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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Most Effective Injection Site For Mounjaro
The injection site you choose for Mounjaro doesn't significantly affect how well the medication works — all approved sites deliver the same clinical results. However, rotating injection sites properly and using correct technique are important for comfort, safety, and avoiding skin irritation. This guide explains the three recommended injection sites and answers common questions about injection technique and how Mounjaro compares to Wegovy when prescribed in the UK.
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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.
Approved Mounjaro Injection Sites and Technique
Mounjaro comes as a pre-filled KwikPen and can be injected into three areas of your body:
- Stomach (abdomen) — the most commonly used site. Inject at least 5cm away from the belly button and avoid areas with scars, bruises, or hardened skin
- Upper thigh (front or outer area) — a good alternative that many people find convenient
- Upper arm (back of the upper arm) — also approved, though requires assistance to inject properly
Research shows that rotation between injection sites is important for preventing lipohypertrophy (fatty tissue buildup). Use a different site each week rather than repeatedly injecting in the same spot. All three sites absorb tirzepatide at similar rates, so absorption speed is not affected by your choice of location. The dose titration schedule (starting at 2.5mg and increasing to a maximum of 15mg weekly) remains the same regardless of injection site.
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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 — Injection Site Comparison
Mounjaro (tirzepatide) is injected once weekly using a pre-filled KwikPen with approved sites of stomach, thigh, or upper arm. Wegovy (semaglutide) is also a weekly injection pen using the same three injection sites. Both medications follow identical site rotation principles, and both have similar absorption characteristics across all approved locations.
The key difference between these weight loss injections is not the injection site, but the mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 only receptor agonist. Clinical trials (SURMOUNT for Mounjaro and STEP for Wegovy) showed both can support weight loss when combined with diet and exercise. Individual suitability depends on your health profile, and a prescriber conducting a clinical assessment will determine which weekly weight loss injection, if either, is appropriate for you in the UK.
- 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 injection site affect how well Mounjaro works?
No. All three approved injection sites (stomach, thigh, upper arm) absorb tirzepatide at similar rates. Clinical results are the same regardless of location. What matters is rotating sites each week to prevent skin issues and using correct injection technique.
Which injection site is best for Mounjaro?
The stomach (abdomen) is most commonly used and easiest for self-injection. The thigh is a good alternative. The upper arm requires assistance. Choose based on comfort and convenience — all deliver identical clinical effectiveness.
Can I inject Mounjaro in the same spot every week?
No. Rotating injection sites is important to prevent lipohypertrophy (fatty tissue buildup) and maintain skin health. Use a different site each week, marking each location to ensure proper rotation.
How do Mounjaro and Wegovy compare regarding injection sites?
Both medications use the same three approved injection sites (stomach, thigh, upper arm) and both require weekly injections. The main differences are the mechanism of action (Mounjaro is dual GIP+GLP-1, Wegovy is GLP-1 only) and the dose ranges, not the injection location.
What should I do if I have bruising or pain at my Mounjaro injection site?
Minor bruising is normal. Rotate to a different site for your next injection and allow the area to heal. Avoid injecting near scars or areas with existing bruising. If you experience severe pain, persistent swelling, or signs of infection, contact your GP or prescriber.
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