What are the most common side effects of Mounjaro and Wegovy?
Stomach and bowel symptoms are by far the most common side effects of both medicines. The Wegovy SmPC names nausea, diarrhoea, constipation and vomiting as the most frequently reported reactions in its trials, and Mounjaro's SmPC describes the same gastrointestinal effects as the most common.
These effects tend to be most noticeable in the days after an injection and after each dose increase. If you have vomiting or diarrhoea, keep drinking fluids, and tell your prescriber if you cannot keep fluids down, because dehydration can make you unwell quickly.
These effects tend to be most noticeable in the days after an injection and after each dose increase. Practical advice on eating during treatment is in our guide on eating well on weight loss injections.
If a side effect is bothering you, tell your prescriber rather than stopping or changing the dose yourself.
Are side effects different at higher Wegovy doses?
Higher doses can bring different side effects. In the trial of Wegovy 7.2 mg, the SmPC reports dysaesthesia (abnormal skin sensations) in 21.6% of people on the 7.2 mg dose, compared with 0.3% on placebo. Hair loss was reported by 5.3% on 7.2 mg compared with 1.0% on placebo. These figures come from a search extract of the SmPC, so check the full table before relying on them.
The higher dose is only licensed for adults with obesity and is started only after a minimum of 4 weeks on 2.4 mg. Your prescriber should explain the change before you move up. See our dose schedule guide for the steps.
Can Mounjaro or Wegovy cause pancreatitis?
Yes, pancreatitis, which is inflammation of the pancreas, is a recognised risk of GLP-1 and GIP receptor medicines. On 29 January 2026 the MHRA published a Drug Safety Update strengthening the warnings for these medicines, including semaglutide and tirzepatide. Acute pancreatitis is described as uncommon, but some reports have been severe.
The MHRA's update gives the following figures from the Yellow Card scheme for GLP-1 medicines between 2007 and October 2025: 1,296 reports of pancreatitis, 19 of them fatal and 24 necrotising. A report is not proof that a medicine caused an event.
Seek urgent medical help if you have severe or persistent pain in the upper abdomen that may spread to your back, particularly if it comes with nausea or vomiting. Do not wait for it to pass. Tell the doctor or NHS staff that you are taking a weight loss injection.
Can Mounjaro cause low blood sugar?
Mounjaro can increase the risk of low blood sugar (hypoglycaemia) when it is used with sulfonylureas, a group of diabetes medicines. This is one reason the prescriber should know about every medicine you take, including those for diabetes.
If you have diabetes and take insulin or another blood sugar lowering medicine, your prescriber may need to adjust those doses. Know the warning signs of a hypo, such as shakiness, sweating and confusion, and make sure you know how your treatment for it works.
Do Mounjaro and Wegovy affect contraception or pregnancy?
Both medicines carry guidance on contraception, and the rules differ slightly between them.
- Mounjaro: the SmPC advises switching to a non-oral contraceptive method, or adding a barrier method, when you start tirzepatide and after each dose increase, for 4 weeks. Mounjaro should not be used during pregnancy.
- Wegovy: the SmPC recommends that women of childbearing potential use contraception during treatment.
Mounjaro should be stopped at least one month before a planned pregnancy. Its half-life is around 5 days, so the medicine remains in the body for some time after the last injection. If you might become pregnant, talk to your prescriber before starting and keep them updated.
What should you tell the surgical or anaesthetic team?
In January 2025 the MHRA issued an alert about aspiration, where stomach contents enter the lungs, during general anaesthesia or deep sedation with GLP-1 and dual GIP and GLP-1 medicines. Anaesthetists are advised to assess each patient's risk individually.
A multidisciplinary consensus statement from January 2025, which includes the Association of Anaesthetists, recommends continuing these medicines before elective surgery in most cases. Tell your surgical and anaesthetic team early, before the procedure, so they can plan. Do not stop or change your medicine before an operation without advice from your prescriber or the anaesthetic team.
Are fake weight loss pens a side effect risk?
Counterfeit and illegal weight loss medicines are a serious and growing risk. In July 2026 the MHRA warned that prescription weight loss medicines were being advertised illegally on social media, often alongside quick-fix claims. Warning signs include unusually low prices, "miracle results" and offers that need no prescription.
The MHRA has also reported fake Mounjaro 15 mg KwikPen pens dispensed by a pharmacy in Birmingham. Testing found tirzepatide in the pens, but sterility could not be confirmed. Injecting a medicine you cannot trust carries risks that no side effect table covers. Read our guide to buying safely before you order.
Who should talk to a doctor before starting?
Some people need a more careful review before they start, or while they are taking these medicines. Tell your prescriber before treatment begins if you:
- have had pancreatitis before, or have a history of problems with your pancreas
- are taking, or planning, a pregnancy, or are breastfeeding
- take insulin or a sulfonylurea, or any other medicine that lowers blood sugar
- take an oral contraceptive pill and have not yet discussed contraception advice
- have a planned operation or procedure that needs a general anaesthetic
This list is a starting point, not a full assessment. Your prescriber will review your medical history and the SmPC before deciding whether these medicines are suitable for you.
When should you get medical help?
Some reactions need urgent care. Get emergency help if you have severe or persistent stomach pain, a severe allergic reaction with swelling of the face, lips or throat, or difficulty breathing. Contact your GP, out-of-hours service or prescriber for side effects that are troubling you but not an emergency, including vomiting that stops you keeping fluids down.
You can report suspected side effects to the MHRA through the Yellow Card scheme. Reporting helps the regulator track problems across all medicines, not only the ones you suspect.