UK Guide · Updated August 2026 · Reviewed by our superintendent pharmacist
Nausea, constipation, tiredness and the dreaded sulphur burps. What's normal, what helps, and the symptoms that mean stop and get checked.
The short answer
Most side effects of Mounjaro (tirzepatide) and Wegovy (semaglutide) are gastrointestinal — nausea, constipation, diarrhoea, vomiting, indigestion and burping — and they cluster in the first one to two weeks after every dose increase, then usually ease. They are classed as "very common", but they are mostly mild to moderate and short-lived, and in the Wegovy trials only about 4.3% of people stopped treatment permanently because of them. The three things that genuinely help are going up in dose slowly, eating smaller and less fatty meals, and drinking more than you think you need. Stop and seek urgent help for severe stomach pain that spreads to your back, signs of severe dehydration, or sudden vision changes. At our Timperley pharmacy every patient is reviewed face-to-face monthly, which is when most side-effect problems get solved.
Nothing has gone wrong when you feel queasy in week one. Both Mounjaro and Wegovy work partly by slowing how quickly your stomach empties and by acting on appetite signalling in the brain. Food sits in the stomach for longer, you feel full sooner and for longer, and you eat less. That is the therapeutic effect — and the side effects are the same mechanism turned up a notch.
Which is why side effects follow a predictable rhythm: they spike when your body meets a new dose and fade as it adapts. Understanding that pattern is most of the battle, because it tells you when to hold your nerve and when something genuinely needs attention.
Honest numbers beat reassurance. These come from the licensed product information and the published phase 3 trials — not from what people say online, where the miserable weeks are always over-represented.
| Side effect | Wegovy 2.4mg (68 wks) | Mounjaro (SURMOUNT-1, 72 wks) |
|---|---|---|
| Nausea | 43.9% | 24.6–33.3% (by dose) |
| Diarrhoea | 29.7% | 18.7–23.0% |
| Vomiting | 24.5% | 8.3–12.2% |
| Constipation | 24.2% | 11.7–17.1% |
| Stopped treatment for side effects | 4.3% (gut side effects) | 4.3–7.1% (all adverse events) |
A word of caution on reading that table sideways: these were separate trials with different participants and designs, so the columns are not a fair head-to-head. Use them to calibrate your expectations, not to pick a medicine. Tiredness, headache, dizziness, indigestion, reflux, burping, bloating and hair loss are also listed as common with semaglutide, and the pattern is similar for tirzepatide.
This is the shape most of our patients follow. It is a guide, not a promise — some people sail through and some need longer at each step.
Week 1
The introduction
Appetite drops noticeably within a day or two. Mild queasiness, early fullness and burping are typical, often peaking 24–72 hours after the injection. Some people feel nothing at all — that is fine and does not mean it is not working.
Weeks 2–4
Settling in
Nausea usually softens as your body adapts. Constipation tends to arrive now rather than in week one, because you are simply eating and drinking less. This is the point to build the habits — smaller plates, more fluid, daily movement — that carry you through every later dose step.
The dose-step weeks
The reset button
Every increase restarts a smaller version of week one. Expect one rougher week, then improvement. If it is not improving by the end of week two on a new dose, that is your signal to talk to your pharmacist rather than push on.
Months 2–3
The stable stretch
For most people the gut settles into a new normal: smaller appetite, occasional reflux or burping, manageable bowels. Tiredness in this phase is often about what you are eating rather than the drug — too few calories, too little protein, not enough fluid.
Month 4 onwards
Maintenance
Side effects that persist this far are usually fixable with diet timing, dose pacing or a change of medicine — they are not something you have to accept indefinitely. New or worsening symptoms after months of stability always deserve a proper review.
Our health advice hub covers the eating, protein and activity side in more depth, and there is more on dosing pace in our Wegovy 7.2mg high-dose guide.
Seek urgent medical advice — call 111, or 999 if you are struggling to breathe — if you get:
Pancreatitis deserves a paragraph of its own, because the guidance changed recently. On 29 January 2026 the MHRA strengthened the warnings on GLP-1 and dual GLP-1/GIP medicines after reviewing Yellow Card data: between 2007 and October 2025 it received 1,296 reports of pancreatitis linked to this class, including 24 necrotising cases and 19 with a fatal outcome. For scale, roughly 25.4 million packs were dispensed in the UK over five years — so this remains an infrequent event. But it is one where acting quickly matters, which is exactly why we brief every patient on the warning signs before their first dose.
Gallstones are the other one worth knowing about: they are a recognised side effect of both medicines and of rapid weight loss generally. Pain in the upper-right abdomen, particularly after fatty meals, should be assessed. And if you take a combined oral contraceptive alongside Mounjaro, read our contraception and pregnancy guide — that interaction catches people out. The full list of situations where treatment isn't suitable is on our contraindications page.
The single most common mistake we see is treating the dose schedule as a timetable you must keep up with. It isn't. It is the fastest reasonable pace, and there is no prize for reaching the top dose first. If a step is making you miserable, the options a pharmacist will discuss with you include staying at your current dose for another month, stepping back down a level, extending the gap before the next increase, or — occasionally — switching medicine entirely. Some people also do better on a gentler introduction; we cover that approach on our microdosing page.
What we'd discourage is the DIY response: skipping a week, splitting doses yourself, or quietly stopping and restarting. Erratic dosing gives you the side effects of starting over without the benefit of steady treatment. Bring it to your review instead — see our treatments and how it works pages.
WeightGone UK is a GPhC-registered pharmacy (reg. 1118728) on Stockport Road in Timperley, Altrincham, serving Sale, Hale, Brooklands, Stretford, Wythenshawe and the wider Greater Manchester area. Side effects are precisely where face-to-face care earns its keep: every patient is seen in person at least once a month, weighed on the same scales, and asked in detail how the last four weeks actually felt — not asked to tick a box on a form.
That means a dose step can be paused the same day you tell us it isn't working, and someone who knows your history is looking at you when you describe a symptom. If you're struggling on treatment started elsewhere, we're happy to review it. Prices for both medicines start at £100 per month, including those monthly in-person reviews.
Sources
Published 3 August 2026. Written and clinically reviewed by the WeightGone UK pharmacy team. This article is information, not a prescription — suitability and side-effect management are always confirmed face-to-face. If you think you have had a side effect from a medicine, you can also report it through the MHRA Yellow Card scheme.
For most people the worst of it is short-lived. The product information for both medicines describes gastrointestinal effects as mostly mild to moderate, of short duration, and reported most often while the dose is being increased. In practice that means the first week on a new dose is usually the roughest, and things settle over the following two to three weeks as your body adapts. Side effects that are still severe after a month at the same dose are worth reviewing rather than enduring.
Common enough that you should plan for it. In the Wegovy trial programme nausea was reported by 43.9% of people, diarrhoea by 29.7%, vomiting by 24.5% and constipation by 24.2% over 68 weeks. In the SURMOUNT-1 trial of tirzepatide (Mounjaro), nausea was reported by 24.6% on 5mg, 33.3% on 10mg and 31.0% on 15mg. Both are classed as 'very common' side effects — but the majority of people carry on with treatment, and permanent discontinuation because of gut side effects was around 4.3% in the Wegovy trials.
Smaller portions eaten more slowly, stopping at the first sign of fullness, and going easy on fried, fatty and very rich food — dietary fat slows stomach emptying further, on top of what the medicine is already doing. Keep fluids up, don't lie down straight after eating, and avoid alcohol, which the NHS specifically notes can make nausea and vomiting worse. If nausea is still unmanageable after a couple of weeks, speak to your pharmacist rather than skipping doses — holding at your current dose for longer is usually the better answer.
Two reasons stack up. The medicine slows gut transit, and you're eating and drinking considerably less, so there's less fibre and less fluid going through. The fix is boring but effective: deliberately drink more than feels necessary, keep fibre in the diet even when appetite is low, and move daily. If nothing has shifted in three days or you have abdominal pain and bloating with it, ask a pharmacist before reaching for anything off the shelf — some laxatives are a poor choice on a GLP-1.
Severe, persistent stomach pain that spreads through to your back — with or without nausea and vomiting — needs urgent assessment, because the MHRA strengthened its warnings on acute pancreatitis in January 2026 after reports that included necrotising and fatal cases. Also seek urgent help for signs of severe dehydration, sudden vision changes or loss of vision, and symptoms of an allergic reaction such as swelling of the face or throat and difficulty breathing. Call 111, or 999 for breathing difficulty.
There's no fair head-to-head answer from the trials, because the studies were run separately with different populations and designs. The type of side effect is near-identical — nausea, diarrhoea, vomiting, constipation, indigestion, burping and tiredness lead the list for both. Reported nausea rates were numerically higher in the Wegovy trials, but that comparison isn't reliable. What we see at the pharmacy is that tolerance is highly individual: people who struggle on one sometimes do fine on the other.
Usually not. The dose schedule is a maximum pace, not a target you have to hit. If you're still queasy at the end of a four-week step, staying where you are for another month is a completely legitimate plan — and often a lower dose you actually take every week beats a higher dose you keep skipping. That's a decision to make with your pharmacist at your monthly review, not alone.
No — and this is a myth worth killing. Plenty of people lose weight steadily with barely any nausea, and having a rough week doesn't mean you're losing more. Feeling unwell is not a progress marker, it's a signal to review how the dose is being introduced. Equally, no medicine can guarantee a particular result; trial averages describe groups, not individuals.
Medically reviewed by Muhammad Adnan, GPhC-registered Superintendent Pharmacist · Last updated June 2026.
This article is general information and isn't a substitute for personal medical advice. Your pharmacist will assess what's right for you at a face-to-face consultation. More health advice · Book a consultation.
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