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 Monjaro — Safe Administration Sites
Monjaro is a prescription-only, once-weekly weight loss injection containing tirzepatide. It's administered as a subcutaneous injection, meaning it goes just under the skin rather than into muscle or a vein. Knowing the correct injection sites is important for safe administration and to minimise injection site reactions. Your prescriber will provide detailed guidance, but understanding the approved sites and rotation schedule helps ensure you get the most from this GLP-1 receptor agonist and dual GIP receptor treatment. This guide covers the recommended injection areas for UK users.
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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 Monjaro Injection Sites
Monjaro can be injected into four main areas of the body:
- Stomach (abdomen) — the most commonly used site. Inject at least 2 inches away from the belly button and avoid areas with scars, bruises, or hardened skin
- Upper thigh (front or outer) — a convenient alternative for self-injection
- Upper arm (back) — suitable for this weekly weight loss injection, though you may need assistance
- Buttocks — less commonly used but an approved site
Always use a different injection site each week to reduce the risk of lipohypertrophy (lumps under the skin). This rotation schedule helps maintain consistent absorption of tirzepatide and reduces injection site discomfort. Never inject into areas where skin is tender, bruised, hardened, or scarred.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Injection Site Reactions and Tips for Comfort
Common injection site reactions include redness, bruising, pain, or mild swelling — these typically resolve within a few hours or days. To minimise discomfort: allow the KwikPen to reach room temperature for 15–20 minutes before injecting, use a fresh needle for each injection, and inject slowly. If you experience persistent redness, warmth, or signs of infection at any site, contact your GP. Always wash your hands before injection and ensure the injection area is clean and dry. Proper technique and site rotation are key to comfortable administration of this weekly GIP and GLP-1 receptor agonist.
- 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 Monjaro into the same site every week?
No. It's important to rotate your injection sites each week to reduce the risk of lipohypertrophy (lumps or thickened skin) and to maintain consistent absorption of tirzepatide. Using a different area each week helps avoid injection site reactions and ensures the medication works effectively.
What should I do if I have bruising or pain at my injection site?
Minor bruising and discomfort are normal and usually resolve within hours. Apply a cold compress if needed. However, if you experience persistent pain, redness, warmth, or signs of infection, contact your GP or healthcare provider. These could indicate an injection site reaction requiring medical attention.
Is the stomach the best place to inject Monjaro?
The stomach is the most commonly used site because it's accessible for self-injection and generally comfortable. However, the upper thigh and upper arm are equally approved sites. Choose based on personal preference and comfort — the most important thing is rotating sites each week to avoid skin thickening.
Can I inject Monjaro through clothing?
No. Always inject through clean, bare skin. Remove or lift your clothing away from the injection site and ensure the area is clean and dry before injecting. This reduces the risk of infection and ensures accurate needle placement through the skin.
How long does it take for injection site reactions to go away?
Most mild injection site reactions (redness, bruising, slight swelling) resolve within hours to a few days. If reactions persist beyond a week, worsen, or show signs of infection (increasing warmth, pus, or spreading redness), contact your GP for advice.
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.