GLP-1 Risks: What the UK Evidence Says (October 2026)

01 · The core answer

Is taking a GLP-1 worth the risk?

For most suitable adults the answer depends on your starting health, your medical history and what you expect from treatment. The risks of Mounjaro are mostly manageable, and the serious ones are uncommon. It is licensed in the UK for weight management in adults and for type 2 diabetes, and it is supplied only after a prescriber has reviewed your history.¹ The honest picture has three parts: effects you are likely to notice, rarer problems that need action, and groups of people for whom the medicine is not suitable.

The effects you are likely to notice are digestive and short-lived. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.¹³ The serious problems are uncommon but need urgent attention. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of the skin or eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹³ Our risks and benefits comparison sets these against the trial results.

02 · What to decide

The decisions you are actually facing

Reading about risk is only useful if it leads to a decision. Most readers face one of three questions, and each has a clear next step.

• Is this relevant to me? A history of pancreatitis, gallbladder problems, severe gut disease, diabetic retinopathy, pregnancy or planning pregnancy all change the assessment.
• Can I live with the common effects? Most people can, because treatment begins on 2.5mg, a dose chosen to let your body adjust rather than for weight loss itself.¹
• Where should I get it? The safest route is a regulated pharmacy where a prescriber reviews you. The riskiest is a seller offering to supply without a consultation or prescription.

None of this is a reason to skip a conversation with a clinician. It is a reason to have a better one. Bring your full medicine list, mention any family history of thyroid cancer, and tell the prescriber how you have felt in the past if you have had low mood.

03 · Getting treatment

NHS or private: how the risk review works

Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

Either way, the safety assessment is the same in principle: a qualified prescriber checks your history before the first dose. With us, the free online consultation is reviewed by a GPhC-registered prescriber, not a scoring script. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full. Ongoing monthly reviews are included, and repeat orders re-check suitability rather than auto-shipping.

If you are weighing up the whole picture, our weight loss clinic page explains the pathway from consultation to delivery.

Related guide: Agonisti glp 1.

04 · Common side effects

Common side effects of GLP-1 medicines

The commonest effects are digestive and tiredness, and they cluster in the days after each injection. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.¹³ Tirzepatide slows how quickly the stomach empties and acts on appetite, which is why you feel fuller sooner.¹

Experiences differ widely, and that is normal. Some people notice appetite change in the first week, others feel little until a higher dose. Neither pattern signals that something is wrong. Because early loss includes water weight, the trend over several weeks matters more than any single weigh-in.¹³

Why the starting dose is deliberately low

Everyone starts on 2.5mg once a week. This starting dose is about letting your body adjust, not about weight loss itself. After at least 4 weeks the dose increases to 5mg, and can then step up by 2.5mg at a minimum of 4-week intervals, up to a maximum of 15mg weekly, depending on how you respond and tolerate it.¹ Higher is not automatically better. Many people settle at 5mg, 10mg or 15mg, and your prescriber will guide the right pace.

Practical ways people reduce discomfort

Pick a day and time you will remember. Many people choose a weekend morning so early side effects fall on days off, and there is no best time of day because Mounjaro works the same morning or evening, with or without food.¹ Rotate the injection site with each dose. Keep fluids up, especially if you have been sick or had loose stools. If nausea is persistent or you are struggling to keep fluids down, contact your clinical team rather than waiting it out.

Side effects you should report

If you notice a symptom that is not on the leaflet's list, it is not a listed side effect, but tell your prescriber about any new or lasting symptom. You can also report suspected side effects through the MHRA Yellow Card scheme, which helps regulators monitor safety across the UK.¹⁹ Reporting is open to patients as well as clinicians and takes only a few minutes online.

For a Mounjaro-specific view, see our guide to Mounjaro risks and side effects.

05 · Serious side effects

Serious risks: pancreatitis, gallbladder and allergy

Serious side effects are uncommon but need urgent attention. Knowing the warning signs in advance is the single most useful safety step you can take.¹³

Possible pancreatitis

Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting. This pattern could indicate pancreatitis, inflammation of the pancreas. It is different from the mild, passing queasiness most people feel after an injection: it is severe, it does not ease, and it may move through to the back. Stop using Mounjaro and get medical help straight away.

If you have a history of pancreatitis, your prescriber will take extra care or may recommend against treatment. This is why the consultation asks about it directly, and why you should answer completely.¹

Possible gallbladder problems

Pain in the upper right abdomen, fever or yellowing of the skin or eyes may indicate gallbladder problems and needs medical help straight away.¹³ Previous gallbladder problems are a reason for your prescriber to take extra care.

Severe allergic reaction

Signs of a severe allergic reaction include swelling of the face or throat and difficulty breathing: get medical help straight away. Anyone allergic to tirzepatide or any of its ingredients should not take Mounjaro.¹

Low blood sugar with other medicines

If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar: shakiness, sweating and confusion. Your prescriber may need to adjust those medicines. This is a dose-adjustment issue, not a blanket ban, which is why people with diabetes should tell the consultation team exactly what they take and not stop or change anything on their own.¹³

What to do

• Treat severe, persistent abdominal pain as urgent: stop using Mounjaro and get medical help straight away.
• Keep the leaflet to hand and know the warning signs before your first injection.
• Tell any doctor or pharmacist treating you that you are on Mounjaro.
• Report suspected side effects through the Yellow Card scheme.¹⁹

For a wider look at how these risks compare with day-to-day experience, our overview of GLP-1 health risks goes through the categories in one place.

06 · Kidneys and dehydration

Kidneys, liver and dehydration

Mounjaro is not known to damage the kidneys or liver directly, and no dose adjustment is needed for mild to moderate kidney or liver impairment.

The indirect risk

The practical risk is indirect. Severe vomiting or diarrhoea can cause dehydration, which can strain the kidneys. So keep fluids up, and get medical help straight away if you cannot keep fluids down.¹

Practical steps

• Sip fluids steadily through the day, particularly in the days after an injection.
• If you have a stomach bug, tell your clinical team rather than carrying on regardless.
• If vomiting or diarrhoea is severe or you cannot keep fluids down, contact your clinical team — dehydration needs medical help straight away.
• Watch for signs of dehydration such as dark urine, dizziness or an unusually dry mouth.

People with existing kidney or liver conditions

People with severe impairment need individual prescriber assessment.¹ If you have a kidney or liver condition, say so in the consultation and list every medicine you take, including over-the-counter products. The prescriber can then decide whether treatment is appropriate and what monitoring makes sense. Mild to moderate impairment does not by itself require a different dose, but it is still information the prescriber needs.

Why this matters for how you start

If a dose increase brings on more vomiting or loose stools than you can manage, your prescriber can hold you at the current dose. That is a better choice than pushing through. The BNF entry for tirzepatide describes the licensed dosing and safety information that prescribers follow.¹¹

In short, the kidneys are not a target of the medicine, but they can be affected indirectly if you become dehydrated. Hydration and early contact with your team are the control measures.

07 · Thyroid safety

Thyroid safety and the rodent finding

The thyroid warning comes from animal studies, not from human experience. In rodent studies tirzepatide caused thyroid C-cell tumours; it is not known whether this applies to humans, and no increase has been shown in human trials.¹

Why the question is still asked

It is not a prediction that a patient will develop a tumour. Equally, it is not something to wave away. That is why the consultation asks about it.

Who should speak up

As a precaution, tell your prescriber if you or a close relative have had medullary thyroid cancer or the inherited condition MEN 2. They will weigh this in your assessment.¹ It is a family-history question as well as a personal one, so ask relatives if you are unsure. If you do not know your family history, say so in the consultation instead of guessing.

Monitoring

Routine thyroid monitoring is not required for most people.¹ You do not need scans or regular blood tests for the thyroid just because you are taking the medicine. Your prescriber can decide whether anything needs looking into. Raising a symptom early is always reasonable.

Keeping the risk in proportion

The way to read this finding is as a reason to take a careful history at the outset, not as a reason to avoid the medicine altogether if the history is clear. Prescribers work through exactly this kind of balance every day.

If you want to read the product information yourself, the Mounjaro SmPC and patient information leaflet are public on the electronic medicines compendium, and the NHS also publishes a plain-English medicine guide.⁵ Our wider piece on weight loss risks with GLP-1 treatment covers how these items fit together.

08 · Mood and mental health

Mood, mental health and GLP-1 medicines

Regulators have looked at this directly and found no causal link. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link between GLP-1 medicines and low mood or suicidal thoughts, and the MHRA keeps this under continuous review.¹⁷ It is also not the end of the conversation.

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.

Why mood can still change

Any big change in weight, appetite and routine can affect how you feel.

What to tell your prescriber

Tell your prescriber about any history of depression.

When to seek help promptly

Seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷ You do not need to work out whether the medicine is responsible before asking for support. If you are in crisis, use emergency services immediately.

A balanced reading

It helps to hold two statements together. First, formal review has not found that the medicines cause these problems. Second, treatment is a significant change in your life and deserves attention to how you feel, not only to the scales.

Any suspected side effect can be reported through the Yellow Card scheme.¹⁹

09 · Who should not take it

Who should not take a GLP-1 medicine

Mounjaro is not suitable for everyone, and this is where risk is best controlled. You should not take it if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive.¹

Extra care, or a recommendation against

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.¹³ Pancreatitis and gallbladder history link to the serious side effects. Retinopathy is a diabetic eye condition your prescriber needs to know about.

Pregnancy and contraception

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. Because the medicine slows how quickly the stomach empties, it can reduce absorption of oral contraceptives. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ The MHRA has issued guidance on contraception with these medicines, so ask your prescriber what protection suits you.⁷ Do not rely on assumptions here, because it matters for you and for any future pregnancy.

Insulin and other diabetes medicines

If you take insulin or a sulfonylurea, the risk is low blood sugar. The medicines do not have to be stopped, but doses may need adjusting under your prescriber's guidance.¹³ Never change diabetes medicines on your own.

Being honest in the consultation

The consultation is free, short, and reviewed by a GPhC-registered prescriber. Its purpose is to find reasons why treatment might be unsuitable.

The question of being at a healthy weight

Mounjaro is licensed for weight management in adults with a BMI of 30 or above, or a BMI between 27 and 30 with at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, cardiovascular disease, prediabetes or type 2 diabetes. It is used alongside a reduced-calorie diet and increased physical activity.¹ If treatment is not appropriate for your circumstances, a prescriber will say so.

10 · Unregulated supply risks

The avoidable risks: unregulated supply and altered doses

Some of the largest risks around GLP-1 medicines have nothing to do with the molecule. They come from where the pen comes from and how it is used.

Counterfeit and unregulated products

The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷ Be wary of any seller offering to supply without a consultation or prescription, or promising dramatic weight-loss results. A genuine prescription-only medicine requires a clinical review. You can check any UK pharmacy on the GPhC public register; ours is registered under GPhC 9012878.¹²

Missed doses

If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time. If more than 4 days have passed, skip the missed dose entirely and take the next one on your normal day. Never take two doses within 3 days of each other.¹

Storage errors

Keep Mounjaro pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded. Keep the cap on to protect it from light, and never use a pen that has been frozen.¹³ For travel, carry pens in hand luggage, because hold luggage can fall below freezing.

If an unopened pen has been out of the fridge, check with the pharmacy before using it rather than relying on a remembered day limit. Figures circulating online often come from other countries' labels.

Injecting safely

Inject under the skin once a week on the same day. Licensed sites are the stomach area, the front of the thigh, or the back of the upper arm if someone else gives the injection. Rotate the site each time. Needles, a sharps bin and injection guidance are included with every order.

11 · Benefits and stopping

Weighing benefits, pace and stopping

Risk only makes sense next to benefit. In trials, weight loss on tirzepatide came with average improvements in waist size, blood pressure, cholesterol and blood-sugar measures, and most participants with prediabetes returned to normal blood-sugar levels. These are trial-average secondary outcomes, not promises, but they are why prescribers talk about weight management as health management rather than appearance.²

A gradual pace is part of the safety design

Weight loss on Mounjaro is gradual by design. The first weeks on 2.5mg are an adjustment phase and losses are usually modest; most measurable progress builds from 5mg onward, month by month. Trials measured results at 72 weeks, so think in months, not days, and judge progress by trend, not individual weigh-ins.² Rapid, dramatic loss is not the expected pattern. If you notice something far faster than this, tell your prescriber.

Review points

Treatment response is reviewed by your prescriber. If you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right for you.⁴ This is a safeguard against staying on a treatment that is not helping you.

Why people stop

People stop for several reasons: side effects that do not settle, cost, reaching their goal, or a review that finds the medicine is not working for them. Stopping is always a conversation to have with your prescriber rather than a solo decision. After a final dose, tirzepatide 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

The trial literature on continued treatment, such as SURMOUNT-4, looked at what happens when treatment continues compared with when it is withdrawn, which is a useful topic to discuss at review.¹⁴ Individual results vary, and trial averages do not predict your own result.

Comparing with other options

Wegovy is another licensed weight-management option, with its own dose ladder and product information; Ozempic is semaglutide licensed for type 2 diabetes and is not a UK weight-loss medicine. Our Wegovy pen page sets out its licensed dose ladder if you want to compare.

To decide whether treatment fits you, begin with the free consultation.