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 to Inject Mounjaro — Safe Sites and Rotation Guidance
Mounjaro (tirzepatide) is administered as a once-weekly weight loss injection using a pre-filled KwikPen device. In the UK, Mounjaro is prescribed through NHS specialist tier 3 weight management services or private prescribing following clinical assessment. Understanding the correct injection sites and rotation technique is essential for safe, effective use and minimising injection site reactions. This guide covers NHS-aligned administration guidance for adults taking this weekly weight loss 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.
Approved Mounjaro Injection Sites and Rotation
Mounjaro can be injected into three safe sites on your body:
- Stomach (abdomen) — the most commonly used site, inject into the fatty tissue 5cm away from your belly button or previous injection marks
- Upper thigh — on the front or outer side of the thigh, into fatty tissue
- Upper arm — back of the arm between shoulder and elbow (requires help from another person to administer safely)
Rotate your injection site each week to reduce local reactions and maintain consistent absorption. Never inject into areas with active bruising, swelling, hardness, redness, or scarring. If you have concerns about injection site reactions, discuss them with your GP or prescribing clinician. The KwikPen device is designed for subcutaneous injection only — never inject into muscle or vein.
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.
NHS Administration and Safety Guidance
The NHS recommends following the standard subcutaneous injection technique: clean the skin with an alcohol wipe, allow it to dry, pinch the skin gently, insert the needle at a 90-degree angle, inject slowly, and withdraw the needle. Dispose of used needles safely in a sharps container — never in household waste. Store unused Mounjaro pens in the refrigerator at 2–8°C in the original carton. After first use, pens can be kept at room temperature (below 30°C) for up to 30 days. If you experience persistent injection site pain, significant bruising, or signs of infection, contact your GP promptly. Your prescriber should provide detailed training on injection technique before your first dose.
- 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 — always rotate your injection site each week to different areas (stomach, thigh, or arm) or different locations within the same area. Rotating sites reduces risk of injection site reactions, lipohypertrophy (thickened tissue), and ensures consistent medication absorption. Using the same spot repeatedly can cause pain, bruising, and reduced effectiveness.
Is the upper arm injection site suitable for self-injection?
The upper arm is technically an approved site, but it's difficult to self-inject safely into the back of your upper arm. Most people use stomach or thigh sites for self-injection. If you choose the arm, ask another person (partner, family member, or healthcare professional) to administer the injection for you. Your prescriber can advise which site suits you best.
What should I do if I develop redness or swelling at my injection site?
Mild redness or small bruises are common and usually resolve within a few days. If you develop significant swelling, warmth, hardness, or signs of infection (increasing redness, pus, fever), contact your GP or NHS 111 for advice. These may indicate lipohypertrophy or infection, which require medical assessment. Avoid injecting into affected areas until they've fully healed.
How deep should the needle go when injecting Mounjaro?
Mounjaro is a subcutaneous injection, meaning it goes into the fatty layer under your skin — not into muscle. Insert the needle fully at a 90-degree angle into the pinched skin fold. The needle should penetrate the skin and subcutaneous fat but not reach muscle. Your prescriber or nurse will demonstrate correct technique before you begin — ask for a demonstration if unsure.
Can I reuse Mounjaro injection needles?
No — each KwikPen contains four pre-attached needles (one per weekly dose). Never reuse needles. Using the same needle multiple times increases infection risk, dulls the needle, and can cause tissue damage. After each injection, dispose of the entire pen in a sharps container. Your GP practice or pharmacy can advise where to dispose of sharps safely in your area.
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.