31.0% | 9.5% |
| Diarrhoea | 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% |
Nausea is the single most common effect, reported by roughly a quarter to a third of people depending on the dose, against about one in ten on placebo.[3] Diarrhoea and constipation are next, and vomiting is less frequent. Because these effects are dose-related, they tend to be at their strongest in the days after a dose increase and often ease again once the body has adjusted to the new level.
Other commonly reported effects
Beyond the four effects above, the UK product information lists other common effects including indigestion (dyspepsia), abdominal pain, decreased appetite, fatigue, injection-site reactions and hair loss.[1] Decreased appetite is, of course, part of how the medicine works rather than only a side effect. Injection-site reactions are usually mild; our guide on how to inject Mounjaro covers rotating sites to reduce them.
Mounjaro is started at a low 2.5 mg once-weekly dose for four weeks, then increased to 5 mg, and thereafter in 2.5 mg steps no more often than every four weeks, up to a maximum of 15 mg — as tolerated and directed by the prescriber.[5] This slow, stepwise titration exists specifically to reduce the dose-related gastrointestinal side effects above.[3] The full schedule is on our Mounjaro doses and titration page.
Even with these effects, most people who start Mounjaro keep taking it. In SURMOUNT-1, the share of participants who stopped because of side effects was 4.3% on 5 mg, 7.1% on 10 mg and 6.2% on 15 mg, compared with 2.6% on placebo.[3]
Serious risks to be aware of
Serious side effects are much less common than the gastrointestinal ones, but they are the reason Mounjaro needs a prescriber's oversight. The product information sets out several warnings and precautions.[2][4]
Acute pancreatitis
Inflammation of the pancreas (acute pancreatitis) has been reported with this class of medicine. The label advises that Mounjaro should be stopped if pancreatitis is suspected.[2] The classic warning sign is severe, persistent pain in the upper abdomen, which may spread to the back and may come with vomiting. This needs urgent medical attention.
Gallbladder problems
Acute gallbladder disease — including gallstones and inflammation of the gallbladder — is listed among the warnings.[2] Rapid weight loss of any kind can raise the risk of gallstones, so this is worth knowing about. Symptoms to report include pain in the upper-right abdomen, fever, or yellowing of the skin or eyes.
Low blood sugar (hypoglycaemia)
On its own, Mounjaro carries a low risk of hypoglycaemia. That risk rises when it is combined with insulin or a sulfonylurea (a type of diabetes tablet), and the label advises considering a lower dose of those medicines when Mounjaro is added.[2] This is mainly relevant to people taking Mounjaro alongside other treatments for type 2 diabetes, and it is one reason a prescriber needs a full picture of your current medicines.
Dehydration and kidney injury
Acute kidney injury has been reported, usually in the setting of dehydration caused by vomiting and diarrhoea.[2] So the gastrointestinal effects are not only uncomfortable — if they are severe enough to leave you dehydrated, they can affect the kidneys. Keeping up fluids matters, and prolonged or severe vomiting or diarrhoea should be reported rather than pushed through.
Other precautions
The product information also flags severe gastrointestinal disease, serious hypersensitivity (allergic) reactions, and — in people with a history of it — monitoring for diabetic retinopathy.[2]
The thyroid C-cell warning (rodent studies)
In two-year animal studies, tirzepatide caused thyroid C-cell tumours — both adenomas and carcinomas — in rats, in a way that increased with the dose and the length of exposure. Whether this happens in humans is not known.[2] Because of that uncertainty, the US prescribing information carries a prominent boxed warning about the risk, and lists a personal or family history of medullary thyroid carcinoma (MTC), or the genetic condition Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), as reasons the medicine must not be used.[2]
UK labelling does not use the US "boxed warning" format, so the presentation differs — but the underlying caution is the same, and a prescriber will ask about your and your family's thyroid history before considering Mounjaro. The key point to take away is that this specific finding comes from rodent studies and its relevance to people is unknown; it is not evidence that Mounjaro causes thyroid cancer in humans.
Who should not take Mounjaro?
If you have searched "can I take Mounjaro", the honest answer is that it depends on your medical history, and only a prescriber can decide. But the product information is clear about some situations where it must not be used, or used only with particular caution.
- A known serious allergy to tirzepatide or any of the ingredients — Mounjaro is contraindicated.[1]
- A personal or family history of medullary thyroid carcinoma (MTC), or MEN 2 — listed in the US prescribing information as a reason not to use it.[2]
- A history of pancreatitis, gallbladder disease or severe gastrointestinal disease — these need careful consideration and monitoring.[2]
- Using insulin or a sulfonylurea — because of the higher risk of hypoglycaemia, doses may need adjusting.[2]
There is also a contraception point that is easy to miss. According to the product information, Mounjaro is reported to reduce how well oral hormonal contraceptives (the combined pill) work, because it slows stomach emptying. Anyone using the pill is advised to add a barrier method — such as condoms — or switch to a non-oral contraceptive for a period after starting Mounjaro and after each dose increase.[1] Check the exact advice with your prescriber or pharmacist.
No medicine is risk-free, and Mounjaro is no exception. For most people the side effects are gastrointestinal and manageable, and the serious risks above are uncommon — but they are real, which is exactly why Mounjaro is a prescription-only medicine and not something to obtain or self-manage on your own. Whether it is safe for you is a judgement a prescriber makes with your full history in front of them.
Buying Mounjaro online: a safety note
Because Mounjaro is prescription-only, it cannot legally be sold in the UK without a prescription. If you are looking to buy Mounjaro online, the only lawful route is through a GPhC-registered pharmacy following a consultation with a prescriber — the same clinical checks described on this page apply whether you go through the NHS or a regulated private service. We do not name or recommend any provider.
Steer clear of any seller offering Mounjaro without a consultation, or "no prescription needed". Medicines bought outside the regulated supply chain may be falsified, wrongly dosed or unsafe, and you lose every safeguard this page describes. The UK's #FakeMeds campaign explains how to spot illegal sellers of weight-loss medicines.
If you experience side effects from Mounjaro or any medicine, you can report them through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, and speak to your GP or pharmacist. Seek urgent medical help for signs of a serious problem — for example severe, persistent stomach pain (with or without vomiting), which can be a sign of pancreatitis.[6]
Frequently asked questions
What are the most common Mounjaro side effects?
They are gastrointestinal and usually mild to moderate. In SURMOUNT-1, nausea affected 24.6% to 33.3% of people depending on the dose, diarrhoea 18.7% to 23.0%, constipation 11.7% to 17.1% and vomiting 8.3% to 12.2%, all higher than on placebo.[3] They are usually strongest just after a dose increase and often settle over time.
Is Mounjaro safe?
No medicine is risk-free. Most side effects are gastrointestinal and manageable, and in SURMOUNT-1 between 4.3% and 7.1% of people stopped because of side effects.[3] There are also rarer serious risks, including pancreatitis, gallbladder problems, low blood sugar with insulin or a sulfonylurea, and dehydration affecting the kidneys.[2] A prescriber decides whether it is safe for you.
Who should not take Mounjaro?
Anyone with a serious allergy to tirzepatide or its ingredients must not use it.[1] The US prescribing information also lists a personal or family history of medullary thyroid carcinoma (MTC), or MEN 2, as reasons not to use it.[2] People with a history of pancreatitis, gallbladder or severe gastrointestinal disease, or using insulin or a sulfonylurea, need extra caution.[2]
Do Mounjaro side effects go away?
For many people the common gastrointestinal effects ease as the body adjusts. Mounjaro is started at 2.5 mg for four weeks and increased slowly, no more often than every four weeks, because gradual increases reduce dose-related gastrointestinal side effects.[5][3] Persistent or severe symptoms should be discussed with a prescriber, who can slow or pause the increases.
How do I report a Mounjaro side effect in the UK?
Report suspected side effects from any medicine through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, and speak to your GP or pharmacist.[6] Seek urgent medical help for signs of a serious problem such as severe, persistent stomach pain.
For the wider picture, see our dosage guide, the page on how to inject Mounjaro, and the main Mounjaro FAQ.
References
- electronic medicines compendium (UK). "Mounjaro KwikPen — Summary of Product Characteristics." medicines.org.uk
- US Food and Drug Administration. "Mounjaro (tirzepatide) prescribing information, NDA 215866." accessdata.fda.gov
- Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). New England Journal of Medicine. nejm.org
- DailyMed (US National Library of Medicine). "Mounjaro — prescribing information." dailymed.nlm.nih.gov
- MHRA / GOV.UK. "MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss." gov.uk
- MHRA. "Yellow Card — report a side effect." yellowcard.mhra.gov.uk
- Eli Lilly and Company. "FDA approves Lilly's Mounjaro™ (tirzepatide) injection, the first and only GIP and GLP-1 receptor agonist." investor.lilly.com