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
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Considering Mounjaro but feeling anxious? Here's what you need to know
It's completely normal to feel apprehensive about starting a new prescription medicine like Mounjaro (tirzepatide). Understanding how it works, what to expect, and how it compares to alternatives like Wegovy can help ease concerns. Both are prescription-only weight management injections licensed in the UK, but they work differently and affect people in different ways. This guide compares both options so you can make an informed decision with your prescriber.
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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 in mechanism and trial outcomes
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously, which is what distinguishes it from Wegovy. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15mg lost an average of 20.9% of their body weight over 72 weeks when combined with a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. In the STEP-1 clinical trial, participants lost an average of 14.9% of their body weight over 68 weeks on the maximum dose of 2.4mg, also alongside lifestyle changes.
Both medications can cause similar side effects like nausea, diarrhoea, and constipation, especially during the first few weeks of treatment. The key difference is that Mounjaro's dual mechanism may produce stronger effects — both benefits and side effects — in some people. Individual responses vary considerably; what suits one person may not suit another.
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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.
Why some people feel nervous — and what actually helps
Common anxieties about starting Mounjaro include fear of side effects, worry about injections, and uncertainty about whether it will work. The good news: side effects are typically mild and temporary, particularly if your dose is increased slowly (which is standard practice). Most people adjust well after the first 4–8 weeks. You're not injecting into a vein — the injection goes under the skin (subcutaneously) in your stomach, thigh, or upper arm, which is straightforward and painless for most users.
Your prescriber will choose between Mounjaro or Wegovy based on your individual health, BMI, and preferences. Both require clinical assessment before prescribing. Use our GLP-1 weight loss calculator to explore potential outcomes with both medications, and discuss any concerns honestly with your GP or prescriber — they can address your specific worries and may suggest starting Wegovy if Mounjaro feels too daunting.
- 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 Mounjaro scarier than Wegovy?
No — both have similar side effect profiles. The difference is that Mounjaro's dual mechanism may produce stronger effects in some people, but this means it may also work more effectively for weight loss. Side effects are usually mild and improve after a few weeks. Your prescriber will choose the medication best suited to you.
Can you stop Mounjaro if you feel scared?
Yes. You can pause or stop Mounjaro at any time, though you should discuss this with your prescriber. If you experience severe side effects, contact your GP. Most people find that side effects settle after the first few weeks as their body adjusts to the dose.
Which has fewer side effects — Mounjaro or Wegovy?
Side effects are comparable between both medications. Nausea, diarrhoea, and constipation are common in the first 4–8 weeks on either drug. Clinical trial data shows similar frequencies, though individual experiences vary. Slow dose increases help minimise early side effects.
Is the Mounjaro KwikPen injection painful?
No — the Mounjaro KwikPen is designed for subcutaneous injection (under the skin) and most users report it is virtually painless. The needle is very fine. You inject in your stomach, thigh, or upper arm — areas with plenty of tissue. Rotating injection sites each week reduces discomfort.
How do I decide between Mounjaro and Wegovy?
Your prescriber will assess your individual health, BMI, and weight-related conditions to recommend the most suitable option. Both require a UK prescription and clinical assessment. If you have concerns about either medication, discuss them with your GP — they can explain which aligns better with your health profile and address your specific worries.
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.