Updated November 2026 · Based on the UK Mounjaro patient leaflet, the SURMOUNT-1 trial and MHRA safety advice
The short version: most side effects of Mounjaro (tirzepatide) affect the stomach and gut: nausea, diarrhoea, constipation and vomiting. In the main weight loss trial, about 1 in 4 to 1 in 3 people felt sick at some point, against about 1 in 10 on a placebo. Side effects are usually mild to moderate, they're most likely when you start and each time the dose goes up, and they tend to ease as your body gets used to each dose.
The most common side effects
The UK patient leaflet lists these as very common, meaning they can affect more than 1 in 10 people:
- nausea (feeling sick)
- diarrhoea
- vomiting
- constipation
- low blood sugar (hypoglycaemia), mainly in people with type 2 diabetes who also take insulin or a sulfonylurea
Common side effects (up to 1 in 10 people) include stomach pain, indigestion, bloating, burping, wind, acid reflux, reduced appetite, tiredness, dizziness, hair loss and reactions where you inject, such as redness or itching. Check the leaflet in your pack for the full list.
How often they happened in the main trial
SURMOUNT-1 followed 2,539 adults with obesity or overweight (without diabetes) for 72 weeks. People took 5mg, 10mg or 15mg of Mounjaro, or a placebo.
| Side effect | 5mg | 10mg | 15mg | 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% |
| Stopped because of side effects | 4.3% | 7.1% | 6.2% | 2.6% |
Most of these effects were mild or moderate and happened while the dose was being increased. Most people who got them carried on with treatment.
When side effects are worst (and when they ease)
Mounjaro is started at 2.5mg once a week for four weeks, then usually increased by 2.5mg every four weeks or more, up to a maximum of 15mg. The start-up dose of 2.5mg is there to help your body adjust; it isn't expected to cause much weight loss on its own.
For most people:
- Week 1 to 2 of each new dose is when nausea, fullness and stomach upset are most likely, often in the day or two after the injection.
- Weeks 3 to 4 are usually easier as your body adjusts.
- Each dose increase can bring symptoms back for a while.
If side effects are hard to manage, talk to your prescriber. They may keep you on the same dose for longer before going up, or lower the dose. You don't have to go up to 15mg if a lower dose is working.
Tips to manage common side effects
Nausea and feeling full
- Eat smaller meals and stop when you feel full; your appetite signals change on Mounjaro.
- Go easy on fatty, fried and very sugary foods, which often make nausea worse.
- Eat slowly and don't lie down straight after eating.
Diarrhoea and vomiting
- Sip water or oral rehydration drinks through the day. Losing fluid can, rarely, affect your kidneys.
- Seek advice if you can't keep fluids down.
Constipation
- Drink plenty of water, add fibre gradually and keep active.
- Speak to a pharmacist about a suitable laxative if it doesn't settle.
Protect your muscle
- Aim for protein at every meal and do some strength training. See what to eat on Mounjaro.
Alcohol
- Alcohol can make nausea and low blood sugar more likely. See can I drink alcohol on weight loss injections?.
Serious side effects: when to get help
These are uncommon or rare, but you should know the signs. Stop taking Mounjaro and get medical help straight away if you have:
- Severe, ongoing stomach pain that may spread to your back, with or without vomiting. This can be a sign of pancreatitis (inflamed pancreas).
- Pain in the upper right of your tummy, fever, or yellowing of your skin or eyes. These can be signs of gallstones or an inflamed gallbladder, which are more likely with fast weight loss.
- Signs of a serious allergic reaction, such as swelling of the face, lips or throat, difficulty breathing or a widespread rash.
- Severe or prolonged vomiting or diarrhoea with signs of dehydration, such as passing very little urine or feeling dizzy.
Call 999 for breathing problems or swelling of the face or throat. For other urgent concerns, call NHS 111 or your prescriber.
Tell your doctor before an operation
Mounjaro slows how quickly your stomach empties. The MHRA has warned of a possible risk of food or fluid getting into the lungs (pulmonary aspiration) during a general anaesthetic or deep sedation. Always tell the doctor, dentist or anaesthetist that you're taking Mounjaro before any procedure.
Mounjaro and contraception
This is the side effect many people don't hear about. Mounjaro can make the contraceptive pill less effective, particularly when you start treatment and after each dose increase, because it slows down how your stomach empties.
The MHRA advises that if you're overweight or obese and take an oral contraceptive, you should also use a barrier method (such as condoms) or switch to a non-oral method (such as a coil or implant) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.
You shouldn't take Mounjaro if you're pregnant, trying to get pregnant or breastfeeding. If you're planning a pregnancy, stop Mounjaro at least 1 month before trying to conceive. (For Wegovy, the advice is at least 2 months.)
Who shouldn't take Mounjaro
Your prescriber will check your medical history, but Mounjaro may not be suitable if you have had pancreatitis, have severe stomach or gut problems (such as gastroparesis), have diabetic eye disease, or are pregnant or breastfeeding. Be honest in your consultation, including about other medicines you take.
Reporting side effects
You can report suspected side effects to the MHRA through the Yellow Card scheme. Reports help the MHRA spot new safety issues.
Does the cheaper Mounjaro cause more side effects?
No. Every UK provider supplies the same Eli Lilly KwikPen, so side effects don't depend on where you buy it. What can differ is how quickly your prescriber reviews you and how easy it is to pause or adjust your dose. Prices currently start at £139.99 a month at the 2.5mg dose, at providers such as GenMeds and IQ Doctor. Avoid Mounjaro sold without a prescription or through social media, which may be fake.
Compare Mounjaro prices at every dose
Keep reading
- Cheapest Mounjaro UK: every dose priced
- Mounjaro on the NHS in 2026: who qualifies
- Coming off Mounjaro or Wegovy: what happens
- Wegovy side effects: the complete UK guide
- Mounjaro and Wegovy: what to expect
Frequently asked questions
What are the most common side effects of Mounjaro?
Nausea, diarrhoea, vomiting and constipation. In the main trial, 25% to 33% of people on Mounjaro had nausea, against about 10% on a placebo. Most cases were mild to moderate.
How long do Mounjaro side effects last?
They're usually worst in the first week or two of each new dose and ease as your body adjusts. If they don't settle, your prescriber may keep you on the same dose for longer.
Do Mounjaro side effects get worse at higher doses?
Nausea, diarrhoea and vomiting were more common at 10mg and 15mg than at 5mg in the trial, and they're most likely just after a dose increase.
Does Mounjaro stop the pill working?
It can make the pill less reliable. The MHRA advises using condoms or a non-oral method for 4 weeks after starting and 4 weeks after each dose increase if you're overweight or obese and take an oral contraceptive.
When should I worry about Mounjaro stomach pain?
Get urgent help for severe stomach pain that doesn't go away, especially if it spreads to your back or comes with vomiting, as it can be a sign of pancreatitis.
Can I drink alcohol on Mounjaro?
Small amounts are not forbidden, but alcohol can worsen nausea and dehydration. See our alcohol guide.
This article is for information only and is not medical advice. CompareTheJab is an independent comparison website, not a pharmacy or medical service. Always speak to a qualified prescriber before starting, stopping or switching any medicine.
Prices were checked on each provider's own website on 3 November 2026 and change often. Always confirm the price at checkout. Some links on this page are affiliate links, which never affect how we rank providers. Read our affiliate policy and how we compare providers.
