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
Can I Switch From Mounjaro To Wegovy?
Yes, switching between Mounjaro (tirzepatide) and Wegovy (semaglutide) is possible in the UK, but the decision must be made with your prescriber based on your individual response, health goals, and any side effects you've experienced. Both are prescription-only weight management injections licensed in the UK, but they work through different mechanisms—Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Any switch requires a clinical assessment and a new prescription.
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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 Differ Mechanistically
The key difference between these two medications lies in their mechanism of action. Mounjaro (tirzepatide) activates two hormone pathways—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1—making it a dual receptor agonist. Wegovy (semaglutide) activates only the GLP-1 pathway. This dual mechanism in Mounjaro may explain why some clinical trial data showed slightly higher average weight loss: in the SURMOUNT-1 trial, tirzepatide achieved approximately 15–clinically studied weight loss depending on dose, while Wegovy's STEP-1 trial showed around clinically studied weight loss at the highest dose. However, clinical trial results do not directly predict individual outcomes—weight loss varies significantly between people.
- Mounjaro: GIP + GLP-1 agonist, once-weekly injection, maximum dose 15 mg
- Wegovy: GLP-1 agonist only, once-weekly injection, maximum dose 2.4 mg
- Both are prescription-only and require a clinical assessment before switching
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.
When and Why People Consider Switching
People switch between these medications for several reasons. You might consider switching if you're experiencing unmanageable side effects on one medication that don't improve with time, or if you're not achieving the weight loss results you and your prescriber hoped for after a reasonable trial period at your current dose. Some people find one medication suits their body better than the other. Important: Any switch must be discussed with your prescriber—never stop one medication and start another without clinical guidance. Your GP or the prescriber managing your weight loss treatment will assess whether switching is appropriate, discuss the transition process (which typically involves stopping one medication and waiting an appropriate interval before starting the other), and monitor you closely during the change.
- 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.
Can I switch from Mounjaro to Wegovy immediately?
No. You cannot switch immediately between medications. Your prescriber will typically advise stopping Mounjaro and waiting a period of time (usually several days) before starting Wegovy. This allows your body time to clear the previous medication. Never overlap doses or switch without medical guidance.
Is Wegovy or Mounjaro "better" for weight loss?
Neither is universally "better"—both are effective, and individual results vary significantly. Clinical trials show slightly different average outcomes (Mounjaro ~15–21%, Wegovy ~14.9%), but these are population averages. Your prescriber will recommend whichever is most appropriate based on your health profile, previous response, and side effects experienced.
What if I've had side effects on Mounjaro—will Wegovy be better?
Side effects often improve with time and dose titration on the same medication. However, if side effects remain unmanageable, switching may help—some people tolerate one medication better than another. Your prescriber will help determine if switching is worth trying or if a different approach is needed.
How long should I try one medication before considering a switch?
Most guidance suggests giving a medication at least 12–16 weeks at an effective dose to assess full benefits and side effect tolerance. If you're experiencing severe side effects earlier, discuss with your prescriber immediately rather than waiting.
Is switching available on the NHS or only privately?
Availability depends on your region and the tier 3 or tier 4 specialist weight management service managing your care. NHS provision varies significantly by area. If accessing privately, discuss switching options with your online prescriber, who will arrange the transition and monitor you appropriately.
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.