Mounjaro Risks: What the UK Evidence Says (October 2026)

01 · The short answer

The main Mounjaro risks at a glance

The risks worth knowing fall into a handful of groups: digestive effects, pancreas and gallbladder problems, allergic reaction, dehydration and the kidneys, thyroid questions, and mood.

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.

02 · What to do next

Is this relevant to you, and what should you do next?

Whether a risk matters to you depends mostly on your medical history, which is why Mounjaro is only supplied after a prescriber has reviewed your answers. If you have had pancreatitis, gallbladder disease, severe digestive illness or diabetic retinopathy, say so clearly. It does not automatically rule you out, but it changes the assessment.

If you are weighing up treatment, the benefits and risks side by side are set out on a companion page, and the longer-term picture has its own guide. Every consultation is reviewed by a GPhC-registered prescriber, and orders that are unsuitable are refunded automatically.

03 · Serious side effects

Serious side effects and when to get help

Serious side effects 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, which could be pancreatitis.¹³

Pain in the upper right abdomen, fever or yellowing of the skin or eyes can point to gallbladder problems; get medical help straight away.¹³ Signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing, also need immediate medical help.¹³

The full list of common and less common effects, including the everyday digestive ones, is in the patient information leaflet supplied with every pen.¹³ For a timeline of what tends to matter at each stage, see our week-by-week risks guide.

04 · Who should avoid it

Who should not take Mounjaro

Mounjaro is not suitable for everyone. 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.¹³ 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.

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.¹³ Always share your full medical history and medicine list during consultation.¹³

Two practical points are worth adding. If you take insulin or a sulfonylurea, watch for low blood sugar, with shakiness, sweating or confusion, because your prescriber may need to adjust those medicines.¹³ And if you use an oral contraceptive, Mounjaro slows stomach emptying, which can reduce how well the pill is absorbed.⁷ 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

This is also why the consultation matters. It is not a formality. It is the main safeguard against the people for whom the risks outweigh the benefits.

05 · Thyroid and cancer

Mounjaro cancer risk and the thyroid warning

The cancer question is almost always about the thyroid, and the honest answer is that it is a precaution based on animal data. 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

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 Routine thyroid monitoring is not required for most people.¹

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.¹ People with severe impairment need individual prescriber assessment.¹

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

Sip fluids steadily, and if you cannot keep them down, get in touch rather than pushing through. Our team offers side-effect support as part of ongoing monthly reviews, so you do not have to judge that line alone.

07 · Mood and mental health

Mood and mental health

Regulators have looked specifically at whether GLP-1 medicines are linked to low mood or suicidal thoughts. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link, and the MHRA keeps this under continuous review.¹⁷

That said, any big change in weight, appetite and routine can affect how you feel. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷ If you notice a symptom that is not listed in the leaflet, tell your prescriber and report it through the Yellow Card scheme.¹⁹

08 · Long-term and stopping

Long-term effects and what happens if you stop

Long-term safety is a fair question for a medicine that is designed to be used over an extended period. We cover the wider picture on our page about long-term Mounjaro risks, and the NHS side on NHS-related risk questions.

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.² These are trial-average secondary outcomes, not promises. They are why prescribers talk about weight management as health management rather than appearance.

If you stop, the medicine does not vanish overnight. Tirzepatide has a half-life of about five days, so after a final dose it 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 Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued lost a further 5.5% on average.¹⁴

09 · Weight loss expectations

What weight loss is realistic, and why it belongs in a risk discussion

Unrealistic expectations are a risk in themselves, because they push people towards higher doses or unsafe shortcuts.

In the SURMOUNT-1 trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages — individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

Our guide to weight loss risks on Mounjaro covers the problems that come with losing weight quickly, and the broader weight loss injection risks page compares the whole class of treatments.

10 · Mounjaro vs Wegovy safety

Is Ozempic or Mounjaro safer?

Ozempic is semaglutide licensed for type 2 diabetes only; it is not a UK weight-loss medicine. The licensed weight-management options are Mounjaro (tirzepatide) and Wegovy (semaglutide 2.4mg), and both carry serious-but-uncommon warnings in their product information.¹¹⁸

Mounjaro activates both GIP and GLP-1 receptors, whereas semaglutide, sold as Wegovy and Ozempic, is a single-action GLP-1 medicine.¹ Which suits you depends on your history, previous response and tolerance, so the safer choice is the one your prescriber judges appropriate for you.

11 · Buying safely

Reducing risk when you buy

A large and avoidable risk is where the pen comes from. Counterfeit and unregulated weight-loss injections are the subject of MHRA alerts, and anyone offering to supply without a consultation or prescription should be avoided.⁷ Check the pharmacy on the public register; ours is GPhC-registered, number 9012878.¹²

Store pens in the fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge at a temperature not above 30°C for up to 30 days after first use, and must then be thrown away.¹³

12 · NHS access and risk

NHS or private: does the risk change?

The medicine and its risks are the same whichever route you use; what differs is access and monitoring. Mounjaro is available on the NHS, but access is being phased in over several years.⁴ 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).⁶

Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.⁶ Whichever route you choose, a prescriber should review your history first and follow your progress afterwards.