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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Feeling Sick On Mounjaro vs Wegovy — What You Need to Know
Nausea and sickness are among the most commonly reported side effects when starting weight loss injections in the UK. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause nausea, particularly during dose escalation. Understanding how these medications differ, why sickness occurs, and when it typically improves can help you decide which treatment might suit you better. This guide compares the two based on clinical trial data and UK prescribing experience to help you make an informed decision with your GP.
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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 Cause Nausea — Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single pathway. Both mechanisms slow gastric emptying (how quickly food leaves your stomach), which triggers nausea signals as your digestive system adjusts.
In the SURMOUNT-1 clinical trial for Mounjaro, nausea was reported by over 25% of participants at the 15mg dose, with severity usually decreasing after the first 2–4 weeks on each new dose. In the STEP-1 trial for Wegovy, nausea affected approximately 25% of participants on the 2.4mg dose. Importantly, nausea often improves as your body adjusts — most participants reported that symptoms peaked in week 1–2 of each dose increase, then gradually resolved.
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Which Is More Likely to Cause Nausea — Individual Factors Matter
Clinical trial data shows similar nausea rates between the two medications, but individual responses vary significantly. Some people tolerate Mounjaro better; others find Wegovy easier. This is because individual gut sensitivity, dose escalation speed, and meal size and content all affect how much nausea you experience.
Your GP or prescriber will assess your medical history, current medications, and any previous experiences with GLP-1 medications to recommend the most suitable option for you. Neither medication is "better" universally — the right choice depends on your personal health profile. If you're experiencing severe sickness on either medication, contact your prescriber immediately; they may slow your dose escalation, adjust timing, or recommend dietary strategies to reduce symptoms.
- 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 make me feel sick than Wegovy?
Clinical trials show similar nausea rates (around 25% at maximum doses for both). Individual responses vary significantly — some people tolerate Mounjaro better, others prefer Wegovy. Your prescriber will assess your personal health profile to recommend the most suitable option. Neither is universally "more sickening."
How long does nausea last when starting these injections?
Nausea typically peaks in the first 1–2 weeks after starting or increasing your dose, then improves gradually over 2–4 weeks as your body adjusts. Most people report that sickness becomes much less noticeable by week 3–4 on each dose level.
What should I do if I feel very sick on Mounjaro or Wegovy?
Contact your GP or prescriber. They can slow your dose escalation, adjust your injection timing, recommend eating smaller meals, or advise dietary changes. Never stop your medication without medical guidance — your prescriber can help you manage side effects effectively.
Can I switch from Mounjaro to Wegovy if the sickness doesn't improve?
Yes, but any switch must be managed by your prescriber. There is no official conversion chart — your prescriber will assess whether switching is appropriate and plan the transition carefully. Do not attempt to switch medications on your own.
Does nausea mean the injection isn't working for weight loss?
No. Nausea is a side effect of how the medication works, not a sign of effectiveness. Many people experience reduced appetite and steady weight loss even while managing nausea. Nausea often improves as your body adjusts, whilst weight loss benefits continue.
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