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GLP-1 Side Effects: What Is Common, How Long It Lasts and What Helps
The common side effects of every GLP-1 medicine are the same, because they come from the same action: a slowed stomach and a smaller appetite. Nausea, diarrhoea, constipation and vomiting lead the list for tirzepatide (Mounjaro), semaglutide (Wegovy, Ozempic) and liraglutide (Saxenda) alike, and in the trials they were mostly mild to moderate, clustered in the first weeks and after each step-up, and settled with time.1, 2, 3 Behind them sit the effects of eating much less: tiredness, headaches, feeling cold, dry mouth and, a few months in, hair shedding. This page is the index: the figures across the class, the food fixes that shorten each side effect, the rarer effects the MHRA lists with the symptoms to act on, and the full page for each one.
- Common to the class: nausea, diarrhoea, constipation, vomiting, burps, reflux, tiredness, headaches
- Usually: weeks one to six, and about a week after each step-up, then easing
- What helps: small low-fat meals, fibre and fluid, protein; a pharmacist or prescriber for anything beyond food
- Report it: the MHRA Yellow Card scheme, for any medicine
What are the side effects of GLP-1 medicines?
Gut effects first, in every trial. In STEP 1, the 68-week semaglutide trial of 1,961 adults, nausea affected about 44% of people on the medicine against 17% on placebo, diarrhoea about 32% against 16%, vomiting about 25% against 6% and constipation about 23% against 10%; the events were typically transient and mild to moderate, and 4.5% of people stopped because of them against 0.8% on placebo.1 In SURMOUNT-1, the 72-week tirzepatide trial of 2,539 adults, nausea affected roughly a quarter to a third, diarrhoea about a fifth, constipation about one in six and vomiting about one in ten, again mostly mild to moderate and mostly during the step-up weeks.2 In the 56-week liraglutide trial, nausea and diarrhoea led the list in the same way.3 A 2025 pooled analysis of the class in people without diabetes confirmed it: every GLP-1 medicine raised the risk of nausea, and most raised vomiting, diarrhoea and constipation.4 The reason is mechanical. The medicines slow the stomach's emptying and quieten appetite, so a meal sits longer, fullness tips into queasiness, and food moves through more slowly: constipation when fibre and fluid are short, diarrhoea when a fatty or large meal is released all at once.5 Ozempic and Wegovy are the same medicine, semaglutide, so their side effects are one list. What is GLP-1 explains the hormone the medicines copy.
How long do GLP-1 side effects last?
The trials report side effects as totals over a year or more, not as timelines, but they agree on the shape: most gut effects appeared during the step-up weeks and subsided with time.1, 2 On the forums it reads "worst on days one to three after the injection, better by week four to six, a brief return after each step-up". The side effects that come from eating less are different: tiredness, feeling cold, constipation and hair shedding last as long as intake stays too low, and food shortens them, not patience. How long Mounjaro side effects last gives the week-by-week version, and the same timeline applies to semaglutide.
What helps with sickness, vomiting and diarrhoea?
Food and fluid, before anything else. Small portions, low in fat, nothing fried, eaten slowly and not close to bed; something plain rather than nothing, because an empty stomach makes nausea worse; and fluid sipped through the day rather than drunk in one go.5 Diarrhoea is usually a fatty or oversized meal arriving all at once, so the fix is the same meal, smaller. On a day of vomiting or diarrhoea you lose salt and water together; the NHS advice is to keep drinking and to seek help if you cannot keep fluids down.6 If you are keeping fluids down but eating little, salts come from soup, a salted meal or an electrolyte sachet: Wellgard's ElectroMight Daily Electrolytes gives 300 mg of sodium, 200 mg of potassium and 62.5 mg of magnesium per 5 g sachet, and magnesium contributes to electrolyte balance. What to avoid on Mounjaro is the list of foods and habits that restart the stomach symptoms at any stage.
Which side effect, which fix, which page?
Every page below was written for Mounjaro; the food fixes apply to any GLP-1 medicine, because the mechanism is shared.
| Side effect | Usually | Food and routine fix | Full page |
|---|---|---|---|
| Nausea | Days 1 to 3 after the injection; weeks 1 to 6 | Small, low-fat, plain meals; never an empty stomach | Feeling sick |
| Constipation or diarrhoea | Weeks 1 to 6; constipation while fibre is short | 30 g of fibre built up over a fortnight, 1.5 to 2 litres of fluid | Constipation and diarrhoea |
| Sulphur burps | First weeks and after fatty meals | Less fat, smaller portions, no fizzy drinks | Sulphur burps |
| Heartburn | First weeks; evenings | Finish eating three hours before bed; stay upright | Heartburn |
| Tiredness | Weeks 2 to 6; longer if under-eating | Enough food, protein, fluid; blood tests after a month | Tiredness and on Wegovy |
| Headaches and dizziness | First weeks | Fluid, regular meals, a steady caffeine routine | Headaches |
| Feeling cold | As long as intake is low | Eat enough; warm meals and drinks | Feeling cold |
| Dry mouth, taste changes | First weeks; taste up to three months | Fluid through the day, sugar-free gum, oral hygiene | Dry mouth |
| Poor sleep, vivid dreams | Injection nights in month one | Do not skip the evening meal; a protein snack | Insomnia |
| Hair shedding | Months 2 to 4, then regrowth | Protein and iron; slower loss where possible | Hair loss |
| Muscle loss | Throughout, unless protected | 1.2 to 1.6 g of protein per kg a day; resistance training | Muscle loss |
Women report a few extra, around periods, iron and the pill: does Mounjaro affect your periods covers them. The sunken look some people get in the face is weight loss, not the medicine: Mounjaro face. All of these are chapters of our guide to living well on Mounjaro, and what is the GLP-1 diet turns the fixes into a way of eating.
What are the dangers and downsides of GLP-1 medicines?
The downsides most people meet are the ones above, plus the fact that appetite returns when the medicine stops. The dangers are rarer and belong to the MHRA, whose safety reminder for the class lists pancreatitis and gallstone disease (1 in 100 to 1 in 10,000 people, depending on the medicine), serious allergic reactions, low blood sugar, which can occur without diabetes, and dehydration from vomiting and diarrhoea severe enough to damage the kidneys; it also warns of falsified pens sold outside a registered prescription.7 Longer-term questions, about organs, mood or anything else, are not for a nutrition blog to speculate on: the MHRA reviews every Yellow Card report and publishes what it finds in its Drug Safety Updates.7, 8 One recently studied example is hair loss, which a 2026 review of 24 studies linked most often to semaglutide and tirzepatide, usually as the temporary shedding that follows fast weight loss.9
When should you contact your prescriber or 111?
The same day, for any of these: severe or persistent stomach pain, especially spreading to the back, with or without vomiting; vomiting that stops you keeping fluids down for more than a day; yellowing of the skin or eyes, pale stools or dark urine; pain under the right ribs after meals; a racing heart, fainting or confusion; passing much less urine than usual; blood in vomit or stools; symptoms of low blood sugar if you take insulin or a sulfonylurea; or signs of an allergic reaction.7, 10, 11 None of these is a side effect to manage with food. For anything not on that list, including a side effect still not easing after six weeks, speak to your prescriber: any change to your prescription is their call, and medicines for nausea or constipation are a pharmacist's conversation, not a forum's.
How do you report a GLP-1 side effect?
Through the MHRA's Yellow Card scheme, at yellowcard.mhra.gov.uk or its app, which takes reports from patients as well as prescribers, for any medicine and for supplements too.8 You do not need to be certain the medicine caused it; suspicion is enough, and the scheme exists to find patterns no single trial could.
Frequently asked questions
What is the downfall of taking GLP-1 medicines?
That the appetite effect lasts only as long as the medicine does. In the trial extensions, people regained much of the weight within a year of stopping, which is why protein, resistance training and a diet that gets the most from your own gut hormones matter from the start. What happens when you stop taking Mounjaro and stopping Wegovy or Ozempic have the figures.
What are the side effects of Ozempic?
The same as Wegovy's, because both are semaglutide: nausea, diarrhoea, vomiting and constipation most often, mostly in the first weeks, with tiredness and headaches from eating less.1
Do GLP-1 side effects go away?
For most people, yes, over the first one to three months, with a brief return after each step-up; the ones tied to eating too little last as long as the under-eating does.1, 2
Are GLP-1 side effects different in women?
A few extra ones, around periods, iron and contraception. Mounjaro side effects in women sets them out.
Do supplements help with GLP-1 side effects?
Protein, fibre, fluid and salts do most of the work, from food where appetite allows and a supplement where it does not. What supplements to take on a GLP-1 medicine goes through them with the authorised wording for each.
Sources
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 2021. PubMed 33567185.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022. PubMed 35658024.
- Pi-Sunyer X, et al. A randomized, controlled trial of liraglutide in weight management (SCALE). New England Journal of Medicine, 2015. PubMed 26132939.
- Ismaiel A, et al. Gastrointestinal adverse events associated with GLP-1 RA in non-diabetic patients with overweight or obesity: a systematic review and network meta-analysis. International Journal of Obesity, 2025. PubMed 40804463.
- Bellavance D, et al. Gastrointestinal motility effects of GLP-1 receptor agonists. Current Gastroenterology Reports, 2025. PubMed 40622491.
- NHS. Dehydration. nhs.uk/conditions/dehydration.
- MHRA. GLP-1 receptor agonists: reminder of the potential side effects and to be aware of the potential for misuse. Drug Safety Update. gov.uk/drug-safety-update.
- MHRA. Yellow Card: report a suspected side effect. yellowcard.mhra.gov.uk.
- Gupta AK, et al. GLP-1 therapies and hair loss: a systematic review of current evidence and implications for counseling. Science Progress, 2026. PubMed 41998799.
- NHS. Tirzepatide. nhs.uk/medicines/tirzepatide.
- NHS. Semaglutide. nhs.uk/medicines/semaglutide.
This article is general information, not medical advice. Tirzepatide, semaglutide and liraglutide are prescription medicines: follow your prescriber's instructions, never change how you take them because of something you read, and contact your prescriber or NHS 111 the same day about any of the symptoms listed above.
