Diarrhoea is a very common side effect of Mounjaro and Wegovy, and reflux, bloating, wind and burping are common. They are usually mild to moderate, tend to be most noticeable when you start treatment or increase your dose, and often settle as your body adjusts. What helps most: smaller meals, less fat, spice and alcohol, avoiding fizzy drinks, staying upright after eating, and keeping up fluids. Speak to your pharmacist or prescriber if symptoms persist, if you cannot keep fluids down, or if you notice blood in your stool, black stools or severe stomach pain.
Mounjaro (tirzepatide), Wegovy (semaglutide) and Foundayo (orforglipron) work in part by slowing gastric emptying, so food stays in the stomach longer and you feel full sooner. They also act on receptors in the gut and brain that influence appetite and digestion. This is helpful for appetite, but it can also cause digestive side effects, because the whole digestive tract is working at a different pace.
The UK product information lists nausea, vomiting, diarrhoea and constipation as very common side effects, and dyspepsia (indigestion), abdominal distension (bloating), burping, wind and gastro-oesophageal reflux as common. In most people these are mild to moderate, and they tend to ease after the first weeks or after each dose increase. Full lists are in our Mounjaro, Wegovy and Foundayo side effects guides.
| Symptom | Frequency in UK product information | Typical pattern |
|---|---|---|
| Diarrhoea | Very common | Loose or frequent stools, often worst after a dose increase |
| Bloating (abdominal distension) | Common | Feeling full, tight or gassy, often after meals |
| Wind (flatulence) and burping (eructation) | Common | More wind or burping, sometimes with an eggy or sulphur smell |
| Reflux and indigestion | Common | Burning behind the breastbone, sour taste, worse after large meals or lying down |
| Nausea, vomiting, constipation | Very common | See our nausea and constipation guides |
In the trials, around 30% of people taking the highest dose of Wegovy reported diarrhoea, which shows how frequently it occurs. The same trials also showed that digestive side effects were mostly mild to moderate and tended to settle over time. Our tolerability guides for Mounjaro and Wegovy explain the pattern in more detail.
Why it happens
Changes in gut movement and secretion can lead to looser or more frequent stools. It is most often seen when you first start, after a dose increase, or after a meal that is large or rich in fat. Some people alternate between diarrhoea and constipation, which is common as the gut adjusts.
Taking a weekly injection? See the injection technique guide and Mounjaro storage and Wegovy storage guides. Constipation is the flip side: our constipation guide and nausea guide cover it.
What helps
- Eat smaller meals and avoid very large or high-fat meals, which are a common trigger.
- Limit greasy, fried and very spicy food while symptoms last.
- Reduce caffeine, alcohol and sugar-free sweeteners such as sorbitol, which can loosen stools. See our alcohol guides for Mounjaro, Wegovy and Foundayo.
- Choose simple, bland foods such as rice, bananas, toast, plain potatoes and cooked vegetables until things settle.
- A fibre supplement that bulks the stool, such as psyllium, helps some people. Introduce it gradually and ask your pharmacist first.
- Ask your pharmacist about short-term anti-diarrhoeal medicines if you need relief. Do not use them for more than a short period without advice.
Diarrhoea can cause dehydration, particularly if you are also eating and drinking less because your appetite is low. The UK product information notes that digestive side effects may lead to dehydration, which in rare cases can affect kidney function, and advises taking precautions against fluid loss. Sip fluids steadily through the day, and consider an oral rehydration solution if diarrhoea is more than mild.
| Sign of dehydration | What it means |
|---|---|
| Dark urine, passing little urine | Your body needs more fluid |
| Dry mouth, thirst, headache | Mild dehydration, increase fluids |
| Dizziness on standing, fast heartbeat | Seek advice Possible significant dehydration |
| Unable to keep fluids down | Promptly Seek medical advice |
Because food moves more slowly through the stomach and gut, it sits longer and is fermented by gut bacteria, which can produce gas. This is thought to explain why many people notice bloating, wind and burping, including the eggy or sulphur-smelling burps that people often describe. They are unpleasant but usually harmless, and they tend to improve as your body adjusts.
What helps
- Eat slowly and chew well, which reduces swallowed air.
- Choose smaller portions and stop at the first sign of fullness.
- Limit fizzy drinks, chewing gum and drinking through straws, which all add air.
- Notice trigger foods. Beans, lentils, onions, cabbage and broccoli are healthy but gassy, so build them up gradually.
- Sugar-free sweeteners and sugar alcohols can cause gas and loose stools.
- A gentle walk after meals helps move gas through.
- Peppermint or ginger tea settles some people’s stomachs.
Some people notice more sulphur burps after protein-rich foods and drinks, such as eggs or protein shakes. If you notice a pattern, try varying your protein sources rather than cutting protein out. Our nutrition guide has ideas.
Reflux and IndigestionSlower stomach emptying can increase pressure in the stomach, which makes acid reflux more likely, particularly after large meals. Heartburn, a sour taste and burning behind the breastbone are common descriptions.
| Do | Why it helps |
|---|---|
| Eat smaller meals, and stop before you are full | Less volume means less pressure on the valve at the top of the stomach |
| Avoid eating within 2 to 3 hours of lying down | Gravity helps keep stomach contents down |
| Raise the head of your bed slightly | Reduces night-time reflux |
| Limit alcohol, coffee, spicy and fatty foods | These are common triggers |
| Avoid tight clothing around the waist | Reduces pressure on the stomach |
Over-the-counter antacids or alginates can give short-term relief, and your pharmacist can advise which suits you and your other medicines. If you need them most days, mention this to your prescriber.
When to get advice
Reflux that is severe, keeps coming back, or comes with difficulty swallowing, vomiting blood, black stools or unexplained weight loss needs medical review. Chest pain should never be assumed to be reflux. If you have chest pain, especially with breathlessness or sweating, call 999.
| If you mainly have... | First steps | Speak to someone if... |
|---|---|---|
| Loose stools | Smaller, lower-fat meals; bland foods; fluids; limit caffeine and alcohol | It lasts more than a few days, or you feel dehydrated |
| Bloating and wind | Slow eating; fewer fizzy drinks; walk after meals; vary protein sources | It is severe or painful |
| Reflux | Smaller meals; stay upright; avoid late eating and alcohol | You need antacids most days |
| A mixture | Keep a short food and symptom diary | Symptoms are not settling after a dose increase |
Digestive effects are most likely when you start treatment and after each dose increase, then tend to settle. If a dose step is not settling, your prescriber may suggest staying on the current dose for longer before moving up, or in some cases stepping back. This is a normal and sensible adjustment, not a failure. See the Mounjaro, Wegovy, Foundayo and Wegovy pill dose guides. If you are on a tablet such as the Wegovy pill or Foundayo, follow the instructions for taking it, because timing and what you take it with can affect how you feel.
Other Things to Be Aware Of- Contraceptive pills. Severe diarrhoea or vomiting can reduce how well an oral contraceptive pill works, whatever medicine you take. Follow the missed-pill advice in your pill’s leaflet and use extra protection if it applies.
- Other oral medicines. Diarrhoea can affect how well tablets are absorbed. Tell your pharmacist if you take medicines where timing matters, such as thyroid tablets, or medicines with a narrow safety margin.
- Existing digestive conditions. If you have irritable bowel syndrome, inflammatory bowel disease, coeliac disease or significant reflux, mention it in your consultation.
- Food intolerance. Lactose and gluten sensitivity can look like GLP-1 side effects, so a diary helps separate them.
| Warning sign | What to do |
|---|---|
| Diarrhoea lasting more than a few days, or with signs of dehydration | Soon Contact your prescriber or pharmacist |
| Blood in your stool, or black, tarry stools | Promptly Seek medical advice promptly |
| Severe, persistent abdominal pain, sometimes spreading to your back, with vomiting | Urgent Seek urgent medical attention, as it can be a sign of pancreatitis |
| Pain in the upper right abdomen, fever or yellowing of skin or eyes | Same day Seek medical advice, as it can be a sign of gallbladder problems |
| Unable to keep fluids down, or very little urine | Urgent Seek urgent advice, as dehydration can affect the kidneys |
| Swelling of the face, lips or throat, or difficulty breathing | 999 Call 999 |
- Is what I am experiencing expected at my dose?
- Which anti-diarrhoeal or antacid is suitable alongside my other medicines?
- Should I stay on my current dose for longer before increasing?
- Could any of my other medicines or supplements be contributing?
The bottom line
Diarrhoea, bloating, burping and reflux are common and usually temporary on GLP-1 medicines. Smaller, lower-fat meals, fewer triggers, steady fluids and good timing around dose increases help most. Get advice if symptoms persist or any warning sign appears. Related reading: Mounjaro vs Wegovy, Foundayo vs Mounjaro, Mounjaro plateaus, Wegovy plateaus, what happens when you stop, microdosing and our full guide library. Not sure a pharmacy is genuine? See Is Happy Pharmacy a genuine UK pharmacy?
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Start Your Free Online Consultation → Use code SWITCH10 if you're switching from another provider Happy Pharmacy is a GPhC-registered pharmacy (No. 9013192) — Superintendent Pharmacist: Palvinder Deol — Rated 4.8★ Excellent on Trustpilot- Mounjaro® (tirzepatide) KwikPen: Summary of Product Characteristics, sections 4.4 and 4.8 (gastrointestinal adverse reactions, dehydration). electronic Medicines Compendium (emc).
- Wegovy® (semaglutide) FlexTouch: Summary of Product Characteristics, sections 4.4 and 4.8. emc.
- Foundayo® (orforglipron): Summary of Product Characteristics. emc.
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021;384:989-1002.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022;387:205-216.
- Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society. 2025.
- NHS. Diarrhoea and indigestion: self-help and when to get help.