- Gut symptoms are the most common side effects of GLP-1 weight-loss medicines and tend to ease over the first weeks to months.
- They are usually worst on starting and after each dose step-up — which is exactly why the dose is normally started low and increased gradually by your prescriber.
- Ease nausea with smaller, plainer, lower-fat meals, eating slowly, and sipping fluids — not by changing your dose on your own.
- Red flags: severe, persistent stomach or back pain (possible pancreatitis or gallbladder problems), and dehydration from vomiting or diarrhoea you cannot keep on top of — seek urgent help.
- Report side effects via the MHRA Yellow Card scheme, and speak to your pharmacist or GP if anything bothers you.
Which medicines this covers
“Weight-loss jabs” usually means the GLP-1 receptor agonists. They share the same broad side-effect profile because they act on the same gut–brain pathways and slow how quickly your stomach empties. In the UK the main ones are:
| Brand | Active ingredient | Licensed UK use |
|---|---|---|
| Mounjaro | Tirzepatide | Type 2 diabetes and weight management |
| Wegovy | Semaglutide | Weight management (obesity) |
| Ozempic | Semaglutide | Type 2 diabetes |
For how these differ, how to access them and current UK supply, see our guides to Mounjaro availability, price and NHS rollout and Wegovy & Ozempic availability. You can follow official supply signals on our pages for tirzepatide and semaglutide.
Why the gut side effects happen — and why they usually settle
GLP-1 medicines slow down how fast food leaves your stomach and act on appetite signalling. That is part of how they work — but it is also why food can sit heavier, why large or rich meals feel worse, and why nausea, being sick, constipation and diarrhoea are so common at the start.
The NHS lists the common side effects of tirzepatide as feeling sick (nausea) or being sick (vomiting), constipation or diarrhoea, stomach pain, feeling tired, itchy or red skin where you inject, and hair loss. For semaglutide the NHS list is very similar. These are typically worst when you first start and shortly after each dose increase.
The reassuring part: for most people the symptoms are strongest in the early months and then improve. NHS obesity-service patient information describes side effects such as nausea being more noticeable in roughly the first six months and generally getting better over the following six to twelve months as “your body will have adjusted to the medicine.” This is also why the dose is normally started low and stepped up gradually by your prescriber — a slower climb gives your gut time to adapt.
Easing nausea and feeling sick
The single most common complaint is nausea. NHS weight-management and diabetes services suggest practical, non-medicine steps that many people find help:
- Eat small, regular meals and stop as soon as you feel full — that may be a much smaller portion than you are used to. An empty stomach can also make nausea worse, so try not to skip meals entirely.
- Choose plain, dry, lower-fat foods when you feel sick — things like toast or crackers. Fried, fatty, very sweet, spicy or strong-smelling foods tend to make it worse.
- Eat slowly and sit upright and relaxed for a while after eating rather than lying down.
- Sip fluids between meals rather than gulping large amounts with food; sugar-free drinks are fine. Some people find ginger or peppermint tea settles the stomach.
- Go easy on alcohol. The NHS notes alcohol can increase side effects like feeling or being sick while on these medicines.
Managing diarrhoea and constipation
Both ends of the spectrum are common, and both respond to simple changes. NHS patient guidance suggests:
| Symptom | What often helps |
|---|---|
| Diarrhoea | Keep well hydrated — aim for at least around 2 litres of fluid a day to avoid dehydration. Have small, frequent meals rather than large ones, and choose soft, easily digested foods (for example white fish, mashed potato, yoghurt). Very fatty, fried or spicy foods can make it worse. |
| Constipation | Aim for plenty of fluid (again around 2 litres a day), plenty of fruit and vegetables, wholegrains, beans and pulses, and gentle activity. Some services suggest adding a tablespoon of ground flaxseed/linseed. Ask your pharmacist before using any laxative. |
If a common side effect is severe, does not settle, or you are unable to eat or are eating very little, don’t just push through it — contact your prescriber, pharmacist or GP for advice.
Red flags: when gut symptoms signal something serious
Most gut side effects are unpleasant but not dangerous. A few are, and they need prompt attention. Contact your GP, NHS 111, or emergency services as appropriate if you notice:
| Warning sign | What it may mean | What to do |
|---|---|---|
| Severe pain in your stomach or back that does not go away, often with being sick | Inflamed pancreas (acute pancreatitis) | Get urgent medical help |
| Severe tummy pain, sometimes with a high temperature or yellowing of the skin or eyes | Gallstones or gallbladder inflammation (cholecystitis) | Get medical advice promptly |
| Persistent vomiting or diarrhoea you cannot keep on top of — very little urine, dark urine, dizziness, extreme thirst | Dehydration (which can also strain the kidneys) | Contact your GP or NHS 111; seek urgent help if severe |
| Swollen throat or tongue, a raised itchy rash, or difficulty breathing | Serious allergic reaction (anaphylaxis) | Call 999 / emergency help |
| Shakiness, sweating, confusion, fast heartbeat, feeling very hungry (mainly if you also take insulin or a sulfonylurea) | Low blood sugar (hypoglycaemia) | Treat the low; speak to your diabetes team |
Vision changes: a separate warning worth knowing
Gut symptoms are not the only side effect to watch for. On 5 February 2026 the MHRA issued a drug safety update warning that semaglutide may very rarely be associated with an eye-nerve condition (NAION), and advises that any sudden change in your eyesight during treatment needs urgent eye care (attend eye casualty or A&E). That is covered in detail in our guides to GLP-1 medicines and NAION vision loss and semaglutide side effects.
Reporting side effects
Report any suspected side effect through the MHRA Yellow Card scheme. Reporting helps the MHRA monitor the safety of these medicines. For plain-English overviews, see the NHS pages on tirzepatide and semaglutide, and always follow the patient information leaflet in your pack.
Related reading
Mounjaro (tirzepatide) in the UK
Availability, price changes and the NHS rollout.
Wegovy & Ozempic in the UK
The difference, access routes, shortages and safety essentials.
Does it cause muscle loss?
What the trials show about GLP-1 medicines and lean mass.
Orforglipron: the oral GLP-1
Where the once-daily tablet stands for UK availability.
GLP-1 and pancreatitis
The safety signal and what the red-flag symptoms are.
Tirzepatide side effects
The fuller UK side-effect picture for Mounjaro.
Stay ahead of weight-loss jab supply changes
MediWatch checks official DHSC and NHS data daily and alerts you if your medication is affected.
Search shortages free →Official sources: NHS: tirzepatide (common and serious side effects, alcohol) · NHS: semaglutide (side effects) · Cambridge University Hospitals NHS FT: dietary advice for managing tirzepatide side effects · Cambridge University Hospitals NHS FT: obesity treatment with tirzepatide (side effects over time) · Guy’s & St Thomas’ NHS FT: dietary advice on weight-loss medicines · MHRA Yellow Card scheme
MediWatch is not medical advice. Always follow your prescription label and patient information leaflet, and ask a pharmacist, GP, specialist, NHS 111, or emergency services if you are unsure or unwell. Data checked daily against official sources.