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 and Wegovy: Nausea and Sickness Compared
Nausea and feeling sick are among the most commonly reported side effects with both Mounjaro (tirzepatide) and Wegovy (semaglutide) weight loss injections. While both medications can cause these effects, clinical trial data shows some important differences in how often and severely they occur. Understanding which side effect profile might suit you better requires looking at the evidence alongside your individual health circumstances and GP guidance.
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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.
Clinical Trial Evidence: Nausea Rates Compared
Both medications are GLP-1 or GIP/GLP-1 receptor agonists that slow stomach emptying, which commonly triggers nausea—especially during the first few weeks or when doses increase. In the SURMOUNT-1 trial for Mounjaro (tirzepatide), nausea affected approximately 25–33% of participants, with most cases mild to moderate. The STEP-1 trial for Wegovy (semaglutide) reported similar nausea rates of around 25–42% depending on the dose stage. Importantly, nausea typically improves over time as your body adjusts to the medication—most people find it reduces within the first 4–8 weeks of treatment.
Key differences: Mounjaro's dual GIP+GLP-1 mechanism may cause different symptom patterns than Wegovy's single GLP-1 pathway, though both medications produce very similar nausea frequencies overall. Nausea severity generally increases with higher doses, particularly during the dose titration phase when you step up to a new strength each week.
Your GP or prescriber will discuss these risks before starting either medication and can help manage side effects if they occur.
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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 Might Be Better for You?
Neither Mounjaro nor Wegovy is inherently "better" for avoiding nausea—individual responses vary significantly. Some people report minimal sickness with one medication but more with the other; others experience the opposite. Factors affecting your experience include your dose, how quickly you escalate, whether you take it with food, and your individual tolerance. Both medications require a clinical assessment by a UK-licensed prescriber who will review your health history and help you decide which option suits you best.
If nausea becomes severe or persistent, your GP can suggest practical strategies like taking the injection at different times, adjusting meal timing, or in some cases reducing your dose temporarily. Most people find these side effects manageable and worth the weight loss benefits they achieve.
- 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 nausea worse with Mounjaro or Wegovy?
Clinical trials show similar nausea rates for both—around 25–42% of users experience it. Individual responses vary significantly. Some people tolerate one medication better than the other, which is why a prescriber assesses your specific health profile before choosing which to prescribe.
How long does nausea last with these injections?
Nausea typically peaks during dose increases and improves within 4–8 weeks as your body adjusts. Most people find it reduces substantially after the first month, though some experience mild nausea throughout treatment. Consistent food intake and timing can help manage symptoms.
Can I reduce my dose if I'm feeling very sick?
Yes—contact your GP or prescriber if nausea is severe. They may recommend staying on your current dose longer before escalating, or temporarily reducing it while your body adjusts. Never change your dose without guidance from your healthcare provider.
What's the difference between Mounjaro and Wegovy's mechanisms?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. This means Mounjaro targets two appetite-regulating pathways. Both slow stomach emptying, which can cause nausea, but individual tolerance varies—your prescriber will help choose the right option for you.
Will sickness from these injections affect my weight loss progress?
Mild to moderate nausea often naturally supports weight loss by reducing appetite further. However, severe nausea that prevents eating properly can be counterproductive. If you're struggling, contact your GP—they can adjust your dose or suggest management strategies to keep you on track safely.
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