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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Mounjaro Feeling Sick vs Wegovy: What's the Difference?
Nausea and sickness are common side effects of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but the two medicines work differently and may affect individuals in different ways. This guide compares what clinical trials showed about nausea with each medication, how their mechanisms differ, and what you should know before starting treatment. If you're considering weight loss medication in the UK, understanding these differences helps your GP or prescriber find the best option for you.
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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.
How Mounjaro and Wegovy Differ in Causing Nausea
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only GLP-1 receptor agonist medications licensed for weight management in UK adults, but they use different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. Wegovy is a single GLP-1 agonist, activating only the GLP-1 pathway.
In clinical trials, nausea was reported as a very common side effect with both medications — affecting more than 1 in 10 people. However, the timing and severity may differ. With Mounjaro, gastrointestinal effects including nausea, diarrhoea, and constipation tend to emerge during dose titration (the gradual increase from 2.5 mg up to 15 mg over weeks). With Wegovy, nausea often peaks during the first 4–8 weeks on each new dose.
The key difference is that Mounjaro's dual mechanism may produce more pronounced appetite suppression, which some users experience as stronger nausea. Wegovy's single-pathway approach may cause milder but more persistent nausea for some individuals. Neither is "better" — individual responses vary considerably.
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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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Which Medication Is Right for You?
Choosing between Mounjaro and Wegovy is not based on nausea alone — your GP or specialist prescriber will assess your overall health, BMI, weight-related conditions, and any medications you take. Both require UK prescription and clinical assessment before being dispensed. If you have a history of severe nausea, gastroparesis, or gastrointestinal conditions, inform your prescriber; they may recommend one over the other or suggest managing nausea with anti-sickness medication during titration.
Try our free GLP-1 weight loss calculator above to explore potential weight loss outcomes with either medication. Many people find that nausea improves significantly after the first 2–4 weeks on a stable dose. Your prescriber can adjust your dose, slow titration, or switch medications if side effects are unmanageable.
- 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 Mounjaro more likely to cause nausea than Wegovy?
Clinical trials show both cause nausea as a very common side effect (affecting over 1 in 10 people). Mounjaro's dual mechanism may cause more pronounced appetite suppression, which some experience as stronger nausea, but individual responses vary widely. Neither is definitively "worse" — prescriber assessment and personal tolerance matter most.
How long does the feeling sick sensation last with Mounjaro?
Nausea typically peaks during dose titration (weeks 1–4 on each new dose) and often improves after stabilising on a maintenance dose. Most people experience significant improvement after 2–4 weeks. If nausea persists beyond 4 weeks on a stable dose, contact your prescriber — medication adjustments may help.
Can I switch from Mounjaro to Wegovy if I feel sick?
Yes, switching is possible and should be managed by your prescriber. Approximate dose equivalence exists (5 mg Mounjaro ≈ 0.5 mg Wegovy, 10 mg ≈ 1.7 mg, 15 mg ≈ 2.4 mg), but any switch requires professional guidance. Your prescriber will advise on timing and how to transition safely.
Does Mounjaro nausea mean it's not working?
No — nausea does not indicate whether the medication is working for weight loss. Clinical trials showed both medications produce significant weight loss (average 15–21% body weight) regardless of nausea severity. Nausea is a side effect that often improves, while weight loss benefits typically continue.
What can I do to manage feeling sick on either medication?
Eat smaller, lighter meals; avoid fatty or spicy foods; stay hydrated; and take anti-sickness medication if prescribed. Slow dose titration may help — discuss with your GP or prescriber. Ginger supplements or anti-nausea bands may provide some relief. If side effects are severe, your prescriber may adjust your dose or suggest alternative treatment.
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