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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Injecting Mounjaro in Your Thigh — What You Need to Know
Injecting Mounjaro in your upper thigh is one of three approved injection sites, alongside your stomach and upper arm. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are injected once weekly using pre-filled pens, but they work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. This guide explains safe thigh injection technique, how the two medications compare, and what eligibility criteria mean for your prescription. Use our free BMI calculator to check your starting point, and our GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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 to Safely Inject Mounjaro in Your Thigh
Your upper thigh is an excellent injection site for Mounjaro — it's easy to access, rotates between left and right weekly, and requires no assistance unlike your upper arm. Injection technique matters: clean the area with soap and water (alcohol swabs are optional), allow skin to dry completely, pinch the skin gently, insert the needle at a 90-degree angle, and hold for 3 seconds before releasing. Do not massage the injection site afterwards — this can affect medication absorption. Rotate between your three approved sites (stomach, upper thigh, upper arm) each week to reduce injection site reactions like redness or bruising.
- Allow Mounjaro to reach room temperature for 15–30 minutes before injecting if taken from the fridge
- Never inject into areas that are bruised, hardened, or tender
- Use a fresh needle for each injection — never reuse
- Dispose of used pens in a sharps bin or return to your pharmacy
Both Mounjaro and Wegovy use similar injection techniques, though Mounjaro's KwikPen may feel slightly different to handle than Wegovy's auto-injection pen.
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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.
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Mounjaro vs Wegovy — Mechanism and Efficacy Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural appetite-regulating pathways simultaneously. Wegovy (semaglutide) activates GLP-1 only. In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight across doses, while Wegovy showed approximately 15% in the STEP-1 trial. Both medications require a prescription following clinical assessment and are used alongside reduced-calorie diet and increased physical activity. Mounjaro reaches a maximum dose of 15 mg weekly; Wegovy maxes at 2.4 mg weekly. Neither assures specific outcomes — individual results vary significantly depending on diet adherence, exercise, genetics, and metabolism.
- Your prescriber decides which medication is suitable based on your health history
- Neither is available on NHS prescription for most people outside specialist tier 3/4 services
- Both require dose titration starting at lower doses and increasing weekly
- 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 you inject Mounjaro into your thigh every week?
Yes. Your thigh is one of three approved injection sites for Mounjaro. Rotate between your stomach, both thighs, and upper arms each week to reduce injection site reactions. The upper thigh is convenient because it requires no assistance and is easy to access. Always use a fresh needle and clean the area beforehand.
Is Mounjaro injection technique different from Wegovy?
The injection technique is very similar — both are once-weekly injections using pre-filled pens. The main difference is the pen device itself (Mounjaro uses a KwikPen, Wegovy uses an auto-injection pen). Both allow you to inject into the thigh, stomach, or upper arm. Your prescriber or pharmacist will show you the correct technique when you start.
Why is Mounjaro more effective than Wegovy?
In clinical trials, Mounjaro showed slightly higher average weight loss than Wegovy, but this varies between individuals. Mounjaro's dual GIP and GLP-1 mechanism may contribute to greater appetite suppression for some people. However, results depend on many factors including diet, exercise, genetics, and how long you've been taking it. Your prescriber can advise which is more suitable for you.
Do I need a prescription to inject Mounjaro in the UK?
Yes. Mounjaro is prescription-only in the UK. You need a clinical assessment from a licensed prescriber before a prescription is issued. This can happen through NHS specialist weight management services (tier 3/4) or through private online prescribing following an online clinical assessment with a UK-licensed prescriber.
What happens if you inject Mounjaro into the same spot twice?
Injecting repeatedly into the same site increases risk of injection site reactions including redness, bruising, hardening, or tenderness. Rotating between approved sites (stomach, thighs, upper arms) weekly reduces these risks significantly. If a site is bruised or tender, skip it and use a different site that week.
Ready to take control of your weight loss journey?
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