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
Where Do I Inject Mounjaro?
Mounjaro is injected under the skin (subcutaneously) using a pre-filled KwikPen device once weekly. The three approved injection sites are your stomach (abdomen), upper thigh, or upper arm — you should rotate between sites each week to prevent skin irritation. This guide explains safe injection technique, site rotation, and how Mounjaro compares to Wegovy in terms of injection practicality. Always follow your prescriber's instructions and read the patient information leaflet before your first injection.
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.
Mounjaro vs Wegovy — Injection Site Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injectable medications for weight management in the UK, but they differ in delivery systems and ease of use. Mounjaro comes as a pre-filled KwikPen with four doses per pen, whilst Wegovy uses a pre-filled auto-injection pen. Both medications use the same three approved injection sites: stomach, upper thigh, and upper arm. The key practical difference is that Mounjaro's KwikPen requires you to dial the dose before injecting, whereas Wegovy's auto-pen injects automatically after you press the button — some users find Wegovy's system slightly simpler, though both are straightforward. Site rotation is essential with both medications to prevent lipohypertrophy (thickened skin at injection sites). Clinical trial data (SURMOUNT for Mounjaro, STEP for Wegovy) showed similar injection-related side effects — mild redness or bruising at the injection site occurred in around 10–15% of users with both medications.
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.
Safe Mounjaro Injection Technique and Site Rotation
Always inject Mounjaro at room temperature (allow the pen to sit for 15 minutes if refrigerated). Clean the skin with an alcohol wipe and let it dry completely before injecting. Inject at a 90-degree angle into a skin fold, and hold the needle in place for 6 seconds after pressing the injection button. Rotate your injection site each week using this pattern: week 1 stomach, week 2 thigh, week 3 arm, week 4 opposite side of stomach, then repeat. Never inject into areas where skin is bruised, tender, or hardened. If you experience severe pain, swelling, or signs of infection, contact your GP or prescriber immediately. Proper technique and rotation reduce the risk of injection site reactions and ensure consistent medication absorption.
- 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 inject Mounjaro in the same spot every week?
No. The NHS and manufacturers recommend rotating injection sites weekly to prevent lipohypertrophy (thickened or lumpy skin). Alternate between stomach, thigh, and upper arm in a planned pattern. This also helps ensure consistent medicine absorption.
Is Mounjaro injected at home or at a clinic?
In the UK, after your initial prescriber assessment and training, Mounjaro is self-injected at home once weekly. Your prescriber will show you the correct technique before your first injection. If you're uncomfortable self-injecting, discuss alternatives with your GP.
Does Mounjaro injection hurt more than Wegovy?
Both medications use thin needles and most users report minimal discomfort. Mounjaro's KwikPen needle is slightly thinner than Wegovy's. Pain depends on needle sensitivity, injection technique, and individual variation — both are generally well-tolerated.
What's the difference between Mounjaro and tirzepatide?
Mounjaro is the brand name; tirzepatide is the active ingredient. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1-only agonist. Both are prescription-only in the UK and require clinical assessment before prescription.
Can I use the BMI calculator to check if I'm eligible for Mounjaro?
Our free BMI calculator can show your current BMI, which is one eligibility factor for weight management medications in the UK. However, actual prescription eligibility depends on your full health picture — your GP or prescriber must assess you individually to determine suitability.
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.