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
NHS Weight Loss Injections: Mounjaro vs Wegovy
If you're exploring weekly weight loss injection options in the UK, you've likely encountered both Mounjaro and Wegovy. Both are prescription weight loss injections approved by MHRA for UK use, but they work differently and suit different people. This guide compares the clinical evidence, mechanisms, doses, and eligibility so you can understand what your GP or prescriber might recommend. Both require a clinical assessment before prescription — individual results vary significantly.
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 Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the body. This dual mechanism may help regulate appetite more comprehensively and support feeling fuller after meals. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single pathway.
In the SURMOUNT-1 clinical trial, Mounjaro at the maximum dose of 15 mg produced average weight loss of approximately 20.9% body weight over 72 weeks. Wegovy, based on STEP-1 trial data, showed average weight loss of approximately 14.9% body weight over 68 weeks at the maximum 2.4 mg dose. However, these are averages — individual results vary considerably, and many people achieve meaningful health benefits at lower doses. Both are used alongside a reduced-calorie diet and increased physical activity.
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.
Doses, Side Effects, and Prescriber Considerations
Mounjaro starts at 2.5 mg once weekly, escalating to a maximum of 15 mg. Wegovy starts at 0.25 mg, escalating to a maximum of 2.4 mg. Both are once-weekly injections administered via pre-filled pens (Mounjaro KwikPen, Wegovy auto-injection pen). Common side effects for both include nausea, diarrhoea, vomiting, and constipation — these typically reduce over time.
Your prescriber will consider your medical history, other medications, and individual response to determine which suits you. Neither medication is "better" — suitability is personal. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and our BMI calculator to establish your starting point for clinical assessment.
- 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.
Which weight loss injection is available on the NHS?
Both Mounjaro and Wegovy are now approved by NICE and available through NHS specialist tier 3/4 weight management services. However, NHS availability varies significantly by region and ICB, with some areas having long waiting lists. Your GP can refer you to your local specialist service for assessment.
Can I switch from Mounjaro to Wegovy or vice versa?
Any switch between medications must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist (for example, Mounjaro 15 mg is roughly equivalent to Wegovy 2.4 mg), but individual response differs. Your prescriber will assess whether switching is appropriate.
How much weight will I lose on Mounjaro or Wegovy?
Clinical trials show average weight loss of 15–21% body weight with Mounjaro and approximately 15% with Wegovy, but individual results vary considerably. Some people lose more, others less. Weight loss depends on your starting weight, adherence to diet and exercise, dose tolerance, and individual metabolism. Your prescriber can discuss realistic expectations.
What happens if I stop taking these injections?
If you stop either injection, your appetite regulation gradually returns to baseline, and weight regain is common. These medications work best as part of a long-term weight management strategy alongside sustained lifestyle changes. Your prescriber will discuss whether long-term use is appropriate for you.
How do I access these injections — NHS or private?
NHS access is through specialist weight management services (referral required, regional variation in availability). Private access is through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber. Private costs range from approximately £129–£320 per month depending on dose and provider.
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.