The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Vomiting With Mounjaro vs Wegovy — What You Need to Know
Nausea and vomiting are among the most commonly reported side effects with GLP-1 and GIP/GLP-1 weight loss medications in the UK. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause these gastrointestinal symptoms, particularly during the initial dose titration phase. Understanding how vomiting presents with each medication, and how the two compare, can help you make an informed decision alongside your prescriber. This guide compares the evidence and practical context for both treatments.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Mounjaro vs Wegovy — Vomiting and Nausea: What Clinical Trials Show
In the SURMOUNT-1 clinical trial for Mounjaro (tirzepatide), vomiting was reported by approximately 25–32% of participants at the maximum 15 mg dose, with nausea affecting up to 35% at higher doses. The majority of cases were mild to moderate, and symptoms typically improved after the first few weeks on each dose level.
In the STEP-1 trial for Wegovy (semaglutide), vomiting occurred in approximately 20–26% of participants at the maximum 2.4 mg dose, with nausea affecting around 25–30% of participants. Like Mounjaro, most cases were mild to moderate and tended to diminish over time.
Key differences in mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1-only agonist. The dual mechanism of Mounjaro may theoretically result in slightly higher rates of gastrointestinal symptoms initially, though individual tolerance varies considerably. Both medications slow gastric emptying (how quickly food leaves your stomach), which contributes to nausea and vomiting risk — this effect typically reduces as your body adjusts.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Managing Vomiting and Individual Suitability — Why Prescriber Choice Matters
Vomiting severity and frequency depend heavily on individual factors: starting dose, rate of dose escalation, food intake timing, and personal gastrointestinal sensitivity. Neither Mounjaro nor Wegovy is universally "better" for avoiding vomiting — suitability is a personalised clinical decision.
Your prescriber will consider: your medical history, whether you have a history of gastroparesis or reflux, other medications you take, and your tolerance to gastrointestinal symptoms. Most cases of vomiting resolve within 2–4 weeks on each dose. If vomiting persists or is severe, your prescriber may recommend staying on your current dose longer before escalating, or switching to the alternative medication. Using our GLP-1 weight loss calculator can help you understand dose progression and projected timelines, whilst our BMI calculator provides context for your individual eligibility and health baseline.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is vomiting more common with Mounjaro than Wegovy?
Clinical trials show vomiting rates are similar — approximately 25–32% with Mounjaro and 20–26% with Wegovy. Both medications cause mild to moderate symptoms in most cases. Individual tolerance varies significantly, so either may be better tolerated depending on your personal gastrointestinal sensitivity.
How long does vomiting typically last when starting Mounjaro or Wegovy?
Most nausea and vomiting symptoms improve within 2–4 weeks on each dose. Symptoms are usually most noticeable in the first week after injection and tend to diminish as your body adjusts. If vomiting persists beyond 4 weeks, contact your prescriber.
Can I prevent vomiting when taking Mounjaro or Wegovy?
Practical strategies include: eating smaller, more frequent meals; avoiding fatty or greasy foods; staying hydrated; taking your injection on a day when you can rest; and avoiding sudden dietary changes. Your prescriber may recommend dietary adjustments alongside your medication.
What should I do if I'm vomiting severely on Mounjaro or Wegovy?
Contact your prescriber immediately. Severe or persistent vomiting may warrant staying on your current dose longer, delaying dose escalation, or, in rare cases, switching to the alternative medication. Never stop taking your injection without medical guidance.
Does the dual mechanism of Mounjaro mean more vomiting than Wegovy?
Not necessarily. Whilst Mounjaro's dual GIP and GLP-1 action theoretically increases gastrointestinal effects, clinical trial rates are comparable. Individual factors like dose titration schedule, food intake, and personal sensitivity matter more than mechanism alone.
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