Are Weight Loss Injections Bad for You? The Honest UK Answer (October 2026)

01 · The verdict

Are weight loss injections bad for you? What the evidence supports

For adults who meet the licence and are supervised by a prescriber, weight loss injections are a legitimate medical treatment with a known set of side effects, not a harmful shortcut. For people who should not take them, or who buy them from unregulated sellers, they can do real harm. Both statements are true at once, which is why a simple yes or no is misleading.

Mounjaro (tirzepatide) is licensed in the UK 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. It is used alongside a reduced-calorie diet and increased physical activity.¹ A prescriber assesses your BMI, medical history and current medicines before deciding whether it suits you.¹¹

A supervised route also means someone is watching for these problems. Ongoing monthly clinical reviews, dose-titration and side-effect support are part of our aftercare, and repeat orders re-check suitability with no auto-shipping.

02 · Ordering

How ordering works, step by step

Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.

• Free online consultation. A short health questionnaire with no charge and no obligation. Start your free consultation.
• Prescriber review. A GPhC-registered prescriber reviews your answers, not a scoring script. Unsuitable orders are refunded automatically.
• Dispensing and delivery. Our registered pharmacy dispenses, with same-day dispatch when ordered by 3pm. Free next-day tracked delivery comes in temperature-controlled, discreet packaging; Saturday delivery is £14.99.

All six doses, 2.5mg to 15mg, are in stock, supplied through official LillyDirect supply direct from Eli Lilly's UK distribution. Needles, a sharps bin and injection guidance come with every order.

Related guide: Weight Loss Injections Register Online.

03 · Risk versus benefit

Weighing the risks against the benefits

A medicine is "bad for you" only when its harms outweigh its benefits for you. For weight loss injections that balance differs between people, so it is worth looking at both sides plainly.

On the benefit side, the SURMOUNT-1 trial enrolled 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.

To put this into a worked example, for someone starting at 100 kg, 20.9% is about 21 kg. Your own result could be higher or lower, and trial averages do not predict it.

On the risk side, the most common side effects, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹³

One more point matters when asking if these medicines are bad. Mounjaro is licensed for weight management only in adults who meet the BMI criteria, so using it outside the licence, for example to lose a few pounds with a BMI in the healthy range, moves the balance in the wrong direction. The risks stay the same while the medical benefit shrinks. A prescriber's job is to stop that from happening.

04 · How they work

How the medicine works

Weight loss injections work by acting on gut hormones, and most of their side effects are digestive.

Tirzepatide, sold as Mounjaro, is a once-weekly injectable prescription medicine made by Eli Lilly. It comes as a pre-filled multi-dose KwikPen in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ It is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1.¹

Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin. In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ This dual action distinguishes it from single-action GLP-1 medicines such as semaglutide (Wegovy and Ozempic).

The medicine is licensed in the UK for weight management in adults and for type 2 diabetes.¹ People often ask what GLP-1 medications are used for, and those two uses are the answer for Mounjaro. The same hormone pathway is the reason prescribers want a full medicines list from you: other medicines, such as insulin or sulfonylureas, interact with the effects on blood sugar.

Finally, there is no rush effect to fear. Treatment starts at 2.5mg and the dose ladder is designed to let your body adjust gradually. The starting dose is about tolerability, not weight loss itself.¹

05 · Common side effects

Common side effects and the first weeks

The commonest early effects are mild nausea, altered appetite and tiredness in the first days after each injection, and they usually settle.¹³

The 2.5mg starting weeks are about adjustment, and experiences differ widely. Some people notice appetite change and early loss in week one, while others feel little until a higher dose. Both are normal. Early loss includes water weight, so the trend over weeks matters more than any single weigh-in.¹

Here are practical points for the early period:

• Pick one injection day and stick to it. Many people choose a weekend morning so early side effects fall on days off. There is no best time of day, because Mounjaro works the same morning or evening, with or without food.
• Keep fluids up, particularly if you have been sick or have had diarrhoea.
• Do not change your dose yourself.
• Tell the prescribing team if symptoms are not settling.

Side effects can come back for a short time after a dose increase, since each step is a new adjustment. Doses step up by 2.5mg at a minimum of 4-week intervals, depending on how you respond and tolerate it.¹ Higher is not automatically better. Many people settle at 5mg, 10mg or 15mg.

Skin reactions at the injection site are another thing people ask about. Our page on injection-site skin reactions such as blisters covers what to do. The injection technique itself matters: rotate the site with each dose, and use only the licensed sites, which are the stomach area at least 5cm from the belly button, the front of the thigh, or the back of the upper arm if someone else gives the injection.¹

Report any suspected side effect to the MHRA through the Yellow Card scheme as well as telling your prescriber.¹⁹

06 · Serious side effects

Serious side effects: when to seek urgent help

Serious side effects are uncommon, but you should know the warning signs before your first injection. Seek immediate medical help for any of the following.¹

• Severe, persistent stomach pain that may spread to your back, with or without vomiting. This could be pancreatitis.
• Pain in the upper right abdomen, fever, or yellowing of the skin or eyes — get medical help straight away. These could signal gallbladder problems.
• Signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing.

These are not symptoms to wait out or to report at the next review. If they appear, stop and get medical help straight away, then tell your prescriber.

There is also the risk of low blood sugar for people who take insulin or a sulfonylurea alongside Mounjaro. The signs are shakiness, sweating and confusion. Your prescriber may need to adjust those medicines, which is not the same as saying they cannot be combined.¹ Tell us everything you take during the consultation.

Why do these matter so much when they are uncommon? Because they are the cases where the answer to "are weight loss injections bad for you" is clearly yes, for that person. A history of pancreatitis or gallbladder problems is one of the things your prescriber looks for beforehand, and it is why you should share your full medical history honestly.

What the evidence does not support is the idea that everyone who takes these medicines will have a serious reaction. The sensible response is to know the signs, to take the medicine as prescribed, and to keep in touch with the clinical team.

Side effects can be reported through the Yellow Card scheme, which helps regulators monitor safety across the UK.¹⁹ If you want a wider look at what the injections do to the body, see our page on their effects on your body.

07 · Liver, kidneys, thyroid

Liver, kidneys and thyroid: what is and is not known

No dose adjustment is needed for mild to moderate kidney or liver impairment. People with severe impairment need individual prescriber assessment.

The practical risk is indirect. Severe vomiting or diarrhoea can cause dehydration, and dehydration can strain the kidneys. So the advice is to keep fluids up and to get medical help straight away if you cannot keep fluids down.¹ Treat vomiting that does not stop as a reason to call, not to push on.

On the thyroid, the position is cautious and honest. 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. Routine thyroid monitoring is not required for most people.

If a page tells you a medicine is entirely safe, or entirely dangerous, it is not describing the evidence.

Your prescriber reviews how you are doing during ongoing monthly clinical reviews, and you can raise any new symptom at any time.

For new symptoms that are not described in the product information, the right steps are the same: speak to your prescriber and report it through the Yellow Card scheme.¹⁹

08 · Who should avoid them

Who should not take weight loss injections

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.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
• gallbladder problems

Always share your full medical history and medicine list during the consultation. A history of medullary thyroid cancer, or the inherited condition MEN 2, in you or a close relative should also be mentioned.¹

Eligibility has its own side. For weight management, the licence covers 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.¹ Treatment is used alongside a reduced-calorie diet and increased physical activity. If you fall outside that, a prescriber will usually say no, and that is the system working.

Contraception deserves a specific mention. Mounjaro can reduce absorption of oral contraceptives because stomach emptying slows.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 Discuss this with your prescriber, who can advise on what to do.

Is there a "safest drug for weight loss"? There is no single safest option for everyone. The safest medicine is the one a prescriber has judged suitable for you after reviewing your history, with monitoring afterwards. That is the reason a free consultation reviewed by a GPhC-registered prescriber, not a scoring script, is part of our process.

09 · Poo smell and other gut changes

Smelly poo, diarrhoea and other digestive changes

Smelly stools are not listed as a side effect in the UK patient leaflet, but diarrhoea is very common.

The point to take seriously is not the smell, but the fluid loss. Severe vomiting or diarrhoea can cause dehydration, which can strain the kidneys.¹ Keep fluids up and get medical help straight away if you cannot keep fluids down.

Get medical help straight away if a gut change comes with:

• severe, persistent stomach pain that may spread to your back
• pain in the upper right abdomen with fever
• yellowing of the skin or eyes

Those signs point to possible pancreatitis or gallbladder problems, not to an ordinary change in digestion.¹

The same approach works for any other symptom, such as a change in skin, sleep or mood, that you cannot place. State it to your prescriber and report it through the Yellow Card scheme.¹⁹ Do not take online comments as evidence either way.

Our support team is available Monday to Friday, 9am to 5pm, by email at support@everydaymeds.co.uk, through account portal chat, or by phone on 0333 207 0413. Side-effect support is part of the aftercare we provide.

10 · After you stop

What happens after you stop weight loss injections

After the final dose, tirzepatide takes roughly 4 to 5 weeks to clear the system, because it has a half-life of about five days.¹ There is no way to speed this up, and nothing needs doing while it clears.

What is true is that the appetite effect comes from the medicine's action on gut hormones, and as it clears, that action fades. Planning for that point with your prescriber before you stop is wiser than finding out afterwards.

Results themselves are 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 rather than individual weigh-ins.¹

Your prescriber reviews your response. If you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, they will discuss whether continuing or stopping is right for you.¹ Stopping can therefore be a medical decision, made together, rather than something you do alone.

If you decide to stop for any reason, including side effects, tell the prescribing team. If you simply want a lower dose, the answer is not to alter the pen yourself.

11 · Using it safely

Using the pen safely: doses, missed doses and storage

Most avoidable harm with injectable weight loss medicines comes from using them in ways the pen was not designed for. Following the licensed routine removes much of the risk.

Dose ladder. Everyone starts on 2.5mg once a week. 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 (7.5mg, 10mg, 12.5mg) up to a maximum of 15mg weekly.¹

Injecting. Inject under the skin once a week, on the same day each week. Each KwikPen contains four weekly doses.

Missed 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 it and take the next one on your normal day. Never take two doses within 3 days of each other. You can change your injection day as long as at least 3 days have passed since your last dose.¹

Storage. Keep pens in a fridge at 2 to 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.¹

Travel. You can fly with Mounjaro. Carry pens in hand luggage, because hold luggage can fall below freezing, ideally in a cool bag with the prescription label or a copy of your prescription. If an unopened pen has been out of the fridge, check with the pharmacy before using it.¹³

12 · Buying safely and NHS access

Buying safely, and the NHS route

The most dangerous version of a weight loss injection is the one that comes from an unregulated seller, not the licensed pen from a registered pharmacy. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷

Protect yourself with a few simple checks:

• Use a pharmacy you can find on the GPhC register. We are GPhC-registered, number 9012878.¹²
• Be wary of any seller offering to supply without a consultation or prescription. Mounjaro is a prescription-only medicine.¹
• Be wary of social media offers, and of dramatic weight-loss claims that sound too good to be true.
• Check that you receive genuine KwikPens with the printed expiry date and patient information leaflet.

Our Mounjaro is supplied through official LillyDirect supply, direct from Eli Lilly's UK distribution. Supplements are a separate category: low-cost supplements and colostrum-based products, which we cover in our page on colostrum weight loss injections, come with no clinical oversight and are not equivalent to a licensed medicine.

On the NHS, Mounjaro is available, 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).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting. NHS access to Wegovy runs through specialist weight-management services; ask your GP.

If you are unsure which route fits you, a GP or our free consultation can help you think it through. Our supply is for UK adults and is reviewed by a GPhC-registered prescriber.