Weight Loss Jab Prescription Risks in the UK (October 2026)

01 · The risks

What are the real risks of a weight loss jab prescription?

The real risks fall into three groups: side effects of the medicine, being unsuitable for it, and getting it from an unregulated source. Mounjaro (tirzepatide) is a once-weekly injection licensed in the UK for weight management in adults and for type 2 diabetes.¹ It works by activating both GIP and GLP-1 receptors, which helps regulate appetite and slows how quickly the stomach empties.¹

Those mechanisms explain why gut effects, and the dehydration that can follow vomiting or diarrhoea, are the practical concern for most people.

None of this is a reason to avoid asking. It is the reason a prescriber reviews your history first. Our checks before a weight loss jab guide explains what that review looks at.

02 · Suitability

Who should not take a weight loss jab?

You should not take Mounjaro if you are allergic to tirzepatide or any of its ingredients, or if you are pregnant or trying to conceive.¹ 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.¹

Licensing matters too. Mounjaro is for 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

Our prescription weight loss injections guide covers how prescribing works across the different medicines.

You can start your free consultation to have your suitability reviewed by a GPhC-registered prescriber.

Related guide: Buy Mounjaro.

03 · Serious side effects

Serious side effects and when to get help

Serious side effects are uncommon, but they 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, so get medical help straight away. Swelling of the face or throat and difficulty breathing are signs of a severe allergic reaction, so get medical help straight away.¹³

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.

You can report any suspected side effect through the MHRA Yellow Card scheme.¹⁹ Reports help regulators keep watching the safety of the medicine.

04 · Thyroid and long-term

Thyroid safety and long-term side effects

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. Routine thyroid monitoring is not required for most people.¹

05 · 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.

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

06 · Mood and mental health

Mood and mental health

Regulators have looked 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

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.

Even so, a 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 Our guide to common weight loss jab fears looks at the worries people raise most.

07 · Wegovy or Mounjaro: safer?

Which is safer, Wegovy or Mounjaro?

Neither can be called safer in general. Both are licensed prescription medicines, and the right one depends on your health, history and current medicines.

Tirzepatide activates two gut-hormone receptors, GIP and GLP-1, where semaglutide (Wegovy) is a single-action GLP-1 medicine.¹ Wegovy comes in 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg pens, stepping up after at least four weeks to the 2.4mg maintenance dose from week 17.¹⁸ Mounjaro has six strengths from 2.5mg to 15mg.

If you are weighing them, ask the prescriber which suits your history.

08 · Alcohol, smell and other questions

Alcohol, body odour and other everyday questions

There is no direct interaction between Mounjaro and alcohol, but drinking on a reduced appetite and emptier stomach can hit harder than usual. If you also take insulin or a sulfonylurea, alcohol increases the risk of low blood sugar.

A change in body odour is not a listed side effect in the Mounjaro patient leaflet. Raise any new symptom with your prescriber and report it through the Yellow Card scheme.¹⁹

09 · Unregulated supply risk

The risk of buying without a prescription

Much of the avoidable danger sits outside the medicine: unlicensed sellers, counterfeit pens and no clinical oversight. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme.⁷ Treat any seller offering to supply without a consultation or prescription as a red flag.

Check the pharmacy on the GPhC register. We are registered under 9012878 and supply Mounjaro through official LillyDirect supply, direct from Eli Lilly's UK distribution, as genuine KwikPens.

Demand has also pushed people towards informal routes. Our pages on UK weight loss jab sales and who is using weight loss jabs give the wider picture.

10 · NHS or private?

NHS or private prescription?

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.⁴

The NHS primary-care rollout began in June 2025 with the highest-need groups first, initially BMI 40 and above 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.

The safety standard should be the same either way: a prescriber who assesses you, checks your medicines and reviews you as you go. Mounjaro is a prescription-only medicine, supplied only after a consultation reviewed by a GPhC-registered prescriber. This page is information, not a substitute for medical advice.