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
Metformin vs Mounjaro: How They Compare for Weight Management
Metformin and Mounjaro are both medications used in weight management, but they work in very different ways. Metformin is an oral diabetes medication that may support modest weight loss, whilst Mounjaro (tirzepatide) is a weekly injection specifically licensed for weight management in UK adults. This guide explains the key differences between these two approaches, helping you understand which might be suitable for you. Both require assessment by a healthcare professional before starting.
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 Metformin and Mounjaro Work Differently
Metformin is an oral medication primarily used to manage type 2 diabetes. It works by improving how your body uses insulin and reducing glucose production in the liver. Some patients experience modest weight loss (typically 2–3 kg on average), though this is not its primary purpose.
Mounjaro (tirzepatide) is a weekly injection that works as a dual GIP and GLP-1 receptor agonist. It activates two natural hormone pathways that regulate appetite, reduce food cravings, and support feeling fuller after meals. In clinical trials (SURMOUNT-1), participants lost an average of 15–21% of body weight, significantly more than metformin typically achieves.
- Metformin: oral tablet, daily dosing
- Mounjaro: weekly KwikPen injection, doses from 2.5 mg to 15 mg
- Metformin: primarily for diabetes; weight loss is secondary
- Mounjaro: specifically licensed for weight management in eligible adults
Use our free GLP-1 weight loss calculator to explore potential outcomes with Mounjaro at different doses.
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.
Eligibility, Access, and Which Is Right for You
Metformin is prescribed for type 2 diabetes or prediabetes by your GP. Mounjaro requires a BMI of 30 or above (or 27+ with weight-related conditions) and is accessed through NHS specialist weight management services or private prescription following a clinical assessment.
Individual suitability depends on: your health conditions, medical history, current medications, and treatment goals. Someone with type 2 diabetes might benefit from metformin's dual glucose-control and modest weight-loss effects. Someone seeking significant weight loss support without diabetes might be better suited to Mounjaro after clinical assessment. Your GP or a licensed prescriber will assess you individually. You can also use our free BMI calculator above to check your current category and discuss eligibility with your healthcare provider.
- 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 metformin cause weight loss?
Metformin may support modest weight loss of 2–3 kg on average, but weight loss is not its primary purpose. It works by improving insulin sensitivity and controlling blood sugar. Most weight loss occurs in the first 6–12 months, then typically stabilises.
Is Mounjaro more effective than metformin for weight loss?
Clinical trials show Mounjaro leads to significantly greater average weight loss (15–21% body weight) compared to metformin's typical 2–3 kg. However, Mounjaro is a specialist weight management injection, whilst metformin is a diabetes medication with secondary weight benefits. Individual results vary.
Can you take metformin and Mounjaro together?
Yes, some patients take both. Metformin controls blood sugar; Mounjaro supports weight management. Your prescriber will assess whether combining them is appropriate for your individual health needs and adjust doses accordingly.
How do I know if I'm eligible for Mounjaro on the NHS?
NHS Mounjaro access is through tier 3 specialist weight management services. You typically need a BMI of 30+ (or 27+ with weight-related conditions like high blood pressure or sleep apnoea). Referral comes from your GP. Waiting lists vary by region.
What are the main side effects of each?
Metformin commonly causes stomach upset, nausea, and diarrhoea, especially when starting. Mounjaro's common side effects include nausea, diarrhoea, constipation, and stomach pain. Both typically improve over time. Always discuss potential side effects with your prescriber.
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.