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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Where Is The Most Effective Place To Inject Mounjaro?
Mounjaro (tirzepatide) is injected subcutaneously — under the skin — and the injection site you choose can affect both comfort and absorption. In the UK, this prescription-only medication comes as a pre-filled KwikPen, and rotating your injection sites helps prevent skin irritation and lipohypertrophy (thickened areas of skin). The three approved injection areas are the stomach, upper thigh, and upper arm, with rotation being key to safe, effective use over time.
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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.
Approved Mounjaro Injection Sites And Rotation
The stomach (abdomen) is the most commonly used injection site for Mounjaro in the UK. This area has good subcutaneous tissue and is convenient for self-injection. The upper thigh is the second preferred site and also offers easy access for most people. The upper arm is the third approved location but typically requires assistance from another person.
- Always rotate injection sites weekly — injecting in the same spot repeatedly can cause lipohypertrophy, making medication absorption less predictable
- Keep each new injection at least 1–2cm away from your previous injection site
- Avoid injecting into areas with bruises, lumps, or hardened skin
- Let your skin temperature return to normal before injecting if the area is very cold
Proper site rotation ensures consistent medication absorption and reduces the risk of local skin complications. Your prescriber or pharmacist will demonstrate the correct injection technique when you start tirzepatide treatment through NHS specialist services or private prescription.
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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 Dose Titration And NHS Prescription Context
In the UK, Mounjaro is typically prescribed through NHS tier 3 specialist weight management services following NICE approval, though private prescription is also available. The dose titration schedule — the gradual increase in dose over weeks — means you'll visit different injection sites across your body as you progress through the 2.5mg starting dose through to potentially 15mg maintenance. This built-in rotation during the titration phase itself supports better tolerability.
- Weeks 1–4: 2.5mg injection once weekly
- Weeks 5+: Dose increases by 2.5mg every 4 weeks until therapeutic dose is reached
- Peak absorption occurs 8–72 hours after injection
- Your GP or NHS prescriber can advise on site rotation during your treatment plan
Whether accessed through NHS services or via private online prescribing, always follow your prescriber's guidance on injection technique and site rotation to maximise safety and medication effectiveness.
- 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 — rotating injection sites is essential. Injecting in the same spot repeatedly can cause lipohypertrophy (thickened skin areas) and may reduce medication absorption. Rotate between your stomach, upper thigh, and upper arm weekly, keeping at least 1–2cm distance between injections on the same body area.
Which Mounjaro injection site absorbs fastest?
The stomach (abdomen) is often cited as absorbing slightly faster than the upper thigh due to greater subcutaneous blood flow, though differences are minimal. Consistent rotation is more important than choosing one "perfect" site. Peak plasma concentration occurs 8–72 hours after injection regardless of site.
Is it painful to inject Mounjaro?
Most people report minimal pain — the KwikPen needle is very fine. You may feel a slight pinch or pressure. If injection sites become painful, tender, or show signs of irritation, notify your prescriber. This may indicate improper technique or lipohypertrophy from repeated use of the same site.
Can I inject Mounjaro through clothing?
No — always inject directly into clean, exposed skin. Remove clothing from the injection area, clean the skin with an alcohol wipe if visibly soiled, and allow it to dry completely before injecting. Never inject through fabric.
What should I do if I get a bruise or lump at my Mounjaro injection site?
Small bruises are normal and usually resolve within a few days. However, lumps or hardened areas suggest lipohypertrophy — avoid that site for at least 4 weeks and inform your prescriber. If bruising is severe or accompanied by pain, swelling, or redness, contact your GP or NHS services for advice.
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