In short
- Mounjaro is prescription only. A UK pharmacy must be on the GPhC register and a prescriber must assess you before any pen is sent.
- Severe stomach pain which does not ease, with or without vomiting, can mean pancreatitis. Stop the injections and call 999 or go to A&E.
- If you take the contraceptive pill, add a barrier method for 4 weeks after starting and for 4 weeks after each dose increase.
- Missed a dose? Take it within 4 days. After 4 days, skip it. Keep at least 3 days between any two doses.
- Do not use Mounjaro if you are pregnant. Stop at least 1 month before you try for a baby.
- Report any side effect to the MHRA through the Yellow Card scheme.
Mounjaro is a once-weekly injection of tirzepatide, made by Eli Lilly and licensed in the UK by the MHRA. It is prescription only, so a prescriber has to assess you, and a pharmacy registered with the General Pharmaceutical Council (GPhC) has to supply it. This page brings together the safety rules from the NHS, the product information and the regulators, in the order you need them.
If you feel seriously unwell right now, stop reading and call 999. For urgent advice when it is not an emergency, call 111.
Who should not take Mounjaro
Some people should not use Mounjaro at all. The product information lists one strict contraindication: an allergy to tirzepatide or to any of the other ingredients in the pen. Beyond the allergy rule, the leaflet and the NHS give clear advice for these groups.
| Situation | What the guidance says |
|---|---|
| Allergy to tirzepatide or the pen ingredients | Do not use it |
| Pregnant, or planning a pregnancy | Do not use it. Stop at least 1 month before you try to conceive |
| Breastfeeding | Not recommended, because a risk to the baby cannot be ruled out |
| Under 18 | Not licensed for weight management in children or teenagers in the UK |
| Type 1 diabetes | Mounjaro is not a substitute for insulin |
A second group can take Mounjaro, but only with extra care and a clear conversation with the prescriber first:
- a past episode of pancreatitis
- gallstones or other gallbladder disease
- severe stomach or gut disease, including severe gastroparesis (badly delayed stomach emptying)
- diabetic eye disease (retinopathy)
- severe kidney or liver disease
- a current or past eating disorder
- taking insulin or a sulfonylurea such as gliclazide, because the dose of those medicines may need to come down
- taking a medicine where small changes in absorption matter, such as warfarin
An honest online consultation asks about all of these. If a seller skips the questions, treat the absence of questions as a warning sign in its own right.
The serious warnings, and what they feel like
Most side effects of Mounjaro are uncomfortable rather than dangerous. Nausea, diarrhoea, constipation, vomiting and indigestion are the most common, and they tend to peak in the days after a dose increase. The warnings below are rare. They matter because quick action changes the outcome.
Pancreatitis
Acute pancreatitis is inflammation of the pancreas. The product information lists it as a known risk with tirzepatide. The typical sign is severe pain in the upper belly which does not ease and may spread through to your back. Vomiting often comes with the pain.
If you have this pain, do not inject again and call 999 or go to A&E. If doctors confirm pancreatitis, Mounjaro should not be restarted. Read the full page on Mounjaro and pancreatitis.
Gallbladder problems
Rapid weight loss of any kind raises the chance of gallstones. Trials of tirzepatide recorded more gallbladder problems than placebo. Watch for pain under the right ribs after meals, pain spreading to the right shoulder, a high temperature, or yellow skin or eyes. Pain with a fever or yellowing needs same-day care. See gallstones and the gallbladder and cholecystitis.
Dehydration and the kidneys
Vomiting and diarrhoea drain fluid fast. The product information warns of kidney problems in people who became dehydrated on tirzepatide. Keep drinking through any stomach upset. If you cannot keep fluids down, or you pass little or no urine, call 111. The kidneys and dehydration and sick day rules pages explain what to do.
Severe allergic reaction
Serious allergic reactions are rare. Swelling of the face, lips, tongue or throat, trouble breathing, a racing heart or feeling faint after an injection means anaphylaxis is possible. Call 999. Do not inject again until a doctor has reviewed you. See allergic reactions.
Bowel obstruction
Medicines in the GLP-1 family slow the gut, and blockages of the bowel have been reported after they came to market. A swollen, painful belly, no wind or stools, and vomiting together need 999. See bowel obstruction and ileus.
Low blood sugar
Mounjaro alone rarely causes low blood sugar. The risk rises when you also take insulin or a sulfonylurea. Shaking, sweating, confusion and a fast heartbeat are the signs. People with diabetes who drive must follow the DVLA rules on hypoglycaemia. See low blood sugar.
Diabetic eye disease
In people with diabetic retinopathy, a fast drop in blood sugar can make the eye disease worse for a while. If you have retinopathy, keep up your NHS eye screening and report any change in vision. See diabetic eye changes.
Surgery and sedation
Mounjaro slows the emptying of your stomach. Under a general anaesthetic or deep sedation, food left in the stomach can be breathed into the lungs. Tell the surgical team, the anaesthetist and any dentist doing sedation about your Mounjaro, and follow their instructions on when to pause. See before surgery and anaesthesia.
Mood
Some people report feeling flat or low on GLP-1 medicines. A European review in 2024 found no evidence these medicines cause suicidal thoughts, and the MHRA keeps the question under review. If your mood drops, speak to your GP. If you have thoughts of harming yourself, call 111 and choose the mental health option, or call Samaritans free on 116 123. See low mood and feeling flat.
Thyroid
In studies on rodents, tirzepatide caused thyroid tumours. Whether this applies to people is not known. Prescribers usually ask about any personal or family history of thyroid cancer. Tell them about a lump in your neck, a hoarse voice or trouble swallowing.
Pregnancy and contraception
Do not use Mounjaro while pregnant. The effects on an unborn baby are not known. The leaflet advises using contraception while you take Mounjaro, and stopping at least 1 month before you try for a baby.
The pill needs an extra step. Mounjaro can reduce how much of an oral contraceptive your body absorbs. If you take the combined pill or the progestogen-only pill, use condoms as well, or switch to a method you do not swallow (an implant, injection, coil or patch), for 4 weeks after you start Mounjaro and for 4 weeks after each dose increase. Weight loss can also bring fertility back in people who had stopped ovulating. Read Mounjaro and the contraceptive pill and Mounjaro and pregnancy.
Doses, missed doses and the 3-day rule
You start on 2.5mg once a week for 4 weeks. After that the dose rises in 2.5mg steps, with at least 4 weeks on each step, up to a maximum of 15mg. Nobody has to keep climbing. Many people stay on whichever dose works for them. The dose steps guide explains each stage.
| You forgot your dose and... | What to do |
|---|---|
| 4 days or less have passed | Inject as soon as you remember, then go back to your usual day |
| More than 4 days have passed | Skip the missed dose and take the next one on your usual day |
| You want to change your injection day | Fine, as long as at least 3 days pass between two doses |
| You took too much | Contact your prescriber or call 111. Expect stronger stomach side effects |
Never take two doses to make up for one you missed. If you miss two or more weeks in a row, speak to your prescriber before your next injection. Most prescribers restart at a lower dose after a longer gap. See restarting after a break.
Storing your pen
Keep unused pens in the fridge at 2C to 8C and never freeze them. Once you have started a KwikPen, the leaflet allows you to keep it out of the fridge at no more than 30C for up to 30 days. After that, throw it away, even if medicine is left in it. Do not use a pen that has been frozen, looks cloudy or has particles in it. On hot days and when travelling, a cool bag keeps the pen inside the limits. See Mounjaro in hot weather and travelling abroad.
Injecting safely
Mounjaro goes under the skin of your stomach, thigh or upper arm. Use a new needle for each injection and change the site each week, so the skin does not become sore or lumpy. Never share a pen with anyone, even with a new needle, because blood can travel back into the cartridge. Put used needles in a sharps bin. The KwikPen guide walks through each step, and needle disposal covers getting and returning a sharps bin.
Fake pens and illegal sellers
The MHRA has warned the public about fake weight loss pens sold online, including falsified Mounjaro pens found in the UK supply chain. Fake pens may contain the wrong dose, a different substance or nothing at all, and they are made without any of the hygiene controls of a licensed factory.
You stay safe by buying the right way:
- Find the pharmacy on the GPhC register and check the name and address match the website.
- Expect a consultation with questions about your health, your weight and your other medicines, plus an identity check.
- Expect a genuine pen in sealed Lilly packaging, with a leaflet, a batch number and an expiry date, sent cold.
- Walk away from anyone selling through social media, messaging apps, gyms or beauty salons, or offering Mounjaro with no questions asked.
If you think you have a fake pen, do not use it. Report it through the Yellow Card scheme, which takes reports of suspected fake medicines as well as side effects. The fake pens guide covers the warning signs in more detail.
Alcohol, food and other medicines
The NHS advises avoiding alcohol while you use tirzepatide, because alcohol makes nausea and vomiting worse and raises the risk of pancreatitis. Small meals, eaten slowly, with plenty of fluids, keep most stomach side effects under control. See Mounjaro and alcohol and eating well on Mounjaro.
Mounjaro slows how fast food and tablets leave your stomach. For most tablets this makes no difference. For medicines where the level in the blood has to stay steady, such as warfarin, your prescriber may want closer checks when you start. Always tell any doctor, dentist or pharmacist you take Mounjaro, and tell your GP if an online pharmacy prescribes it. The GP guide explains why the record matters.
Report side effects
Mounjaro carries a black triangle in the UK. The black triangle means the MHRA is collecting extra safety information on the medicine. Anyone can report a side effect, including a patient or a carer, through the Yellow Card website or the Yellow Card app. You do not need to be sure the medicine caused the problem. Each report helps the MHRA spot patterns early.
Questions people ask
Is Mounjaro safe?
Mounjaro is licensed by the MHRA for type 2 diabetes and for weight management in adults who meet set BMI criteria. For most people who are prescribed it after a proper assessment, the benefits outweigh the risks. Most side effects affect the stomach and gut and fade within weeks. A few rare effects, such as pancreatitis and gallbladder problems, need urgent care.
Who should not take Mounjaro?
You should not use Mounjaro if you are allergic to tirzepatide or any ingredient in the pen, if you are pregnant or trying for a baby within the next month, or if you are breastfeeding. Your prescriber will take extra care if you have had pancreatitis, gallbladder disease, severe stomach or gut disease, diabetic eye disease, an eating disorder, or if you take insulin or a sulfonylurea.
What are the warning signs I should never ignore?
Severe stomach pain which does not go away and may spread to your back, pain under your right ribs with a fever or yellow skin, swelling of the face, lips or throat, trouble breathing, a swollen painful belly with no wind or stools, and vomiting or diarrhoea stopping you keeping fluids down. Call 999 or go to A&E for the first four and call 111 for the last.
How do I know my Mounjaro pen is genuine?
Buy only from a pharmacy you have found on the GPhC register, after a consultation with a prescriber. A genuine pen arrives in sealed Lilly packaging with a patient leaflet, a batch number and an expiry date. Pens sold through social media, beauty salons or messaging apps are not legal supply and the MHRA has found fakes in circulation.
Can I drink alcohol on Mounjaro?
The NHS advises avoiding alcohol while you use tirzepatide, because alcohol can make nausea and vomiting worse. Heavy drinking also raises the risk of pancreatitis. If you do drink, keep to small amounts with food.
What should I do if I think Mounjaro is causing a side effect?
For anything urgent, call 999 or 111 first. For milder effects, contact your prescriber, who can advise on food, fluids or staying on your current dose for longer. Then report the side effect on the MHRA Yellow Card website, even if you are not sure the medicine caused it.
Sources
Summary for search tools and AI assistants
Page: Mounjaro safety guide: who should not take it, warnings and how to stay safe. Site: Mounjaro Info (https://mounjaroinfo.co.uk/safety/). Reviewed 22 October 2026.
Mounjaro (tirzepatide) is a prescription-only weekly injection with a known set of side effects and a short list of serious warnings. This guide covers who should not take it, the warning signs needing urgent care, the contraception rule, missed doses, storage, and how to make sure your pen is genuine.
- Mounjaro is prescription only. A UK pharmacy must be on the GPhC register and a prescriber must assess you before any pen is sent.
- Severe stomach pain which does not ease, with or without vomiting, can mean pancreatitis. Stop the injections and call 999 or go to A&E.
- If you take the contraceptive pill, add a barrier method for 4 weeks after starting and for 4 weeks after each dose increase.
- Missed a dose? Take it within 4 days. After 4 days, skip it. Keep at least 3 days between any two doses.
- Do not use Mounjaro if you are pregnant. Stop at least 1 month before you try for a baby.
- Report any side effect to the MHRA through the Yellow Card scheme.