# Nausea, constipation and sulphur burps: managing the gut effects Updated 24 August 2026 ยท Written by the Penwise team Almost every side effect people report on these drugs is gastrointestinal, and almost all of it comes from one mechanism: the stomach empties more slowly. That is not a malfunction, it is a large part of how the drugs work. Knowing which effect comes from which cause makes the difference between coping with something manageable and enduring something that a dose adjustment would have fixed. ## Why the gut is where this lands GLP-1 receptor agonists slow gastric emptying, which is why a smaller meal keeps you full for longer. The same slowing means food sits in the stomach longer than your body is used to, and that produces nausea, early fullness, reflux and belching. Tirzepatide adds GIP receptor activity, which brings its own gut effects on top. Two patterns follow from this and they are worth recognising, because they tell you what is happening: - **The days after a shot are worse than the days before.** Effects cluster in the first two to three days, when the drug level peaks. - **The fortnight after a dose increase is worse than the fortnight before it.** Each new rung reintroduces the effects until the body adapts. Both usually fade with time at a stable dose. The [side effects timeline](https://penwiseapp.com/en/guides/glp-1-side-effects-timeline) covers the full arc, and the [titration schedules](https://penwiseapp.com/en/guides/glp-1-dose-schedule) explain why the steps are four weeks apart. ## Nausea The most common and usually the first. What generally helps, and none of it is exotic: - **Eat less at a sitting, more often.** The stomach is emptying slowly, so a normal portion is now a large one. - **Stop at the first sign of fullness.** On these drugs that signal arrives earlier and is easier to overshoot than it used to be. Overshooting it is the single most common cause of an evening of nausea. - **Eat slowly.** The fullness signal lags, so fast eating means you are past it before you notice. - **Go easy on fatty, greasy and heavily fried food.** Fat slows gastric emptying further, on top of the drug. - **Cold or room-temperature food is often better tolerated than hot food**, largely because it carries less smell. - **Drink between meals rather than with them**, so liquid is not adding to a stomach that is already slow to empty. - **Plan the first two days after a shot.** If your worst day is day two, that is not the day for a heavy dinner out. > If nausea is bad enough that you cannot keep fluids down, that is not a coping problem. Dehydration is the mechanism behind most of the serious complications people end up in hospital with on these drugs, and it needs medical attention rather than patience. ## Constipation The second most common, and the one people are slowest to mention. Slower gut transit plus a sharply reduced food intake plus less fluid is a reliable recipe for it. - **Fluid first.** Appetite suppression reduces drinking as well as eating, and most people on these drugs are drinking less than they think. - **Fibre, increased gradually.** A sudden large increase in fibre without fluid makes constipation worse rather than better. - **Movement.** Even a walk after meals measurably helps transit. - **Ask your pharmacist before reaching for a laxative.** They will tell you which type suits this situation and which to avoid, and it is a thirty-second conversation. Constipation that persists for more than a few days, or comes with severe pain, bloating and vomiting, is worth a call rather than another day of waiting. ## Sulphur burps The one nobody warns people about, and the one that generates the most alarmed searching. Burps that taste and smell of rotten eggs, often with a bad stretch of stomach discomfort or diarrhoea after them. They are most commonly reported on tirzepatide. The mechanism is not formally established, and the leading explanation follows directly from the drug: food sitting longer in a slow stomach gives gut bacteria more time to work on it, and protein-rich food fermented that way produces hydrogen sulphide, which is exactly what rotten eggs smell of. What people generally find helps: 1. **Smaller portions, particularly of protein-heavy and high-fat meals eaten late.** A large steak dinner at nine in the evening is close to the worst case. 2. **Do not eat close to bedtime.** Lying down on a stomach that has not emptied makes everything about this worse. 3. **Watch which foods precede an episode.** For most people it is a short and repeatable list, and it is not the same list for everyone. 4. **Fluids and gentle movement after eating.** 5. **Tell your prescriber if it is frequent.** It is a recognised complaint, and when it does not settle it is often a dose conversation. ## Reflux, burping and early fullness All three come from the same slowed stomach. Eating smaller and earlier helps all of them, and not lying flat within a couple of hours of a meal helps reflux specifically. Persistent reflux is worth raising rather than living with, both because it is treatable and because chronic reflux is not something to leave running for months. ## Diarrhoea Less common than constipation but real, and it sometimes alternates with it. The priority when it happens is fluid, and if it is severe or lasts more than a couple of days it needs a conversation, because dehydration is the thing that turns a tolerable side effect into a hospital visit. ## The symptoms that are not side effects to manage Most gut effects on these drugs are unpleasant and self-limiting. A few are not, and they are worth knowing by name so you recognise them rather than trying to eat around them. - **Severe, persistent abdominal pain**: Particularly pain in the upper abdomen that bores through to the back, with or without vomiting. This is the classic presentation of pancreatitis and it needs urgent medical assessment, not a wait-and-see. - **Pain under the right ribs after eating**: Especially with fever, or yellowing of the skin or eyes. Rapid weight loss raises gallstone risk independently of the drug, and gallbladder problems present this way. - **Vomiting you cannot stop**: If nothing stays down, dehydration follows quickly and it is the route to most of the serious complications. Get help the same day rather than the next one. - **Symptoms that appear suddenly at a stable dose**: The usual pattern is worse after a shot and worse after an increase. Something new and severe at an unchanged dose does not fit that pattern and deserves to be checked. This page is not medical advice and cannot be. It is a description of what commonly happens and what people commonly find helps, so you know what is ordinary and what is not. ## When it is a dose question A great deal of unnecessary suffering on these drugs comes from treating the titration ladder as a target that must be reached. It is not. Both ladders are designed so the dose can stop wherever effect and tolerability balance out. The conversation to have with your prescriber is easier when it is specific. Not "the side effects are bad", but: at 10 mg, days one to three, nausea at six out of ten, three episodes of sulphur burps a week, and it has not improved across four weeks at this dose. That is a description a clinician can act on, and it usually leads to one of three things: staying at the current rung longer, dropping a rung, or slowing the next increase. ## What to track, and why it is the whole point - The symptom, with a severity you use consistently, on the day it happened. - How many days after your shot it fell. Three or four weeks of that shows your own pattern unmistakably. - What dose you were on and how long you had been on it. - What you had eaten, when the effect is food-linked. For sulphur burps particularly, this usually finds the trigger within a fortnight. - Whether it is improving across weeks at the same dose, because that is the single fact that decides whether to wait or to change something. Penwise records symptoms against the day and the dose, so the pattern relative to your injection day is visible rather than remembered, and a report you can hand to a prescriber comes out of the same data. The [injection day](https://penwiseapp.com/en/guides/glp-1-injection-day) page covers placing the difficult days where they cost you least. **Turn "the side effects are bad" into something specific** Symptoms logged against dose and day, with the pattern visible and a doctor-ready report. Free core tracker, no account. ## Common questions **What causes sulphur burps on Mounjaro?** The mechanism is not formally established. The leading explanation is that slowed gastric emptying leaves food in the stomach longer, giving gut bacteria more time to ferment it, and protein fermented that way produces hydrogen sulphide, which is what smells of rotten eggs. Smaller and earlier meals, less high-fat and heavy protein late in the day, and identifying your own trigger foods are what people most often find helps. **How long does nausea last on a GLP-1?** It usually clusters in the first two to three days after a shot and in the fortnight after each dose increase, and it usually fades over a few weeks at a stable dose. Nausea that does not improve after four weeks at the same dose is a reason to talk about the dose rather than to keep waiting. **What helps constipation on Ozempic or Mounjaro?** Fluid first, since appetite suppression reduces drinking as well as eating, then a gradual increase in fibre and regular movement. Ask your pharmacist before choosing a laxative, because which type suits slowed gut transit is worth thirty seconds of advice. **Do GLP-1 side effects get better over time?** For most people, yes. The gut effects are worst in the first weeks and in the fortnight after each dose increase, and they generally settle with time at a stable dose. What does not settle after several weeks at one dose is usually addressed by staying at that rung longer or stepping down. **When should I contact a doctor about GLP-1 side effects?** Urgently for severe persistent abdominal pain, especially upper abdominal pain radiating to the back, for pain under the right ribs with fever or yellowing of the skin or eyes, and for vomiting you cannot stop. Also for anything severe that appears suddenly at an unchanged dose, since that does not fit the usual pattern. **Does eating less fat reduce GLP-1 nausea?** It often helps, because fat slows gastric emptying on top of the drug that is already slowing it. Smaller portions, eating slowly, stopping at the first sign of fullness and not eating late are the changes most people find make the largest difference. ## Sources Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document. - [Mounjaro: EPAR product information](https://www.ema.europa.eu/en/medicines/human/EPAR/mounjaro), European Medicines Agency - [Wegovy: EPAR product information](https://www.ema.europa.eu/en/medicines/human/EPAR/wegovy), European Medicines Agency - [Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)](https://doi.org/10.1056/NEJMoa2206038), New England Journal of Medicine, 2022 - [Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1)](https://doi.org/10.1056/NEJMoa2032183), New England Journal of Medicine, 2021 Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company. ## Keep reading - [The GLP-1 side effect timeline: what lands when](https://penwiseapp.com/en/guides/glp-1-side-effects-timeline): Nausea, fatigue, constipation and reflux do not arrive at random. Here is where they usually sit in the week, in the ladder and in the treatment. - [GLP-1 dose schedules: every ladder, and why the steps are four weeks](https://penwiseapp.com/en/guides/glp-1-dose-schedule): The titration ladder for every GLP-1 product side by side, and the pharmacology that decides how long each step has to last. - [GLP-1 medications before surgery, endoscopy and anaesthesia](https://penwiseapp.com/en/guides/glp-1-before-surgery): A stomach that empties slowly is a real anaesthetic problem. Why holding doses is now standard guidance, how far ahead, and why nobody should stop on their own. - [Alcohol on a GLP-1: what changes and why](https://penwiseapp.com/en/guides/alcohol-on-glp-1): A lot of people find they simply want to drink less. Why that happens, what a slowed stomach does to how alcohol hits, and the situations that need real caution.