Who Shouldn't Take GLP-1 Medications? Contraindications Explained

Most adults who meet the BMI criteria can be considered for a GLP-1 medicine such as Mounjaro (tirzepatide) or Wegovy (semaglutide), but not everyone should take one. The only formal contraindication in the UK product information is an allergy to the medicine or its ingredients. Beyond that, these medicines should not be used in pregnancy, and there is a group of conditions, including past pancreatitis, severe gastroparesis, diabetic eye disease, eating disorders and some medicines, where a prescriber has to think very carefully, seek extra information, or decide treatment is not right. This guide explains each of them and why they matter. It is part of our safety guides; for a wider overview, see all our GLP-1 guides.

Contraindications versus cautions: what the terms mean

Before going through the list, it helps to understand how medicines regulators and prescribers use two words that often get confused.

A contraindication is a situation in which a medicine must not be used, because the risk clearly outweighs any benefit. Contraindications are listed in section 4.3 of a medicine's Summary of Product Characteristics (SmPC), the official technical document approved by the Medicines and Healthcare products Regulatory Agency (MHRA) and published on the electronic medicines compendium (medicines.org.uk).

A caution or special warning (section 4.4 of the SmPC) is a situation in which the medicine may still be used, but the prescriber needs to take extra care. That might mean asking more questions, checking records, arranging monitoring, starting at a lower dose, involving a specialist, or deciding against treatment for that individual.

In practice, a responsible prescriber treats some cautions almost as firmly as contraindications. That is why you can be turned down for treatment even though nothing on the formal contraindication list applies to you.

The formal contraindication: allergy

For both Mounjaro and Wegovy, section 4.3 of the UK SmPC lists a single contraindication: hypersensitivity to the active substance (tirzepatide or semaglutide) or to any of the excipients (the other ingredients in the pen).

Serious allergic reactions, including anaphylaxis and angioedema (swelling of the face, lips, tongue or throat), have been reported with GLP-1 receptor agonists, though they are uncommon. If you have previously reacted to one GLP-1 medicine, your prescriber will want details. A reaction to semaglutide does not automatically mean you would react to tirzepatide, or the other way round, but it is a reason for caution and careful discussion.

Mild redness or itching at the injection site is common and is not the same as an allergy. A spreading rash, hives, wheezing, swelling or feeling faint after an injection is different, and needs urgent medical help.

Pregnancy, trying to conceive and breastfeeding

Pregnancy is the most important situation in which GLP-1 medicines should not be used, even though it sits in the SmPC's pregnancy section (4.6) rather than the contraindications section.

Pregnancy

Animal studies have shown reproductive toxicity with semaglutide, and there is not enough human data to show that either medicine is safe in pregnancy. Weight loss is also not recommended during pregnancy. For these reasons the UK SmPCs say:

The difference in washout times reflects how long each medicine stays in the body. Semaglutide has a longer half-life than tirzepatide.

If you become pregnant unexpectedly while taking a GLP-1 medicine, stop taking it and contact your GP or prescriber promptly. Do not panic, but do get advice. Our guide to GLP-1 medicines, pregnancy and fertility covers this in more detail.

Contraception and Mounjaro

Many women find their fertility improves as they lose weight, particularly those with polycystic ovary syndrome. This can lead to unplanned pregnancy.

Mounjaro adds a specific issue. Tirzepatide has been shown to reduce the amount of oral contraceptive absorbed. The Mounjaro SmPC advises women who use oral contraception to switch to a non-oral method, or add a barrier method such as condoms, for four weeks after starting Mounjaro and for four weeks after each dose increase. Our GLP-1 medicines and contraception guide explains the options.

The same specific advice is not given in the Wegovy injection SmPC, but sickness and diarrhoea from any medicine can reduce how well the pill works. If you vomit or have severe diarrhoea, follow the "missed pill" advice in your contraceptive leaflet.

Breastfeeding

The Wegovy SmPC says semaglutide should not be used while breastfeeding. The Mounjaro SmPC section on breastfeeding has been revised as new data on breast milk levels have emerged. Because this is an area where advice is changing, talk it through with your prescriber or health visitor and check the current SmPC rather than relying on older information.

Pancreatitis

Acute pancreatitis is inflammation of the pancreas. It is uncommon with GLP-1 medicines, but it has been reported with both semaglutide and tirzepatide, and some cases have been severe.

The SmPCs note that people with a history of pancreatitis were not studied in the clinical trials, and advise caution. In practice:

• Many prescribers will not start a GLP-1 medicine in someone who has previously had pancreatitis, especially if the cause was unclear.
• Some may consider treatment after specialist advice, for example if a single episode was clearly caused by gallstones that have since been removed.
• Anyone taking a GLP-1 medicine should know the warning signs: severe, persistent pain in the upper abdomen, sometimes spreading to the back, often with vomiting.

If pancreatitis is suspected, the medicine should be stopped and not restarted if pancreatitis is confirmed. Our guide to GLP-1 medicines and pancreatitis risk looks at the evidence in depth.

Other factors that raise the background risk of pancreatitis, such as very high triglycerides, heavy alcohol use and gallstones, are also worth mentioning in your consultation.

Gallbladder disease

Rapid weight loss of any kind increases the risk of gallstones. In the large trials of semaglutide (the STEP programme) and tirzepatide (the SURMOUNT programme), gallbladder problems such as gallstones and inflammation of the gallbladder (cholecystitis) were reported more often with the medicine than with placebo, although they remained uncommon.

A past history of gallstones is not a formal contraindication, but it is something to discuss. If you still have your gallbladder and have had symptomatic stones before, your prescriber may consider the risk of further episodes. Symptoms include pain under the right ribs (especially after fatty meals), fever, and yellowing of the skin or eyes. See our guide to GLP-1 medicines and gallbladder problems for more.

Severe gastrointestinal disease and gastroparesis

GLP-1 medicines work partly by slowing the emptying of the stomach. That is helpful for appetite, but it can be a problem for people whose stomachs already empty very slowly.

• Gastroparesis: The Wegovy SmPC says semaglutide is not recommended where gastroparesis is severe. The Mounjaro SmPC notes tirzepatide has not been studied in people with severe gastroparesis and advises caution. Gastroparesis is more common in people who have had diabetes for many years.
• Inflammatory bowel disease: Crohn's disease and ulcerative colitis have had limited study. Because GLP-1 medicines commonly cause nausea, diarrhoea and constipation, they could make symptoms harder to manage or to interpret.
• Previous stomach surgery: People who have had bariatric surgery, such as a gastric bypass or sleeve, can sometimes be prescribed GLP-1 medicines, but this needs careful, usually specialist, assessment.

Diabetes-related cautions

Many people who take GLP-1 medicines for weight have type 2 diabetes, or develop better blood sugar control as they lose weight. Several cautions apply.

Low blood sugar with insulin or sulfonylureas

GLP-1 medicines on their own rarely cause hypoglycaemia (low blood sugar). The risk rises when they are combined with insulin or a sulfonylurea such as gliclazide. A prescriber may need to reduce the dose of those medicines, and you may need to monitor your blood sugar more often. This is one reason why online services usually ask for permission to contact your GP if you have diabetes. See our guide to GLP-1 medicines and type 2 diabetes.

Diabetic retinopathy

Both SmPCs warn that a rapid improvement in blood sugar can temporarily worsen diabetic retinopathy (eye damage caused by diabetes). In the SUSTAIN-6 trial of semaglutide in people with type 2 diabetes, complications of retinopathy were seen more often in people already using insulin who had existing retinopathy. Anyone with diabetic eye disease should have their eyes monitored and should tell their prescriber before starting.

Type 1 diabetes

Mounjaro and Wegovy are not licensed for type 1 diabetes, and they are not a substitute for insulin. Their use in people with type 1 diabetes for weight loss is outside the licence and should only happen under specialist care, if at all.

Kidney and liver disease

Severe vomiting or diarrhoea can cause dehydration, and dehydration can harm the kidneys. Both SmPCs warn that this has led to worsening kidney function, including acute kidney injury, in some people.

• Kidney disease: Mild or moderate kidney impairment does not usually need a dose change, but there is limited experience in severe kidney disease and end-stage kidney failure. Prescribers will want to know your latest kidney function results.
• Liver disease: Experience in severe liver impairment is limited. Your prescriber may decide treatment is not suitable or may need specialist input.

Keeping well hydrated is important for everyone on these medicines. Our guide to hydration on GLP-1 medicines has practical tips.

Heart conditions

Wegovy has a licence that covers reducing the risk of heart attacks and strokes in some adults with established cardiovascular disease, based on the SELECT trial. Heart disease is therefore not in itself a reason to avoid treatment. However, the Wegovy SmPC notes limited experience in severe (NYHA class IV) heart failure. GLP-1 medicines can also cause a small increase in heart rate. People with significant heart rhythm problems or advanced heart failure need assessment by a clinician who knows their history.

Thyroid cancer: the UK and US difference

If you have read US information about these medicines, you may have seen a prominent warning about thyroid tumours. In US labelling, semaglutide and tirzepatide carry a boxed warning, and are contraindicated in people with a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia type 2 (MEN2). This is based on thyroid C-cell tumours seen in rodents.

The UK SmPCs for Wegovy and Mounjaro do not list MTC or MEN2 as a contraindication, reflecting a different regulatory judgement about how relevant the rodent findings are to humans. Even so, many UK prescribers still ask about this history, and some will decline to prescribe or seek specialist advice. If you or a close relative has had medullary thyroid cancer or MEN2, mention it.

Eating disorders and mental health

This is one of the areas where responsible online services and clinics take most care, and where honest answers matter most.

Eating disorders

Medicines that reduce appetite and drive weight loss can be harmful for people with anorexia nervosa, bulimia nervosa or other restrictive eating patterns. They can reinforce restriction, mask a disorder, and cause dangerous weight loss. Most prescribers will not prescribe a GLP-1 medicine for weight loss to someone with an active eating disorder, and they will take a history of one very seriously.

Binge eating disorder is more complex, and some research is under way, but GLP-1 medicines are not licensed to treat it. Specialist care is the right starting point.

If you are worried about your relationship with food, the NHS and the eating disorder charity Beat can help. Speaking to your GP first is the safest route.

Depression, anxiety and suicidal thoughts

Regulators in the UK, Europe and the US have reviewed reports of suicidal thoughts in people taking GLP-1 medicines. At the time of writing, these reviews have not established a causal link. Prescribers still ask about mental health, partly because rapid changes in weight, eating and body image can affect wellbeing, and partly to make sure people get the right support. Our guide to GLP-1 medicines and mental health covers this in more detail.

Surgery, sedation and anaesthesia

Because GLP-1 medicines slow stomach emptying, food can remain in the stomach for longer than expected, even after the usual fasting period. The MHRA (in a January 2025 Drug Safety Update) and both SmPCs warn that this has led to cases of pulmonary aspiration (stomach contents entering the lungs) during general anaesthesia or deep sedation.

This does not mean people taking a GLP-1 cannot have surgery, endoscopy or other procedures. It means you must tell the surgical, anaesthetic or endoscopy team that you take one, and follow their specific instructions, which may include stopping the medicine for a period beforehand or changing fasting arrangements. See our guide to GLP-1 medicines, surgery and anaesthesia.

Medicines that need extra thought

GLP-1 medicines have relatively few direct drug interactions, but slowed stomach emptying can affect how quickly some oral medicines are absorbed. Examples prescribers consider include:

• Insulin and sulfonylureas (hypoglycaemia risk, as above)
• Warfarin and other medicines with a narrow therapeutic range, where closer monitoring may be advised when starting
• Oral contraceptives (especially with Mounjaro, as above)
• Levothyroxine, where absorption may change
• Other GLP-1 medicines, such as Ozempic, Rybelsus, Victoza or Trulicity: two GLP-1 medicines should not be taken together

Always give a full list of prescribed medicines, over-the-counter medicines and supplements. Our GLP-1 medication interactions guide goes into detail.

Age: under 18 and older adults

Mounjaro is licensed for weight management in adults only. Wegovy has a licence that extends to adolescents aged 12 and over in some circumstances, but online services in the UK typically treat adults only. Children and teenagers should be assessed through NHS specialist services.

There is no upper age limit, but there is less trial data in older people, particularly those over 85. Older adults may be more vulnerable to dehydration, falls, muscle loss and the effects of eating much less. Prescribers will consider frailty, other medicines and kidney function. Our guide to GLP-1 medicines over 65 explores this further.

Not meeting the BMI criteria

Not having a high enough BMI is not a medical contraindication, but it is a reason prescribers will not supply these medicines. Mounjaro and Wegovy are licensed for adults with a BMI of 30 or more, or 27 to under 30 with at least one weight-related condition. NICE recommends lower thresholds for people from some ethnic backgrounds when assessing risk. You can find out more in our guide to who can get weight loss injections and check your BMI with our BMI calculator.

Summary table: who should not take, or should take extra care with, GLP-1 medicines

Why honesty in the consultation matters

All of these safeguards depend on accurate information. A GLP-1 medicine prescribed to someone who has hidden a pregnancy, a past pancreatitis episode or an eating disorder can cause real harm.

The General Pharmaceutical Council (GPhC) now expects online pharmacies to verify key information, such as weight, rather than relying on questionnaire answers alone, and many services ask to contact your GP. These checks can feel intrusive, but they exist to protect you. Our guide to the online weight loss consultation explains what prescribers ask and why.

If you are comparing services, look for those that take a proper medical history and are willing to say no. You can see how we compare regulated services on our providers page.

What if you have been told a GLP-1 is not suitable?

Being declined can be frustrating, but it is not the end of the road. Options to discuss with your GP include:

• NHS weight management services (tier 2 behavioural programmes and tier 3 specialist services)
• Treating the underlying issue first, for example eating disorder support or gallstone treatment
• Other licensed weight-loss medicines, such as orlistat, where appropriate (see our comparison of orlistat and GLP-1 medicines)
• Bariatric surgery assessment, where criteria are met

Going to a different provider and leaving out the information that led to the refusal is unsafe. If you think a decision was wrong, ask the prescriber to explain their reasoning, and ask your GP for a second opinion.

When to speak to a professional

Speak to your GP, pharmacist or prescriber before starting a GLP-1 medicine if any of the conditions in this guide apply to you, if you take other regular medicines, or if you are planning a pregnancy.

If you are already taking a GLP-1 medicine, seek help as follows:

• Call 999 or go to A&E if you have signs of a severe allergic reaction (swelling of the face, lips, tongue or throat, difficulty breathing, feeling faint), or severe, persistent abdominal pain, with or without vomiting, which could be pancreatitis.
• Contact NHS 111 or your GP urgently if you have pain under the right ribs with fever or yellow skin or eyes, persistent vomiting or diarrhoea leading to signs of dehydration (very dark urine, dizziness, passing little urine), or changes in your vision.
• Contact your prescriber or GP promptly if you become pregnant, if your mood drops or you have thoughts of self-harm (in a crisis, call 999 or Samaritans on 116 123), or if you are due to have surgery or sedation.

Suspected side effects can be reported to the MHRA through the Yellow Card scheme.