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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Mounjaro Injection Sites — Where to Inject for Best Results
The injection site you choose for your weekly Mounjaro dose doesn't significantly affect how well the medication works — all approved sites (stomach, upper thigh, and upper arm) deliver tirzepatide effectively into the subcutaneous tissue. What matters more is rotating your injection site each week to minimise bruising, lipohypertrophy (fatty lumps), and injection site reactions. This UK health guide explains the best practice for administering your weekly weight loss injection and compares Mounjaro's injection approach with alternative GLP-1 options like Wegovy.
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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.
Mounjaro vs Wegovy — Injection Site and Administration Differences
Mounjaro (tirzepatide) is administered as a once-weekly subcutaneous injection using a pre-filled KwikPen. The approved injection sites are the stomach (abdomen), upper thigh, and upper arm — all three deliver the same pharmacokinetic profile, meaning absorption and effectiveness are equivalent. Wegovy (semaglutide) is also a once-weekly subcutaneous injection with identical approved sites.
The key difference between these two weekly weight loss injection options lies in their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whereas Wegovy is a GLP-1 agonist only. Clinical trial data (SURMOUNT-1 for Mounjaro vs STEP-1 for Wegovy) showed Mounjaro achieved approximately 20.9% average body weight loss at the maximum 15 mg dose, compared to Wegovy's approximately 14.9% at 2.4 mg, though individual results vary significantly.
Both are UK prescription-only medicines requiring clinical assessment before dispensing. Neither injection site is "more effective" than another — site rotation is what prevents complications.
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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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Best Practice for Mounjaro Injection Site Rotation and Comfort
To minimise injection site discomfort and localised reactions, rotate your injection site each week following this pattern: Week 1 stomach, Week 2 upper thigh, Week 3 upper arm, Week 4 stomach again. Within each site, inject at a slightly different location (at least a finger's width apart) to avoid repeated trauma to the same area.
Site rotation reduces your risk of lipohypertrophy (lumpy fatty deposits from repeated injections), bruising, and injection site pain. Store your Mounjaro KwikPen at 2–8°C in the fridge until use, then allow it to reach room temperature for 30 minutes before injecting — this can reduce injection discomfort. Always use a fresh needle for each injection. If you experience persistent redness, swelling, or warmth at the injection site lasting more than a few days, contact your GP or the healthcare professional managing your Mounjaro prescription.
- 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 the Mounjaro injection site affect how much weight I'll lose?
No. All approved injection sites (stomach, thigh, upper arm) deliver tirzepatide equally into the subcutaneous tissue, so absorption and weight loss potential are the same. Site choice does not influence results — what matters is dose, diet, activity level, and individual metabolism. Your prescriber will manage your dose titration based on your response and tolerance, not injection location.
Can I inject Mounjaro in the same spot every week?
Not recommended. Injecting in the same location repeatedly increases risk of lipohypertrophy (fatty lumps), bruising, and localised irritation. Rotate sites weekly across your stomach, thigh, and upper arm. Within each site, choose a slightly different spot. This also reduces injection discomfort and improves your overall injection experience over months of treatment.
Is Mounjaro or Wegovy better for weight loss?
Both are UK-licensed weekly weight loss injections with different mechanisms. Mounjaro (dual GIP+GLP-1) showed higher average weight loss in clinical trials (~21% body weight) versus Wegovy (~15%), but individual results vary greatly. Which is suitable for you depends on your medical history, NHS/private access, and prescriber assessment. Neither is universally "better" — suitability is personal.
What if my Mounjaro injection site becomes bruised or swollen?
Minor bruising after injection is normal and usually resolves within a few days. Apply ice immediately after injection if bruising is a concern. Persistent swelling, warmth, or redness lasting more than 3–4 days may indicate infection or a localised reaction — contact your GP or the healthcare team prescribing your Mounjaro. Rotate to a different site for your next weekly dose.
How long does Mounjaro take to work after injection?
Mounjaro begins working within hours of injection, with appetite suppression typically felt within the first 24–48 hours on your dose. Peak effects occur around 3–4 days after injection, which is why you take it once weekly. Weight loss becomes measurable over weeks and months, not days. Use our free GLP-1 weight loss calculator to see projected outcomes at different doses, and our progress tracker to log your actual results.
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.