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 vs Wegovy: What's the Difference?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight loss injections licensed in the UK, but they work in different ways and have different dosing schedules. Understanding the key differences—including how they're administered, their mechanism of action, side effect profiles, and eligibility criteria—can help you and your GP or prescriber decide which might be more suitable for your individual circumstances. Both are taken alongside a reduced-calorie diet and increased physical activity.
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
The most significant difference between Mounjaro and Wegovy is how they work at a hormonal level. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone pathways in your body. This dual action may help regulate appetite, reduce cravings, and support feeling fuller after meals more effectively than a single-pathway medication. Wegovy contains semaglutide, a GLP-1 receptor agonist only—it activates just the GLP-1 pathway. Both medications slow gastric emptying (how quickly food leaves the stomach) and may help reduce hunger signals to your brain.
In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight, whilst Wegovy (semaglutide) showed average weight loss of approximately 15% of body weight. However, individual results vary significantly, and these are average outcomes from controlled trials—not studied results. Both work best when combined with lifestyle changes: a reduced-calorie diet and regular physical activity. Use our free GLP-1 weight loss calculator to explore potential outcomes 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.
Dosing, Administration, and Practical Differences
Mounjaro is administered as a once-weekly injection using a pre-filled KwikPen, starting at 2.5 mg and escalating over weeks to a maximum of 15 mg per week. Wegovy is also a once-weekly injection but uses a different pen device, starting at 0.25 mg and escalating to a maximum of 2.4 mg per week. This means Mounjaro doses are numerically higher but work via a different mechanism—the numbers aren't directly comparable. Both require a minimum of 4 weeks at each dose level before increasing. Storage is similar: refrigerate unopened pens at 2–8°C, and after first use, store at room temperature (below 30°C) for up to 30 days. Both are prescription-only and require an online clinical assessment or NHS specialist referral before being dispensed. Costs, availability, and waiting times vary between NHS and private routes.
- 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 is more effective—Mounjaro or Wegovy?
Clinical trials show Mounjaro may achieve slightly higher average weight loss (15–21%) compared to Wegovy (~15%), but individual results vary. Neither is assured to work; effectiveness depends on your personal response, diet, and activity level. Your GP or prescriber can help assess which may be more suitable for you.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide, but Wegovy is licensed specifically for weight management in the UK at higher doses (up to 2.4 mg). Ozempic is licensed for type 2 diabetes management at lower doses (up to 2 mg). Using Ozempic for weight loss would be off-label in the UK—Wegovy is the licensed weight management option.
Are the side effects of Mounjaro and Wegovy the same?
Side effect profiles are similar—both commonly cause nausea, diarrhoea, vomiting, and stomach pain, particularly in early weeks. These often improve over time. Serious side effects are rare but similar: pancreatitis, gallbladder issues, and allergic reactions. Your prescriber will discuss which risks matter most for your health.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between GLP-1 medications is possible but must be managed by your prescriber. There's no official conversion chart—any change in medication requires a new clinical assessment and dose adjustment. Never switch without medical guidance.
How do I know which one my GP will prescribe?
Both are available on the NHS through tier 3 specialist weight management services, though availability varies by region and waiting lists can be long. Privately, both are available through online clinical assessment. Your prescriber will assess your personal health history, contraindications, and preference to recommend the most suitable option.
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.