Side Effects
Mounjaro side effects. Honestly explained.
We believe in being completely upfront. Here is exactly what to expect, how common each side effect is, and when to seek help.
The side effects you're most likely to notice.
Most side effects are gastrointestinal, tend to be worse in the first few weeks after starting or increasing your dose, and usually settle as your body adjusts.
| Likelihood | Side effect |
|---|---|
| Very common | Nausea Affects around 1 in 4 to 1 in 3 patients, particularly after starting or increasing dose. Usually mild to moderate and improves over the first few weeks. |
| Common | Diarrhoea Affects around 1 in 5 to 1 in 6 patients. Usually manageable with dietary adjustments. |
| Common | Constipation Affects around 1 in 6 to 1 in 7 patients. Often improves with adequate fluid and fibre intake. |
| Common | Vomiting Affects around 1 in 8 to 1 in 10 patients, most often around dose increases. |
| Common | Decreased appetite This is part of how the medication works, but can occasionally feel more pronounced than expected. |
| Common | Indigestion and abdominal discomfort Bloating, fullness or mild abdominal pain - usually related to eating larger or richer meals than your body can currently process comfortably. |
| Uncommon | Fatigue Some patients report feeling more tired than usual, particularly during the first few weeks. |
| Uncommon | Hair thinning Linked more to rapid weight loss itself than the medication directly - usually temporary and resolves once weight stabilises. |
| Uncommon | Injection site reactions Mild redness, itching or bruising at the injection site. Usually settles within a day or two. |
| Uncommon | Gallbladder problems Rapid weight loss of any kind, including with Mounjaro, can increase the risk of gallstones. |
| Rare but serious | Pancreatitis Severe, persistent abdominal pain that may spread to the back. Requires immediate medical attention. |
| Rare but serious | Severe allergic reaction Swelling of the face, lips, tongue or throat, difficulty breathing, or a severe rash. Requires emergency treatment. |
| Rare but serious | Gallbladder attack Sudden, severe pain in the upper right abdomen, sometimes with fever or jaundice. |
Signs that need urgent attention.
Most side effects are mild and manageable. But a small number of symptoms need immediate medical attention - please don't wait for your next scheduled appointment if you experience any of the following.
Seek emergency care immediately if you experience:
- Severe, persistent abdominal pain - particularly if it spreads to your back
- Swelling of your face, lips, tongue or throat
- Difficulty breathing or swallowing
- A severe or rapidly spreading skin rash
- Yellowing of your skin or eyes (jaundice)
- Signs of severe dehydration - dizziness, confusion, or being unable to keep fluids down
- Vision changes, particularly if you have diabetic retinopathy
Practical ways to feel better.
Small adjustments make a meaningful difference for most patients, particularly in the first few weeks of treatment or after a dose increase.
Eat smaller, more frequent meals
Large meals can worsen nausea and bloating. Smaller portions spread across the day are usually much better tolerated.
Avoid rich, fatty or fried food
These are harder to digest and tend to trigger nausea, indigestion and gallbladder discomfort more than lighter meals.
Stay well hydrated
Especially important if you experience diarrhoea or vomiting, to avoid dehydration.
Eat slowly and stop when full
Your body's signals change on Mounjaro - eating too quickly or past the point of fullness is a common trigger for discomfort.
Increase fibre gradually
Helpful for constipation, but introduce it slowly alongside plenty of water to avoid worsening bloating.
Talk to us before stopping
If side effects are difficult to manage, message your clinical team. We may be able to adjust your dose pace rather than stopping treatment altogether.
Experiencing a side effect?
Message your clinical team directly. We respond quickly during clinic hours and can advise on next steps - including whether you need to be seen sooner.