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.

• 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

Sources checked on 29 October 2026.

What are the side effects of GLP-1 medicines?

In the 68-week semaglutide trials pooled in the UK product information, nausea affected 43.9% of people on the medicine against 16.1% on placebo, diarrhoea 29.7% against 15.9%, vomiting 24.5% against 6.3% and constipation 24.2% against 11.1%, mostly mild to moderate and short-lived; in STEP 1 itself, 1,961 adults over 68 weeks, 4.5% stopped because of gut effects against 0.8% on placebo.^{1, 12} In the pooled tirzepatide weight-management trials in the UK product information (SURMOUNT-1 and SURMOUNT-2), nausea affected 24.6% to 29.0% of people on the medicine against 8.5% on placebo and diarrhoea 18.7% to 22.5% against 7.8%; vomiting and constipation were also very common (more than one person in ten); 61% of the gut reactions were mild and 34.4% moderate, and they clustered in the step-up weeks.^{2, 15} In the 56-week liraglutide trial of 3,731 adults, 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, most raised vomiting and diarrhoea, and semaglutide and liraglutide raised constipation.⁴ The medicines copy the gut hormone GLP-1 (what is GLP-1), 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

How long do GLP-1 side effects last?

The trials report totals, not timelines, but they agree on the shape: most gut effects appeared during the step-up weeks and subsided with time.^{1, 2} 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.

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.¹⁴ On a day of vomiting or diarrhoea you lose salt and water together; the NHS says to keep sipping fluids and to get urgent help for dizziness that does not pass, dark urine or passing less than usual.⁶ If you are keeping fluids down but eating little, salts come from soup, a salted meal or an electrolyte sachet, which typically supplies sodium, potassium and magnesium; magnesium contributes to electrolyte balance. What to avoid on Mounjaro lists the foods and habits that restart the stomach symptoms.

Which side effect, which fix, which page?

Periods, iron and the pill: does Mounjaro affect your periods. The sunken face is weight loss, not the medicine: Mounjaro face. All are chapters of living well on Mounjaro; 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 return of appetite 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 One studied example is hair loss: a 2026 review of 24 studies linked it most often to semaglutide and tirzepatide, with the temporary shedding that follows fast weight loss the type most tied to tirzepatide.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 or a pharmacist; any change to your prescription is their call.

How do you report a GLP-1 side effect?

Through the MHRA's Yellow Card scheme, at yellowcard.mhra.gov.uk or its app; tirzepatide, semaglutide and liraglutide are medicines, so Yellow Card is the right route for them.⁸ You do not need to be certain the medicine caused it; suspicion is enough. A reaction to a food supplement goes elsewhere: report it to your local authority's food safety team, which works with the Food Standards Agency.¹³

What we don't know

No trial reports how long each side effect lasts for an individual.^{1, 2} Why a minority never adapt and stop treatment has no identified predictor. Longer-term questions about organs or mood are open: the MHRA reviews every Yellow Card report and publishes what it finds in its Drug Safety Updates.^{7, 8}

Frequently asked questions

What is the downfall of taking GLP-1 medicines?

That the appetite effect lasts only as long as the medicine does, which is why protein, resistance training and your own gut hormones matter from the start. What happens when you stop taking Mounjaro and stopping Wegovy or Ozempic have the regain 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, 12}

Do GLP-1 side effects go away?

For most people, yes, within about six weeks, 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.

Sources

1. Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 2021. Randomised trial, 1,961 adults, 68 weeks; manufacturer-funded. PubMed 33567185.
2. Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022. Randomised trial, 2,539 adults, 72 weeks; manufacturer-funded. PubMed 35658024.
3. Pi-Sunyer X, et al. A randomized, controlled trial of liraglutide in weight management (SCALE). New England Journal of Medicine, 2015. Randomised trial, 3,731 adults, 56 weeks; manufacturer-funded. PubMed 26132939.
4. 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. 39 studies, 33,354 people; no competing interests declared. PubMed 40804463.
5. Bellavance D, et al. Gastrointestinal motility effects of GLP-1 receptor agonists. Current Gastroenterology Reports, 2025. Narrative review; no competing interests declared. PubMed 40622491.
6. NHS. Dehydration. Page last reviewed 1 May 2026. nhs.uk.
7. MHRA. GLP-1 receptor agonists: reminder of the potential side effects and to be aware of the potential for misuse. Drug Safety Update, 24 October 2024. gov.uk/drug-safety-update.
8. MHRA. Yellow Card: report a suspected side effect. yellowcard.mhra.gov.uk.
9. Gupta AK, et al. GLP-1 therapies and hair loss: a systematic review of current evidence and implications for counseling. Science Progress, 2026. Systematic review, 24 studies; one author declares industry fees. PubMed 41998799.
10. NHS. Tirzepatide. Page last reviewed 15 May 2026. nhs.uk.
11. NHS. Semaglutide. Page last reviewed 15 May 2026. nhs.uk.
12. Wegovy, Summary of Product Characteristics, section 4.8. Date of revision 09/2026. medicines.org.uk.
13. Food Standards Agency. How to report a food safety or hygiene issue for members of the public. Updated 5 August 2026. gov.uk.
14. Saha B, et al. GLP-1 and GIP receptor agonists: effects on the gastrointestinal tract and management strategies for primary care physicians. Mayo Clinic Proceedings, 2025. Narrative review; funding not stated on PubMed. PubMed 41324524.
15. Tirzepatide Summary of Product Characteristics, section 4.8 (pooled SURMOUNT-1 and SURMOUNT-2 frequencies), electronic medicines compendium, date of revision of the text 29 August 2026. medicines.org.uk.

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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.