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 Best To Inject Mounjaro
Mounjaro (tirzepatide) comes as a pre-filled KwikPen for self-injection once weekly. The three approved injection sites are your stomach (abdomen), upper thigh, and upper arm — though stomach injection is most commonly used and easiest for self-administration. Rotating your injection site each week is important to avoid skin irritation and lipohypertrophy (thickened tissue). This guide explains the best injection techniques, how to rotate sites safely, and what to expect during self-injection with your Mounjaro KwikPen prescription.
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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
Mounjaro has three approved injection sites under UK prescribing guidance:
- Stomach (abdomen) — the most popular choice. Inject into the area between your ribs and hip bones, at least 5cm away from your belly button. This site is easiest to reach for self-injection and many people find it the most comfortable.
- Upper thigh (front or outer) — a good alternative. You can self-inject here, though some find it slightly less convenient than the abdomen. Inject on the outer or front surface, away from the inner thigh.
- Upper arm (back of the upper arm) — approved but requires assistance from another person for comfortable self-injection, as reaching the correct area is difficult alone.
All three sites are equally effective — the choice comes down to personal preference and comfort. Many people rotate between stomach and thigh, reserving the upper arm for occasions when someone can help.
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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 Rotation and Safety
Rotating your injection site each week is essential to protect your skin health. If you inject in the same spot repeatedly, you risk developing lipohypertrophy — a thickening of fatty tissue that can affect how your body absorbs the medication. Aim to rotate sites systematically: week one in your stomach, week two in your thigh, week three in your stomach again, and so on.
When injecting, ensure the skin is clean and dry. Pinch the skin gently between your thumb and forefinger, insert the needle at a 90-degree angle, and inject slowly. After injection, hold the pen in place for 3 seconds before withdrawing. Avoid injecting into areas with bruising, scars, or broken skin. If you experience persistent redness, warmth, or swelling at any injection site, contact your GP or the prescriber who issued your Mounjaro KwikPen 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.
Can I inject Mounjaro in the same spot twice?
Not recommended. Repeated injections in the same spot increase the risk of lipohypertrophy (thickened tissue that can reduce medication absorption). Rotate your injection site each week between your stomach, thigh, and upper arm to maintain skin health and ensure consistent drug delivery.
Which injection site is best for beginners?
The stomach (abdomen) is easiest for first-time users because it's easy to reach, has more available injection area, and many people find it most comfortable. Inject at least 5cm away from your belly button. You can switch to other sites once you're confident with the technique.
Is the Mounjaro KwikPen injection painful?
Most people report minimal pain — the needle is very thin (29 gauge). You may feel a small pinch, but it's usually less painful than taking blood for tests. Injection discomfort often decreases after the first few weeks as you become familiar with the process. If you experience significant pain, speak to your prescriber.
What should I do if I accidentally inject in the wrong spot?
If you've injected into scar tissue, bruised skin, or broken skin by mistake, it's generally still safe — the dose enters your body. However, monitor the area for unusual redness or swelling. For future doses, choose a cleaner site at least 2cm away. Contact your GP if you're concerned about any injection site reaction.
Can I use the upper arm if I'm alone?
The upper arm (back area) is approved but difficult to self-inject safely without assistance, as it's hard to reach and maintain proper needle angle. If you live alone, stick to your stomach or thigh, which are much easier for solo self-injection. Ask a family member or friend to help if you specifically want to use the upper arm site.
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