Mounjaro and Wegovy side effects: the safety guide.
What the official UK patient leaflets and the MHRA actually say: what is common, what is serious, who should not take these medicines, and the point at which you stop reading and get medical help.
The most common effects of Mounjaro and Wegovy are gastrointestinal: nausea, diarrhoea, vomiting and constipation, usually worst in the first weeks and after each dose increase. The effects that need urgent medical attention are rarer, and the most important is severe, persistent stomach pain that spreads to the back, which can indicate pancreatitis. Tirzepatide can also reduce the effectiveness of the contraceptive pill. These are prescription-only medicines and this page is information, not medical advice.
When to stop and get help.
Most side effects are unpleasant rather than dangerous. A small number are not. This is the part worth knowing before anything else.
Seek urgent medical help if you have
- Severe, persistent stomach pain, particularly if it spreads through to your back and comes with vomiting. This can be a sign of pancreatitis. Stop taking the medicine and get assessed the same day.
- Swelling of the face, lips, tongue or throat, or difficulty breathing. This can be a serious allergic reaction. Call 999.
- Prolonged vomiting or diarrhoea that leaves you unable to keep fluids down. Severe dehydration can injure the kidneys.
- Severe pain in the upper right abdomen, fever, or yellowing of the skin or eyes, which can indicate a gallbladder problem.
- Signs of very low blood sugar such as shaking, sweating, confusion or difficulty concentrating. The risk is higher if you also take insulin or a sulfonylurea.
- Sudden changes in your vision, particularly if you have diabetes.
If you are seriously unwell, do not wait for a pharmacy or an online service to reply. Contact NHS 111, your GP, or 999 in an emergency.
What the leaflets list as common.
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work by slowing stomach emptying and acting on appetite signalling, which is why most reported effects involve the digestive system. The official patient information leaflet supplied with your pen is the definitive list. This is a summary of what it describes.
| Effect | What the leaflets and NHS guidance describe |
|---|---|
| Nausea and vomiting | Among the most frequently reported. Typically strongest in the first weeks and for a few days after each dose increase, then eases for most people. |
| Diarrhoea | Common, particularly early. Persistent diarrhoea that stops you keeping fluids down needs medical attention. |
| Constipation | Common, and can persist longer than the nausea. Fluid and fibre intake are usually the first things a clinician will ask about. |
| Burping, including sulphur-smelling burps | Frequently reported by patients and linked to slowed digestion. Unpleasant but not dangerous in itself. |
| Fatigue | Listed in the leaflets. Can also reflect eating substantially less than before. |
| Indigestion, reflux, bloating | Common, related to delayed gastric emptying. |
| Injection-site reactions | Redness, itching or discomfort where you inject. Rotating the site is standard advice. |
| Dizziness | Listed. Can relate to reduced food and fluid intake. |
| Hair thinning | Reported by some. Generally linked to rapid weight loss rather than the medicine directly, and usually temporary. |
The leaflet for your specific medicine, with the full frequency categories, is published on the electronic medicines compendium. It is the source that matters, not a forum thread or a comparison site, including this one.
The interaction most people miss: the contraceptive pill.
This one is worth its own section because it is genuinely under-discussed and the consequence is significant.
If you take the combined pill or the progestogen-only pill and you are starting or increasing a dose of Mounjaro, raise contraception with your prescriber before you start. A legitimate prescriber should ask about this; if nobody has mentioned it to you, ask.
Other interactions worth telling your prescriber about:
- Insulin or sulfonylureas. Combining these with a GLP-1 medicine raises the risk of low blood sugar, and doses may need review by whoever manages your diabetes.
- Any oral medicine where timing matters. Slower stomach emptying can change how quickly other tablets are absorbed.
- Planned surgery, sedation or an endoscopy. Tell the anaesthetic team you take this medicine. Because the stomach empties more slowly, there is a recognised risk of aspiration under sedation, and they may want to adjust fasting instructions.
Who these medicines are not for.
Whether treatment is appropriate is a clinical decision, and a proper consultation exists precisely to catch these. The leaflets and prescribing information identify, among others:
- Pregnancy, trying to conceive, and breastfeeding. These medicines are not for use in pregnancy. If you become pregnant while taking one, contact your prescriber promptly.
- A personal or family history of medullary thyroid carcinoma, or MEN2. This is a contraindication, based on findings in animal studies.
- A history of pancreatitis. Requires careful clinical judgement.
- Severe gastrointestinal disease, including gastroparesis.
- Type 1 diabetes, for which these medicines are not licensed as a substitute for insulin.
A pharmacy that will supply these medicines without an assessment that asks about the above is not one you should be buying from. Every provider in our directory is verified on the GPhC register, and a real prescriber review is the minimum standard. How to spot a site that skips it →
Living with the common effects.
General guidance from the NHS and the patient leaflets, none of which replaces advice from your own prescriber:
- Eat smaller portions, more slowly. Large meals sit heavily when the stomach empties slowly.
- Go easy on fatty, fried and very rich food, which many people find worsens nausea.
- Keep fluids up, particularly if you have had vomiting or diarrhoea.
- Protein and resistance exercise matter. Weight lost on these medicines includes muscle as well as fat. Discuss protein intake and activity with a clinician or dietitian.
- Expect a dip after each dose increase. Symptoms commonly return briefly when the dose steps up.
Reporting a side effect.
If you experience a side effect, report it through the MHRA Yellow Card scheme, and tell your prescriber or pharmacist.
Anyone can submit a Yellow Card, including patients and carers, and you do not need to be sure the medicine caused it. These medicines are relatively new in widespread weight-management use, and the Yellow Card scheme is how the MHRA detects safety signals that clinical trials were too small or too short to show. It takes a few minutes and it genuinely matters.
If you believe a medicine you were sold is falsified or came from an unregistered seller, report that to the MHRA too. A counterfeit pen carries risks no leaflet covers: unknown contents, unknown dose, no cold chain. Spotting fake weight-loss sites →
Where to read the primary sources.
We would rather you checked the originals than took our summary for it:
- electronic medicines compendium, the official patient information leaflet for every licensed UK medicine.
- MHRA Yellow Card, for reporting and for published safety updates.
- NICE TA1026 for tirzepatide and NICE TA875 for semaglutide, the NHS evidence appraisals.
- NHS obesity guidance, for treatment options in context.
Related reading.
Side effects, answered.
What are the most common Mounjaro side effects?
The official patient information leaflet lists nausea, diarrhoea, vomiting and constipation as the most frequently reported effects. They are usually worst in the first weeks and after each dose increase, and often settle. Fatigue, dizziness, indigestion, burping and injection-site reactions are also listed. Always read the leaflet supplied with your medicine and speak to your prescriber about anything that concerns you.
When are Mounjaro side effects an emergency?
Severe, persistent stomach pain, especially if it spreads to your back and comes with vomiting, needs urgent medical assessment because it can indicate pancreatitis. Swelling of the face, lips or throat or difficulty breathing needs 999. Prolonged vomiting or diarrhoea causing severe dehydration also needs urgent help. Stop the medicine and seek medical advice rather than waiting to see if it passes.
Does Mounjaro stop the contraceptive pill working?
The MHRA advises that tirzepatide, the active ingredient in Mounjaro, may reduce the effectiveness of oral contraceptives. Its guidance is to use a non-oral contraceptive method, or add a barrier method, for four weeks after starting treatment and for four weeks after each dose increase. This is one of the most important and least known interactions, so discuss contraception with your prescriber before starting.
Can you drink alcohol on Mounjaro or Wegovy?
There is no absolute ban, but alcohol can worsen nausea and stomach upset, adds calories, and can increase the risk of low blood sugar, particularly for people also taking insulin or a sulfonylurea. Many people find they tolerate alcohol poorly on these medicines. Discuss it with your prescriber in the context of your own health.
Does Mounjaro cause hair loss?
Hair thinning is reported by some people. It is generally associated with rapid weight loss rather than being a direct effect of the medicine, a pattern called telogen effluvium, and it is usually temporary. Adequate protein and nutrition matter during weight loss. If hair loss is significant or persistent, raise it with your prescriber or GP, as other causes such as iron or thyroid problems can be checked.
Who should not take Mounjaro or Wegovy?
The leaflets state these medicines are not for use in pregnancy, when trying to conceive, or while breastfeeding. They are contraindicated for people with a personal or family history of medullary thyroid carcinoma or MEN2. Caution applies with a history of pancreatitis, severe gastrointestinal disease including gastroparesis, and in type 1 diabetes. Only a qualified prescriber can decide whether treatment is appropriate for you.
How do I report a side effect in the UK?
Report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, or via the Yellow Card app. Anyone can report, including patients and carers, and you do not need to be certain the medicine caused the problem. Reporting is how safety signals for newer medicines are detected. Tell your prescriber or pharmacist as well.
Do I need to tell a surgeon or anaesthetist I take these medicines?
Yes. These medicines slow stomach emptying, which can mean food remains in the stomach longer than expected and raises the risk of aspiration during sedation or general anaesthesia. Tell the anaesthetic or surgical team well before any planned procedure, and tell the clinician if you need emergency treatment.
About this page.
Pharmacy Score is not a pharmacy, a prescriber or a clinical service, and this page is information, not medical advice. It summarises what the official UK patient information leaflets, the MHRA and NHS guidance say, so you know what to look for and when to seek help. It cannot account for your own health, your other medicines or your history.
Mounjaro, Wegovy, the Wegovy pill and Foundayo are prescription-only medicines. Whether one is right for you, at what dose, and whether to continue it, are decisions for a qualified prescriber who has assessed you. If something feels wrong, contact them, NHS 111, or 999 in an emergency. Full medical disclaimer → · Editorial standards →
The latest regulator data, cited to source.
Yellow Card numbers change every month and are quoted loosely all over the web. These are the figures as published, by whom, and when, with links to the original documents.
The product information for every GLP-1 medicine (dulaglutide, exenatide, liraglutide, semaglutide and tirzepatide) was updated to warn of rare necrotising and fatal pancreatitis. An estimated 25.4 million packs were dispensed in the UK over the previous five years, so the risk is small. Patients are told to seek urgent help for severe, persistent stomach pain that may spread to the back, and never to restart the medicine if pancreatitis is confirmed. The update also reminds clinicians that privately prescribed GLP-1s may not appear in NHS records, so they should ask.
The MHRA's Chief Safety Officer described the risk of severe side effects as "very small", while stressing that patients and clinicians need to know the symptoms.
The Royal Pharmaceutical Society's journal analysed Yellow Card data and found that most reports for tirzepatide and semaglutide are non-serious. Its count of acute and chronic pancreatitis reports: 1,176 for tirzepatide (January 2023 to June 2026, 18 fatal) and 383 for semaglutide (January 2019 to June 2026, 5 fatal). Yellow Card reports are suspected reactions, not confirmed causes, and reporting rises with use.
Every figure above is quoted from the named publisher, with the responsible official credited where they put their name to the statement. If a source updates its figures, we update ours and log the change on the corrections page.