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
Free tracker included with every download this week
Mounjaro 5mg Side Effects vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, but they work differently and may cause different side effects. Mounjaro's 5mg dose is typically the first maintenance dose after the 2.5mg starter phase, whilst Wegovy requires dose escalation from 0.25mg upwards. Understanding how their side effect profiles compare can help you and your prescriber decide which medication might suit you better. Both require a clinical assessment before prescription.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 Side Effects Differ
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways, whilst Wegovy is a GLP-1 agonist only. This difference in mechanism can affect how you experience side effects. Both medications cause common side effects like nausea, diarrhoea, vomiting, and constipation—particularly during dose escalation. The key difference is frequency and severity: clinical trial data suggests that Mounjaro may produce stronger initial appetite suppression, which some people find beneficial and others find uncomfortable. Gastrointestinal side effects tend to be most pronounced when starting and during dose increases, then often improve as your body adjusts. At 5mg, Mounjaro is moving into maintenance dosing, so side effects may stabilise compared to the 2.5mg starter phase. Wegovy's slower dose progression (starting at 0.25mg) may result in a gentler side effect experience for some users, though individual responses vary significantly.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Clinical Trial Data and Individual Suitability
The SURMOUNT-1 trial showed that Mounjaro at higher doses produced greater average weight loss (~20.9% at 15mg) compared to Wegovy's STEP-1 trial (~14.9% at 2.4mg), but this doesn't directly predict your personal side effect experience. Some people tolerate Mounjaro 5mg well with minimal nausea, whilst others find Wegovy suits them better despite lower average weight loss outcomes in trials. Your prescriber will assess your individual health history, any existing stomach conditions, other medications, and personal preferences to recommend the most suitable option. Neither medication is universally "better"—suitability depends entirely on you. If you've experienced significant side effects at lower doses, your prescriber may suggest staying at a maintenance dose rather than escalating further, or switching to an alternative. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and discuss specific concerns with your prescriber before starting.
- 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 on Mounjaro 5mg than Wegovy?
Not necessarily. Both cause nausea commonly during dose escalation. At 5mg, Mounjaro is at maintenance dosing so nausea may have already settled if you tolerated earlier doses. Wegovy's slower titration might feel gentler initially, but individual responses vary widely. Your prescriber can advise based on your health profile.
Can you switch from Mounjaro to Wegovy if side effects are intolerable?
Yes, switching is possible and should be discussed with your prescriber. There's no official conversion chart, but approximate equivalences exist (5mg Mounjaro ≈ 0.5mg Wegovy). Any switch requires clinical guidance and a new prescription. Some people tolerate one better than the other despite their similarity.
Does the Mounjaro KwikPen cause injection site side effects?
Injection site reactions like redness, bruising, or mild pain are common with both medications and affect around 1 in 10 users. These are usually mild and resolve quickly. Rotating your injection site (stomach, thigh, upper arm) each week helps minimise irritation.
How long do Mounjaro side effects typically last?
Most gastrointestinal side effects peak during the first week after injection and improve by days 5–7 as your body adjusts. At maintenance doses like 5mg (after dose escalation is complete), many people experience fewer side effects than during the titration phase. However, some mild nausea or changes to digestion may persist.
Will my GP prescribe Mounjaro 5mg or should I ask about Wegovy instead?
Your GP cannot prescribe either on the NHS—both require referral to a specialist tier 3 weight management service, which varies significantly by region. Privately, a licensed prescriber will assess which suits you based on your health, other medications, and preferences. Neither is universally preferable; your clinical assessment will guide the recommendation.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.