Mounjaro Adverse Side Effects: What to Expect and When to Act (October 2026)

01 · The short version

What are the adverse side effects of Mounjaro?

Mounjaro causes side effects like any medicine, and most of them are digestive. ¹ The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹ Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts.¹³

02 · Your next step

Is this relevant to me, and what should I do next?

If you are thinking about starting treatment, the useful question is not whether you will have any side effect, but how likely a bothersome one is and how you would handle it. Digestive symptoms are the most common side effects, and they are most likely when starting or moving up a dose. A smaller group find them hard enough to stop treatment.

If you already take Mounjaro and have a new symptom, compare it with the severity levels in the table. Mild and settling means manage it at home and mention it at your next review. Persistent, worsening or alarming means contact your clinical team. Severe pain, signs of allergy or yellowing of the skin or eyes means get medical help straight away.

Treatment is supplied only after a free online consultation reviewed by a GPhC-registered prescriber, with ongoing monthly reviews and side-effect support from the prescribing team. For wider context on how we support weight loss treatment, see our weight loss clinic.

Related guides: Mounjaro reactii adverse and Mounjaro adverse events and Gastro side effects of mounjaro and Mounjaro psychological side effects.

03 · Common digestive reactions

Common digestive adverse reactions

Digestive symptoms are the adverse reactions you are most likely to meet, because Mounjaro slows how quickly the stomach empties as part of how it works.¹ The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹

In practice these symptoms tend to cluster around dose days. You may feel queasy after eating a large or fatty meal, or feel full far sooner than before. Burping, reflux and indigestion are common companions. None of this means the medicine is failing or harming you; it is the expected consequence of a slower-moving gut and a smaller appetite.

Constipation deserves a separate mention because it is easy to ignore until it is uncomfortable. Constipation is very common on Mounjaro (it may affect more than 1 in 10 people), because food moves through the gut more slowly and many people eat and drink less. Fluids come first, then fibre from vegetables and whole grains, then daily movement. If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.

At the other end, diarrhoea is just as common, and some people swing between the two. Keep drinking water either way, because dehydration is the real risk in both cases.

Practical steps that help

• Eat smaller meals slowly and stop at the first feeling of fullness.
• Favour plain, lower-fat foods while you adjust to a new dose.
• Stay well hydrated through the day.
• Avoid lying down straight after eating.

If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.

04 · Trial rates

How often do side effects happen in trials?

Nausea and diarrhoea are common in the weight-management trials, but most people continue treatment. In the trials supporting Mounjaro's weight-management licence, nausea was reported by 24.6% of people on 5mg, 29.0% on 10mg and 28.0% on 15mg, compared with 8.5% on placebo. Diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹

Importantly, nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ That matches what prescribers see: the early weeks are the hardest, and symptoms often calm once a dose has bedded in.

Stopping because of stomach effects was relatively uncommon. 1.9% of people on 5mg, 3.3% on 10mg and 4.3% on 15mg stopped treatment permanently because of gastrointestinal side effects, compared with 0.5% on placebo.¹ Across all types of side effect, in SURMOUNT-1, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo.²

Read these as trial averages, not personal forecasts. Your own experience depends on your dose, your eating pattern, your medical history and other medicines. The point is that a clear majority of people in the trials tolerated treatment well enough to carry on, and that the higher doses carry somewhat more stomach effects than the lower ones.

05 · Serious side effects

Severe side effects of Mounjaro: when to seek help

Serious side effects are uncommon but need urgent attention.¹ Seek immediate medical help for any of the following.

• Severe, persistent stomach pain that may spread to your back, with or without vomiting — get medical help straight away. This could be pancreatitis.
• Pain in the upper right abdomen, fever or yellowing of the skin or eyes — get medical help straight away. This could point to gallbladder problems.
• Swelling of the face or throat and difficulty breathing — get medical help straight away. These are signs of a severe allergic reaction.

Do not wait to see whether these settle, and do not book a routine appointment. Use urgent care or emergency services (999 or A&E), and tell the clinician you take tirzepatide.¹³

Low blood sugar is a separate concern for people who combine Mounjaro with insulin or a sulfonylurea. Watch for shakiness, sweating and confusion. Your prescriber may need to adjust those medicines, so tell them about everything you take. This is an adjustment, not a ban on combining them.¹

Alongside seeking care, you can report any suspected side effect through the MHRA Yellow Card scheme, which helps regulators monitor safety across the UK.¹⁹

06 · Other recognised effects

Hair shedding, tiredness, injection-site and face changes

Beyond the stomach, several recognised effects are bothersome rather than dangerous, and most settle by themselves.

Hair shedding

Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹ Temporary hair shedding is a recognised side effect, linked to rapid weight loss itself rather than a direct drug effect. For most people it settles as weight stabilises. Protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.

Fatigue and low energy

Fatigue is a recognised common side effect, most noticeable in the first weeks and after dose increases. It usually reflects a mix of the body adjusting and simply eating less. Keep meals balanced (protein especially), stay hydrated and keep gentle activity going. For most people energy returns to normal within a few weeks; tell your clinical team if tiredness is severe or not settling.

Injection-site reactions

Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹ Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days. Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.

Face, loose skin and muscle

"Ozempic face" is not a drug effect. It is what losing a significant amount of weight quickly can do to facial volume, on any method including diet or surgery, and the same applies to loose skin. The counter-measures protect lean mass: adequate protein, resistance exercise, and a loss pace guided by your prescriber rather than maximum speed. Skin commonly adapts gradually after weight stabilises.

07 · Mood and mental health

Mood and mental health reactions

Regulators have looked specifically at whether GLP-1 medicines are linked to low mood or suicidal thoughts. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link, and the MHRA keeps this under continuous review.¹⁷

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

That said, any big change in weight, appetite and routine can affect how you feel. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷ Anything you notice can also be reported through the Yellow Card scheme.¹⁹

08 · Timeline and dose changes

When do side effects start, peak and settle?

Most side effects follow a pattern: they appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts.¹³ This is one reason Mounjaro uses a gradual dose ladder across the 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg strengths.

If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up. Speak to your clinical team rather than pushing through severe symptoms. Holding a dose is a normal clinical decision, not a failure.

A useful habit is to note when symptoms start relative to your injection day, what you ate, and how long they lasted. That gives your prescriber something concrete at a monthly review, and the free GLPTrackr companion app can help you keep that record.

If symptoms are not easing after a dose has had time to settle, or they get worse rather than better, that is the moment to get in touch. Our aftercare includes ongoing monthly reviews and side-effect support from the prescribing team.

09 · Risk, who should be cautious, stopping

Is Mounjaro high risk, who should avoid it, and what happens if you stop?

Mounjaro is a prescription-only medicine, which reflects that it needs a clinical assessment, not that it is unusually dangerous. It is supplied only after an online consultation reviewed by a GPhC-registered prescriber, and it is used alongside a reduced-calorie diet and increased physical activity. Individual results vary.

Who should not take it is a decision for the prescriber, who reviews your medical history, your current medicines and your goals. Mounjaro should not be taken if you are allergic to tirzepatide or any of its ingredients, or during pregnancy or while trying to conceive. Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems. Always share your full medical history and medicine list during consultation. If a consultation shows treatment is unsuitable, the order is refunded automatically. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly.

If you stop, nothing needs doing while the medicine clears. Tirzepatide stays in the body for a while: after a final dose it takes roughly four to five weeks to clear the system, there is no way to speed this up, and nothing needs doing while it clears.¹ Side effects therefore fade gradually rather than overnight. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average. Stopping should always be planned with your prescriber.

Mounjaro is available on the NHS, but access is being phased in over several years.⁴

10 · Other medicines compared

How do other weight-loss medicines compare on side effects?

Other GLP-1 based weight-loss medicines also cause mostly digestive side effects. The details differ by medicine, so check each product's own patient leaflet instead of assuming they match.

Newer oral options are also arriving. Our guide to Foundayo side effects, nausea and common reactions covers its side effects, and Mounjaro's rates should not be read across to it or the reverse.

What matters for you is the profile of the medicine you are actually prescribed, how it was introduced, and how closely you are followed up.