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 vs Wegovy: Sickness and Nausea Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause nausea and sickness as side effects, though rates vary between individuals and doses. Clinical trial data shows important differences in how often these occur with each medication. Understanding these differences can help you and your prescriber decide which option may be more suitable following a clinical assessment. This guide compares the evidence directly so you can make an informed decision about UK prescription weight loss treatment.
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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 Data: Nausea Rates Compared
The SURMOUNT-1 trial for Mounjaro (tirzepatide) and the STEP-1 trial for Wegovy (semaglutide) provide the most direct evidence on sickness rates. In SURMOUNT-1, nausea affected approximately 25–33% of participants on the highest doses of tirzepatide, with vomiting reported in 7–9%. In STEP-1, nausea was reported in approximately 25–26% of participants on 2.4 mg semaglutide, with vomiting in 5–8%.
Key difference: Tirzepatide's dual mechanism (GIP + GLP-1 agonist) may cause slightly different nausea patterns compared to semaglutide's single GLP-1 mechanism. However, individual responses vary widely. Some people experience minimal sickness whilst others find it more problematic. Most nausea improves over the first 4–8 weeks as the body adjusts, particularly if doses are increased gradually. Your prescriber may adjust your dose or timing to manage sickness if it occurs.
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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 Is More Likely to Cause Sickness?
Trial data suggests nausea rates are comparable between Mounjaro and Wegovy at equivalent doses, so neither medication is dramatically "better" or "worse" for sickness. The difference often comes down to individual factors: genetics, how quickly you titrate doses, and whether you take it with food all influence how much nausea you experience.
Both medications are prescription-only and require a clinical assessment before use. Your GP or prescriber will discuss your personal risk factors and tolerability, and can recommend dose timing and management strategies. Using our free GLP-1 weight loss calculator alongside professional guidance can help you understand what realistic expectations look like with either medication when managing side effects properly.
- 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.
Does Mounjaro cause more sickness than Wegovy?
Clinical trials show comparable nausea rates (around 25–33% for Mounjaro, 25–26% for Wegovy at highest doses). Individual experience varies significantly. Some people tolerate one better than the other, but trial data does not show clear superiority. Your prescriber will assess which suits you best following a clinical assessment.
How long does Mounjaro nausea last?
Most nausea improves within 4–8 weeks as your body adjusts to the medication. Gradual dose titration (the standard approach) helps reduce initial sickness. If nausea persists or is severe, inform your prescriber — they may adjust timing, dose, or recommend management strategies.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. This dual mechanism may influence appetite signalling differently, though both cause similar overall nausea rates in clinical trials. The mechanisms do not make one inherently safer than the other.
Can I switch from Mounjaro to Wegovy if I feel sick?
Only your prescriber can advise on switching medications. It is not recommended to switch based on trial data alone — individual tolerance matters more. Your prescriber may first try dose adjustments, timing changes, or waiting longer for adaptation before considering a switch.
Is nausea with GLP-1 injections dangerous?
Nausea itself is not dangerous, though severe or persistent vomiting can lead to dehydration. This is why your prescriber monitors you. Contact your GP if you experience severe, unmanageable sickness, difficulty keeping food or fluids down, or any concerning symptoms whilst taking weight loss medication.
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