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 on Mounjaro vs Wegovy — What You Need to Know
Nausea and vomiting are among the most common side effects reported with both Mounjaro and Wegovy in the UK. Both prescription-only weight management injections can cause gastrointestinal side effects, particularly during dose escalation. This guide compares how often vomiting occurs with each medication, what causes it, and when to seek medical advice. Individual experiences vary significantly, and your GP or prescriber can advise on managing these effects or switching medications if needed.
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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.
Vomiting Rates in Clinical Trials — Mounjaro vs Wegovy
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works on two hormone pathways simultaneously. In the SURMOUNT-1 clinical trial, vomiting was reported by approximately 25–26% of participants at the maximum 15 mg dose. Rates were lower at starting doses (2.5 mg and 5 mg).
Wegovy (semaglutide) is a GLP-1 receptor agonist affecting one hormone pathway. In the STEP-1 clinical trial, vomiting occurred in approximately 9–10% of participants on the maximum 2.4 mg dose, with lower rates at earlier doses (0.25 mg to 1 mg).
- Mounjaro: higher reported vomiting rates overall
- Wegovy: lower reported vomiting rates
- Both: more common during dose titration phase
- Both: usually improve or resolve within weeks as the body adjusts
These are clinical trial averages—individual experiences vary considerably. The dual mechanism of Mounjaro may contribute to higher gastrointestinal side effects, though both medications can cause nausea and vomiting.
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Managing Vomiting and When to Seek Help
If you experience vomiting on either medication, contact your GP or prescriber before stopping treatment. Strategies to reduce symptoms include eating smaller, frequent meals, avoiding fatty or greasy foods, staying hydrated, and taking anti-nausea medication if prescribed. Seek urgent medical attention if vomiting is severe, persistent, accompanied by abdominal pain spreading to your back (possible pancreatitis), or prevents you from keeping fluids or essential medications down.
Your prescriber may recommend delaying dose increases, staying on your current dose longer, or switching to the alternative medication. Individual response differs—some people tolerate one medication better than the other. Both Mounjaro and Wegovy require a clinical assessment before prescription and ongoing GP support.
- 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 or Wegovy?
Clinical trials suggest vomiting is more common with Mounjaro (around 25–26% at maximum dose) than Wegovy (around 9–10%). However, individual responses vary significantly. What causes vomiting in one person may not occur in another. Your prescriber can discuss these risks during your clinical assessment and help you choose the medication most suitable for you.
Why does Mounjaro cause more vomiting than Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1 receptor agonist only, working on one pathway. The dual mechanism of Mounjaro may contribute to stronger gastrointestinal effects, including higher rates of nausea and vomiting. Both medications can cause these side effects, particularly during the dose titration phase.
Does vomiting on these injections go away?
Yes, for most people. Nausea and vomiting typically improve or resolve within 2–4 weeks as your body adjusts to the medication. Eating smaller meals, avoiding fatty foods, and staying hydrated can help. If vomiting persists beyond a few weeks or is severe, contact your GP—they may recommend staying on your current dose longer or exploring alternative medications.
Can I switch from Mounjaro to Wegovy if I'm vomiting?
Yes, switching between medications is possible if one causes intolerable side effects. Your prescriber may recommend switching if vomiting is severe or persistent. There is no official conversion chart, but approximate equivalences exist (e.g., Mounjaro 15 mg ≈ Wegovy 2.4 mg). Your GP or prescriber will manage any switch to ensure safe transition between medications.
When should I see my GP about vomiting on weight loss injections?
Contact your GP if vomiting is severe, persistent after several weeks, preventing you from eating or taking other medications, or accompanied by abdominal pain. Seek urgent care if you experience severe stomach pain spreading to your back (possible pancreatitis) or signs of serious allergic reaction. Your GP can advise on managing symptoms or alternative treatments.
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