GLP-1 Side Effects UK

Most side effects of GLP-1 medicines are about digestion: feeling sick, being sick, loose stools or constipation. They are usually strongest in the first weeks and again when the dose goes up. A few effects are rarer and more serious, and this page shows you which is which. It covers the class as a whole, including semaglutide, tirzepatide and liraglutide, and sends you to the Mounjaro and Wegovy guides for brand detail.

Check if you need help now See the side effects table

Based on MHRA guidance and UK patient leaflets

We compare medicines. We do not prescribe or sell them

Signs that need help now

The first three rows are emergencies. For the others, call NHS 111 or your prescriber the same day.

What you notice	What it may mean	What to do
Severe tummy pain that does not ease, often spreading to your back	Pancreatitis. The MHRA names this as the main sign	Get urgent medical help. Call 999 if the pain is severe
Swelling of the face, lips, tongue or throat, trouble breathing, a fast spreading rash	Serious allergic reaction	Call 999
Sudden loss of sight or eyesight getting worse fast	A very rare eye problem linked to semaglutide	Contact a doctor urgently. If you cannot reach one, call 111, go to an eye casualty or A&E
Sickness, vomiting or diarrhoea that stops you drinking, or you pass very little urine	Dehydration, which can harm the kidneys	Call 111 or your prescriber today
Pain high on the right of your tummy, fever, yellow skin or eyes	Gallbladder problems	Call 111 or your GP today
Shaking, sweating, confusion, a pounding heart	Low blood sugar. The MHRA says this can happen even without diabetes with some of these medicines	Get urgent advice from 111 or your prescriber. Call 999 if you pass out

If your provider only offers a chat form or email, use 111 or 999. Do not wait for a reply.

What are the side effects of GLP-1?

The table groups the side effects of GLP-1 medicines by how often the MHRA and the UK leaflets say people get them. Rates differ by brand, so your own patient information leaflet, or the summary of product characteristics, is the final word.

How often	What people notice	Where it comes from
More than 1 in 10 people	Nausea, vomiting, diarrhoea, constipation	MHRA, October 2024
Common, and listed on some leaflets	Indigestion, reflux, bloating, wind, burping, dizziness, headache, tiredness, soreness at the injection spot, hair loss	Individual UK leaflets, which differ by brand
From 1 in 100 to 1 in 10,000	Pancreatitis, gallbladder problems	MHRA, October 2024
Rare	Serious allergic reaction. A very rare sudden loss of sight with semaglutide	UK leaflets and MHRA guidance
People without diabetes	Low blood sugar, named in the Saxenda and Wegovy product information	MHRA, October 2024

Open the leaflet for your medicine

GLP-1 weight loss side effects

Losing weight fast can bring tiredness and hair thinning, and it is linked to gallstones, so the gallbladder is on the leaflets. Small meals, steady drinks and a slow pace help many people. Tell your prescriber about hair loss, or pain on the upper right of your tummy.

GLP-1 injection side effects

A sore, red or itchy spot, or a small lump where you inject, is common and usually fades. Move the spot each week. A rash that spreads quickly with swelling or breathing trouble is an emergency, so call 999.

Headache, skin and heart

Some leaflets list headache as very common. A faster heartbeat is listed on some, and rashes or itching are most often tied to the injection spot. Chest pain or fainting needs urgent help, so call 999 or 111.

Side effects of GLP-1 receptor agonists: why they happen

GLP-1 is a hormone your gut releases after a meal. It tells the brain you are full, and it slows stomach emptying. These medicines copy that signal, which is why appetite is reduced. The same slowing explains most of the gastrointestinal side effects people feel: fullness, reflux, bloating and sickness. It is also why you should tell an anaesthetist you take one before surgery.

GLP-1 agonist side effects and the GIP link

Tirzepatide acts on a second hormone called GIP as well as GLP-1. The stomach effects are shared, and the details differ by medicine.

GLP-1 only. Semaglutide (Wegovy) and liraglutide (Saxenda)

GLP-1 and GIP. Tirzepatide (Mounjaro)

Compare them in the table below

How long do GLP-1 side effects last?

The MHRA says stomach effects can last several days. Most people build tolerance as the body gets used to the medicine, but a rough patch can return each time the dose goes up on the step up schedule. Drink well, especially after being sick or having loose stools.

When	What to expect	What helps
First weeks	Sickness, fullness and loose or hard stools are most likely	Small, plain meals and steady sips of water
After a dose increase	The same effects can return for a few days	Tell your prescriber if they are hard to cope with. They can hold you on the same dose
Not easing, or getting worse	Effects that last well beyond the usual settling in, or new ones	Speak to your prescriber or pharmacist. Do not change your dose yourself

GLP-1 side effects long term

Many of these medicines have only been used for weight loss in the UK for a few years, so the record over decades is still being built. What is known is that gallbladder problems and pancreatitis can arise during treatment, and hair thinning is often noticed while weight falls. The MHRA keeps watching the safety data and updates the leaflets when it learns more. Tell your prescriber about anything new, keep your review appointments, and report it through Yellow Card.

Yellow Card numbers show what people report, not how common a problem is. The MHRA says so itself, so treat any total you read online with care.

Which GLP-1 has the fewest side effects?

No fair ranking exists. The medicines were tested in different trials on different people, so numbers cannot be lined up honestly. This GLP-1 side effects comparison uses what the MHRA and the leaflets say, not head to head results. A medicine is not safer because it has fewer lines in a table.

GLP-1 drugs side effects compared

What we compare	Semaglutide (Wegovy)	Tirzepatide (Mounjaro)	Liraglutide (Saxenda)
Acts on	GLP-1	GLP-1 and GIP	GLP-1
Stomach effects	Yes, as for the whole class	Yes, as for the whole class	Yes, as for the whole class
Sudden loss of sight	Very rare. The MHRA links it to semaglutide only	Not named by the MHRA	Not named by the MHRA
Low blood sugar without diabetes	In the product information	Not named in the MHRA update	In the product information
Stop before trying for a baby	At least 2 months	At least 1 month	No gap needed. Stop just before trying
Birth control pill	No extra step named	Add a barrier method for 4 weeks after starting and after each dose increase	No extra step named

Source: MHRA guidance updated 5 February 2026 and the October 2024 Drug Safety Update.

Read the Mounjaro guide Read the Wegovy guide

Who should not take GLP-1 medicines?

These medicines are prescription only and should be used under medical supervision. Some people need a doctor’s view before starting, and some should not use them at all.

GLP-1 side effects and pregnancy

The MHRA says do not take a GLP-1 medicine if you are pregnant, trying to get pregnant or breastfeeding. If you could become pregnant, use contraception, and plan the stop period shown in the comparison table. If you are on Mounjaro and use the combined pill, add a barrier method such as condoms for four weeks after starting and after each dose increase. If you find you are pregnant, speak to a healthcare professional straight away.

People who need a doctor’s view first

Tell your prescriber if you have had pancreatitis or gallbladder disease, have thyroid problems, take insulin or other diabetes medicine, are allergic to any ingredient, or are due an operation. The leaflet for your medicine lists the full set.

Specific worries, answered

Cancer. In studies, thyroid tumours appeared in rodents given semaglutide and tirzepatide. It is not known whether people are affected. Tell your prescriber about any thyroid history or a lump in your neck.

Liver. The MHRA guidance does not name liver damage as a main side effect. Yellow skin or eyes can point to the gallbladder, so call 111 or your GP the same day.

Mental health. Mood changes are not named as a side effect in the MHRA guidance. Big changes to eating and weight can still affect how you feel. Tell your GP if your mood, sleep or anxiety shifts.

Depression. If you feel low or hopeless for more than a few days, tell your GP. If you think of harming yourself, call 999, or ring Samaritans free on 116 123.

Hair loss. Some UK leaflets list it as common, and it is often noticed while weight falls. Mention it to your prescriber if it worries you.

Ozempic face. This is a popular name for a thinner, more lined face after fast weight loss. It comes from losing facial fat, and it is not a side effect listed for one brand. Ozempic is licensed for diabetes, and the MHRA has not assessed these medicines for cosmetic use.

Is GLP-1 weight loss safe?

Licensed GLP-1 medicines have been judged by the MHRA to have benefits that outweigh the risks for the people they are licensed for. Safe does not mean free of side effects. The MHRA stresses that they are not a quick way to lose weight, and that benefits and risks outside the licence have not been studied. Your prescriber weighs them with you.

Buy safely and report a side effect

Fake and unlicensed pens exist, and some falsified weight loss pens have been found to contain insulin. Real products come as pre filled pens or tablets. Powder in vials is not authorised. A pen can look genuine and still be fake, so appearance proves nothing. Buy only on a prescription from a registered pharmacy, and check it on the GPhC register. Do not buy from beauty salons or social media sellers.

Anyone can report a suspected side effect, or a suspected fake product, to the MHRA through the Yellow Card scheme. This includes patients and carers.

Open Yellow Card

FAQs

Sickness, vomiting, diarrhoea and constipation. The MHRA says they can affect more than 1 in 10 people, and they usually settle.

The MHRA says they can last several days. They tend to return for a short time after each dose increase. Speak to your prescriber if they hang on or stop you drinking.

No trustworthy ranking exists, because the trials are different. The comparison table on this page shows where the medicines really differ, such as the eye warning and the stop period before pregnancy.

Licensed medicines used under a prescription are judged to have more benefit than risk for the people they are licensed for. They still have side effects, and some are serious.

Anyone who is pregnant, trying to get pregnant or breastfeeding. Others, such as people who have had pancreatitis, should talk to a doctor first.

They can return for a short time after an increase. If they are hard to bear, your prescriber can pause on the same dose. Do not change it yourself.

Search the leaflet for your medicine, the MHRA and NHS pages, or ask a pharmacist. One person’s story on social media does not show how common a problem is.

No. Only licensed pens and tablets are authorised in the UK, and vials of powder are not. Unlicensed products carry none of the safety checks a licensed medicine has.

Sources

Patient information leaflets and summaries of product characteristics, electronic medicines compendium
NHS 111 — when to use it, and how
MHRA Yellow Card scheme — reporting a suspected side effect
Mounjaro side effects and Wegovy side effects — brand detail
Medication safety checklist