Are Weight Loss Injections Safe? Risks, Side Effects and Who They Are Right For

Weight loss injections have gone from a specialist treatment to something almost everyone seems to have an opinion about. That has created a lot of noise, and it can be difficult to tell the everyday side effects from the rarer risks that genuinely need attention.

Here is the short version. For adults who meet the criteria and are properly assessed by a healthcare professional, these medicines have been studied extensively and are licensed in the UK for weight management. They can also cause stomach and bowel side effects, and there are less common but serious risks, including pancreatitis and gallbladder problems. They are not suitable for everyone.

There is another important part of the safety picture: where the medicine comes from. Buying a prescription medicine through an unregulated seller or social media can expose you to products that are fake, incorrectly dosed or contain something different from what is on the label. The MHRA has issued repeated warnings about this.

This guide explains what current UK product information, NICE guidance and MHRA safety advice say, including the figures that help put the risks in context.

Key things to know

• Stomach and bowel side effects are very common with medicines such as semaglutide and tirzepatide, including nausea, diarrhoea, constipation and vomiting
• Serious side effects are uncommon but can happen. Between 2007 and October 2025, the MHRA received 1,296 Yellow Card reports of pancreatitis associated with GLP-1 or GLP-1/GIP medicines, against around 25.4 million packs dispensed over the five years used for comparison. These reports cannot be used to calculate an individual's risk
• In January 2026, the MHRA strengthened the pancreatitis warning. Severe, persistent stomach pain, particularly if it spreads to the back, needs urgent medical attention
• These medicines should not be used during pregnancy. Tirzepatide can also affect the absorption of oral contraceptives
• Tell your healthcare team and anaesthetist you take one of these medicines before an operation or procedure under general anaesthesia or deep sedation
• There is no simple basis for saying these medicines damage the liver. Semaglutide is licensed in the UK for certain adults with a form of fatty liver disease, although people with liver disease still need individual assessment
• Weight regain after stopping is common. In one follow-up study of 327 people treated with semaglutide, much of the weight lost was regained within a year of stopping
• Suitability depends on your health, medicines, medical history and weight-related health risks, and results vary between individuals

The Short Answer

For people who meet the treatment criteria and are assessed by an appropriate healthcare professional, these medicines have an established safety profile and are licensed for specific uses in the UK.

That does not mean they suit everyone, or that side effects can be ignored. Nausea, diarrhoea, constipation and vomiting are common, especially while the dose is being increased, and some people need their dose adjusted or treatment reviewed.

Safety also depends on the route. A prescription medicine supplied by a registered pharmacy is very different from a pen bought from an unknown seller. The MHRA has reported cases involving potentially fake weight loss pens, including products suspected of containing insulin instead of the medicine shown on the label.

What These Medicines Are

A GLP-1 medicine works in a similar way to a natural hormone involved in appetite and blood sugar control. GLP-1 stands for glucagon-like peptide-1.

These medicines slow the movement of food through the stomach and affect appetite, which can make you feel fuller and reduce how much you want to eat. They also affect blood sugar regulation. Tirzepatide works on two hormone pathways, acting on both the GLP-1 and GIP receptors.

Which Are Licensed in the UK

Several are licensed for weight management, including semaglutide, sold as Wegovy, tirzepatide, sold as Mounjaro, and liraglutide, sold as Saxenda. Other medicines and formulations are licensed for different conditions, so the brand and the indication both matter.

For adults, Wegovy and Mounjaro are licensed for weight management in people with:

• A BMI of 30 or above, or
• A BMI of 27 to under 30 with at least one weight-related health condition

They are used alongside a reduced-calorie diet and increased physical activity. Exact eligibility can depend on the medicine and the service. NICE also recommends lower BMI thresholds in some ethnic groups, because health risks associated with excess weight can occur at a lower BMI.

Why the Diabetes Version Is Not the Same Licence

The same active ingredient can be sold under different brand names for different conditions. A medicine licensed for type 2 diabetes does not automatically have the same licence as one licensed for weight management.

Off-label prescribing can be legally possible in some circumstances, but you should know exactly which medicine you are being prescribed, why, and what the prescriber is using it for. If you are unsure, ask. You should never have to guess what is in your injection.

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Side Effects: What Most People Get

Stomach and bowel side effects are the most common problem. In UK product information, "very common" means affecting at least 1 in 10 people.

For semaglutide, very common effects include nausea, diarrhoea, constipation and vomiting, along with headache and fatigue. For tirzepatide, they include nausea, diarrhoea, vomiting, constipation and abdominal pain.

Other effects are listed as common, meaning up to 1 in 10 people. These vary between medicines and can include dizziness, indigestion, gallstones, hair loss and injection site reactions.

Side effects are often more noticeable when treatment starts or the dose increases, which is why these medicines are started low and increased gradually.

Less Common Effects Worth Knowing

Some matter more because they can need medical attention: gallbladder problems, dehydration from vomiting or diarrhoea, low blood sugar when certain diabetes medicines are also used, and eye complications in some people with diabetes.

Hair loss is listed with semaglutide, reported more often in people who lost more weight. The product information says most affected patients recovered while continuing treatment.

Tirzepatide can also increase heart rate slightly. Tell your prescriber about a persistent racing heartbeat, or any new symptom that concerns you.

You should not expect every side effect listed here. Many people have some, and others have few or none. If symptoms make it difficult to eat, drink or manage your day, speak to your healthcare professional rather than putting up with them.

Living With the Stomach Side Effects

Do not change the dose yourself, or take extra medicine to make up for a missed dose. A few habits may make things easier:

1. Eat smaller portions, and slow down when you start feeling full
2. If rich, fatty or fried foods worsen nausea, choose lighter meals while your body adjusts
3. Keep drinking fluids, especially with vomiting or diarrhoea
4. For constipation, regular movement and enough fibre and fluid may help. Speak to a pharmacist or prescriber if it continues
5. If side effects stop you eating or drinking normally, tell your prescriber. Your treatment may need reviewing

The Serious Risks, in Proportion

Pancreatitis

Acute pancreatitis means inflammation of the pancreas. It is an uncommon but potentially serious side effect associated with these medicines.

In January 2026, the MHRA strengthened the warnings after reviewing reports of severe cases, including rare reports of necrotising and fatal pancreatitis. Between 2007 and October 2025 it received 1,296 Yellow Card reports of pancreatitis associated with these medicines. Of those, 24 were recorded as necrotising pancreatitis and 19 were fatal. It also estimated around 25.4 million packs dispensed over the previous five years.

Those figures give context, but Yellow Card reports do not establish the exact incidence, or prove the medicine caused every reported event.

The symptom to know is severe, persistent abdominal pain, which can spread to the back and may come with nausea or vomiting. Seek urgent medical attention if it happens. If pancreatitis is suspected, the medicine should be stopped while it is investigated, and confirmed pancreatitis is a reason not to restart it.

Gallbladder Problems

Gallstones and other gallbladder problems are recognised with these medicines. Rapid weight loss itself also increases the risk of gallstones, so the effect of the medicine cannot always be separated from the effect of losing weight.

For semaglutide 2.4mg, gallstones were reported in 1.6% of patients in the clinical trial information, and cholecystitis in 0.6%. Tirzepatide trials also reported gallbladder-related events.

Warning signs include pain in the upper right abdomen, particularly after eating, along with fever or yellowing of the skin or eyes. Seek medical advice if these develop.

Dehydration and Your Kidneys

This matters particularly if nausea, vomiting or diarrhoea is making it hard to drink enough.

These side effects can cause dehydration, and in some people, particularly those with existing kidney problems, severe dehydration can worsen kidney function.

The medicine does not simply damage the kidneys in everyone. The concern is usually fluid loss. If you have kidney disease, take medicines affecting your kidneys, or are struggling to keep fluids down, contact your healthcare professional. Do not wait until you feel seriously unwell.

Do They Damage Your Liver?

There is no good basis for that claim. Liver safety depends on the individual medicine and the person.

Semaglutide is now licensed in the UK for certain adults with non-cirrhotic metabolic dysfunction associated steatohepatitis, known as MASH, with moderate to advanced liver fibrosis.

That does not mean everyone with liver disease can take it without assessment. Tell your prescriber about liver disease or abnormal liver tests so they can consider your circumstances.

Thyroid Cancer

This concern comes up often online, so it is worth separating the facts from the headlines.

UK product information for semaglutide notes that thyroid C-cell tumours were seen in rodent studies, and that the relevance of those findings to humans is not known. The current UK product information does not list thyroid cancer as a general contraindication.

If you have a personal or family history of thyroid disease or thyroid cancer, tell your prescriber so they can decide whether the medicine is appropriate.

Muscle Loss and Nutrition

When you lose weight, some of what you lose comes from lean tissue as well as fat. That is not unique to these medicines.

Because they reduce appetite, some people find it harder to eat enough protein and other nutrients. A healthcare professional or registered dietitian can help you plan meals that provide enough nutrition while you lose weight.

Strength or resistance exercise also helps maintain muscle. If you are new to exercise, start at a manageable level, and ask for advice if you have a condition affecting what is safe for you.

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Who Should Not Take Them

There is no single answer, because eligibility depends on the medicine, your BMI, related health conditions and your medical history.

For Wegovy and Mounjaro, adult weight management licences generally cover a BMI of 30 or above, or 27 to under 30 with at least one weight-related condition. NICE recommends lower thresholds when assessing people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African Caribbean backgrounds, because weight-related health risks can occur at a lower BMI.

BMI is only part of an assessment. A history of pancreatitis, gallbladder problems, significant kidney disease, liver disease, digestive conditions or an eating disorder needs particular consideration. Those do not all rule treatment out, but they should be discussed.

These medicines should not be used during pregnancy, and current MHRA guidance advises against using GLP-1 medicines while breastfeeding.

You can see the weight loss treatments available and what an assessment covers.

Are They Safe for Diabetics?

They can be used in people with type 2 diabetes, but other diabetes medicines need careful consideration.

Used with insulin or a sulfonylurea, the risk of low blood sugar increases, so your prescriber may adjust the dose of the other medicine. That is not something to change yourself.

People with diabetes should also watch for changes in eyesight. Semaglutide product information includes diabetic retinopathy complications as a possible effect in people with type 2 diabetes, so seek advice promptly for sudden or worsening vision changes.

Type 1 diabetes is different. Wegovy and Mounjaro are not substitutes for insulin, and decisions should be made by the professional managing your diabetes.

Pregnancy, Breastfeeding and Contraception

These medicines are not suitable during pregnancy, and the gap needed between stopping and trying to conceive depends on the medicine. For semaglutide, UK product information advises stopping at least two months beforehand. For tirzepatide, at least one month.

MHRA guidance advises against use while breastfeeding, because there is not enough safety information to know whether these medicines could harm a breastfed baby.

There is also a contraception point that is easy to miss. Tirzepatide can affect the absorption of oral contraceptives, and reduced effectiveness cannot be ruled out in people with overweight or obesity. UK product information advises switching to a non-oral method, or adding a barrier method, for four weeks after starting tirzepatide and for four weeks after each dose increase.

If contraception matters to you, make sure your prescriber knows which method you use before starting.

Are They Safe Long Term?

The longer-term picture is still being studied, because these medicines have not been used for weight management for decades.

There is substantial trial evidence, though. The SELECT trial included more than 17,000 adults with established cardiovascular disease and overweight or obesity, and semaglutide 2.4mg was associated with a lower rate of major cardiovascular events than placebo, with a hazard ratio of 0.80. That was a cardiovascular outcomes trial rather than a general test of every long-term safety question.

The MHRA continues to monitor these medicines and update safety information as evidence emerges, and the January 2026 pancreatitis update is an example of that.

So the accurate position is not that every risk is known. It is that these medicines have a substantial evidence base, while regulators continue to monitor them.

What Happens if You Stop

Weight regain after stopping is common, although the amount varies.

In a follow-up of 327 participants from a semaglutide weight management trial, people regained an average of 11.6 percentage points of body weight over the following year, around two thirds of what they had lost during treatment.

The same study found participants remained, on average, below their original starting weight. So stopping did not mean returning completely to the starting point, but much of the loss was regained.

That is why it is worth discussing the longer-term plan with your prescriber before starting. Weight management does not end when the injections stop.

Mounjaro and Wegovy Compared

There is no simple safer choice for everyone. Both have overlapping side effects, with differences in their product information. The right option depends on your medical history, other medicines, goals and individual risk factors.

These figures come from different studies with different participants and designs, so they are not a direct head-to-head comparison.

The Biggest Practical Risk: Buying Without a Prescription

Prescription weight loss medicines should be supplied through a legitimate healthcare and pharmacy service. Products from unregulated websites or social media may be fake, contaminated, incorrectly dosed, or contain ingredients not listed on the packaging.

The MHRA has reported fake Ozempic and Saxenda pens obtained through illegitimate routes. Some people who used them were hospitalised, and the products were suspected of containing insulin rather than semaglutide.

A low price or an easy purchase is not a reason to skip a proper medical assessment.

Checking a Supplier

1. Check the pharmacy is registered with the General Pharmaceutical Council, using the GPhC register
2. Look for clear information about the pharmacy and the healthcare professionals involved
3. Expect a proper assessment covering your weight, medical history, current medicines and health conditions
4. Check which medicine you are being prescribed, and understand how it should be used
5. Be cautious of anyone offering prescription weight loss medicines through social media or messaging apps

A genuine service should be willing to tell you when a medicine is not suitable for you.

What a Proper Online Consultation Involves

It should ask about your height and weight, weight history, existing conditions, current medicines and supplements, relevant medical history, and pregnancy plans and contraception where appropriate.

The prescriber reviews your answers and decides whether treatment is suitable. They may ask for more information, or decide a different approach would be better.

If treatment is prescribed, it should be supplied through a legitimate pharmacy. Some injections need refrigerating, so you should receive clear instructions about storage and delivery.

When to Get Urgent Medical Help

• Severe or persistent abdominal pain, particularly if it spreads to your back with nausea or vomiting
• Severe pain in the upper right abdomen, particularly with fever or yellowing of the skin or eyes
• Severe vomiting or diarrhoea, or being unable to keep fluids down
• Symptoms of very low blood sugar, such as shaking, sweating, confusion, weakness or feeling faint, particularly if you also take insulin or a sulfonylurea
• Swelling of the face, lips, tongue or throat, or difficulty breathing, which can indicate a serious allergic reaction
• Sudden loss of vision, or a significant change in your eyesight
• A pregnancy while taking one of these medicines, so you can speak to your prescriber about what to do next

If you are unsure whether a symptom relates to your medicine, ask a healthcare professional rather than stopping or changing treatment on your own.

Medical Disclaimer

This page provides general information about the safety of licensed weight loss injections. It does not provide personalised prescribing or dosing instructions, and it is not a diagnosis.

These medicines should be used under the supervision of an appropriately qualified prescriber. Suitability depends on an individual clinical assessment, and results vary between individuals.

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