Safety note
This guide is for common, mild to moderate side effects. It is not a substitute for advice from your own prescriber. If symptoms are severe, persistent or worrying, please seek medical advice.
Last reviewed: 13 July 2026
Side effects on Mounjaro are common, especially when starting treatment or increasing the dose. Many people notice the biggest change when moving from 2.5 mg to 5 mg because the dose doubles. That does not mean you have done anything wrong. It does mean your body may need a bit of support while it adjusts.
The Big Picture
Mounjaro can make appetite quieter and fullness stronger. That can help, but it can also mean your usual meals suddenly feel too much. Many side effects improve when people eat more slowly, reduce portion size, drink regularly, avoid very rich meals and avoid rushing dose increases.
The goal is not to eat as little as possible. The goal is to support your health while using the medication safely.
Nausea
- Eat smaller meals.
- Eat slowly and stop at comfortably full.
- Avoid very greasy, rich or large meals.
- Try bland, simple foods if you feel queasy.
- Sip fluids regularly rather than drinking large amounts at once.
- you are struggling to eat or drink
- nausea is not settling after a few days
- you are due to increase your dose but still feel unwell
Vomiting
Occasional vomiting can happen, especially around dose changes. Repeated vomiting is different because dehydration can become a problem.
- Take small sips of fluid often.
- Use oral rehydration drinks if you are losing fluid.
- Avoid alcohol and heavy meals.
- Do not increase your dose while vomiting is ongoing.
- you cannot keep fluids down
- you feel faint, confused or very weak
- you pass very little urine
- you have severe abdominal pain
Constipation
Constipation is very common in people using Mounjaro for weight management. It can happen because appetite is lower, portions are smaller, fluid intake drops, or fibre intake changes.
- Drink regularly through the day.
- Include fibre: oats, beans, lentils, fruit, vegetables, seeds or wholegrains.
- Increase fibre gradually to avoid bloating.
- Move your body gently each day if you can.
- Speak to a pharmacist about a suitable laxative if needed.
- constipation is painful or persistent
- you have vomiting or significant bloating
- you have blood in your poo
- your bowel habit changes and does not recover
Diarrhoea
Diarrhoea can happen while the gut adjusts. The main thing is to avoid dehydration and watch for symptoms that are not settling.
- Drink enough fluid.
- Use oral rehydration drinks if needed.
- Keep food simple while symptoms settle.
- Avoid alcohol and very rich foods.
- Ask a pharmacist before using anti-diarrhoea medicine if you are unsure.
- diarrhoea is severe or lasts more than a few days
- you have blood in your stool
- you feel dehydrated
- you also have severe abdominal pain or fever
Reflux, Bloating And Sulphur Burps
Mounjaro can slow stomach emptying. For some people this means reflux, bloating, burping or “sulphur burps”, especially after larger or richer meals.
- Eat smaller portions.
- Leave a gap between eating and lying down.
- Reduce greasy, spicy or very rich foods if they trigger symptoms.
- Try a short gentle walk after meals.
- Speak to a pharmacist about reflux treatment if needed.
- reflux is persistent or severe
- you have trouble swallowing
- you are vomiting repeatedly
- you have severe abdominal pain
Dry Mouth
Dry mouth can happen if you are drinking less, eating less, feeling sick or losing fluid through vomiting or diarrhoea.
- Sip water regularly.
- Try sugar-free gum or sugar-free mints.
- Limit alcohol.
- Look after teeth and gums, especially if your mouth feels dry often.
- you also feel dizzy, faint or dehydrated
- you are vomiting or have diarrhoea
- dry mouth is severe or persistent
Tiredness
Tiredness can happen for lots of reasons: eating much less, dehydration, poor sleep, rapid weight loss, low blood sugar in people on some diabetes medicines, or simply the body adjusting.
- Check you are eating enough nourishing food.
- Include protein regularly.
- Drink enough fluid.
- Keep movement gentle if you feel wiped out.
- Prioritise sleep while your body adjusts.
- tiredness is severe or sudden
- you feel faint, confused or shaky
- you are on insulin or sulphonylurea tablets
- you are unable to eat or drink properly
Hair Shedding
Hair shedding can feel really upsetting. It is not always caused directly by the medicine. It can happen after rapid weight loss, low protein, low iron, illness, stress or not eating enough overall.
- Do not aim to eat as little as possible.
- Include protein regularly.
- Include iron-rich foods where you can.
- Avoid crash-diet thinking while appetite is low.
- Ask about blood tests if shedding is significant or persistent.
- hair loss is patchy
- you have scalp symptoms
- you feel unwell, exhausted or cold
- you are worried about low iron, thyroid problems or poor intake
Before You Increase The Dose
If you still have troublesome side effects, it is worth asking before moving up. Higher is not always better if it means you cannot eat, drink, sleep or function properly.
Sometimes the safest plan is to stay where you are for longer, review your food and fluid intake, or discuss whether the medication is right for you.
Do Not Push Through These Symptoms
- severe, persistent abdominal pain, especially going through to the back
- repeated vomiting or inability to keep fluids down
- signs of dehydration such as passing very little urine, confusion, fainting or extreme weakness
- swelling of lips, tongue or throat, breathing difficulty or collapse
- severe right-sided upper tummy pain, fever, yellow skin/eyes or dark urine
Use NHS 111 for urgent advice. Call 999 if there is immediate danger, severe allergic reaction, collapse or severe breathing difficulty.
Prefer To Watch?
A Dr Mel video on managing Mounjaro side effects will sit here once the YouTube channel is live.
Key Messages
- Side effects are often worse when starting or increasing the dose.
- The 2.5 mg to 5 mg increase can feel like a big step for some people.
- Small meals, slower eating, fluids, protein, fibre and gentle movement can help many mild symptoms.
- Do not rush up the dose if you are already struggling.
- Severe pain, repeated vomiting, dehydration or allergic symptoms need urgent advice.
Useful Sources
Mounjaro KwikPen Patient Information Leaflet, emc
Last reviewed: 13 July 2026. Planned review: October 2026, or earlier if MHRA/NICE/product information changes.