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 Should You Inject Mounjaro?
Mounjaro (tirzepatide) is administered as a once-weekly injection, and the injection site you choose plays an important role in how well the medication works. In the UK, the recommended sites are your stomach, upper thigh, or upper arm — each offers reliable absorption of this GLP-1 and GIP receptor agonist. To get the most from your weekly weight loss injection, rotating your sites and following proper injection technique is essential. Your prescriber will show you how to use your Mounjaro KwikPen correctly, but understanding the best practices can help you feel more confident and ensure consistent results.
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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 Injection Sites and What Works Best
Mounjaro can be injected into three main areas:
- Stomach (abdomen): The most commonly used site. Inject into the fatty tissue around your stomach area, at least 5cm (2 inches) away from your belly button. This site typically has good fatty tissue and is easy to access yourself.
- Upper thigh (front or outer surface): Also straightforward to self-inject. Pinch the skin to create a fold and inject into the fatty layer. Both legs can be used, helping you rotate between sites.
- Upper arm (back of the upper arm): Works well but often requires assistance from another person to inject safely. This site is useful for rotating to reduce tissue irritation.
All three sites absorb Mounjaro reliably — none is significantly more "effective" than the others in terms of weight loss outcomes. What matters most is consistent rotation and proper injection technique. Rotating between sites each week reduces the risk of lipohypertrophy (thickened tissue from repeated injections) and potential absorption problems.
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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 Best Practice Tips
To get the best from your Mounjaro dose titration schedule, rotate your injection sites systematically. A simple rotation pattern might be: Week 1 stomach, Week 2 left thigh, Week 3 right thigh, Week 4 upper arm (with help) — then repeat. Always inject at least 2–3cm away from your previous injection site to avoid irritating the same area repeatedly.
NHS prescribers recommend:
- Use a clean, dry area of skin each time
- Inject at room temperature (remove from fridge 30 minutes beforehand)
- Check the solution is clear — do not inject if cloudy
- Let the injection site dry if you've cleaned it with alcohol
- Avoid injecting into areas with bruising, hardness, or redness
Proper technique and site rotation support consistent medication absorption and reduce side effects like injection site reactions. If you experience persistent pain, swelling, or unusual bruising at any site, contact your GP or prescriber.
- 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 much weight you lose on Mounjaro?
No. All three approved sites (stomach, thigh, upper arm) absorb Mounjaro equally well. Weight loss outcomes depend on the dose, your diet, activity level, and individual factors — not which site you choose. What matters is consistency and rotating to avoid tissue damage.
Can you use the same injection site every week?
It's not recommended. Repeatedly injecting in the same spot can cause lipohypertrophy (thickened fatty tissue), which may reduce absorption over time. Rotating sites each week ensures consistent medication delivery and reduces injection site reactions.
What should you do if an injection site becomes red, swollen, or painful?
Minor redness or slight bruising is normal. However, if you experience persistent pain, significant swelling, or hard lumps, stop using that site temporarily and contact your GP or prescriber. They can advise whether to switch sites or adjust your injection technique.
Is the stomach the best site for Mounjaro injections?
The stomach is the most commonly used site because it's easy to access and has adequate fatty tissue. However, the thigh and upper arm are equally effective. Choose based on what feels comfortable and practical for you — the key is rotating between all three.
How long after injecting Mounjaro can you exercise or go swimming?
There's no specific waiting period. You can resume normal activities immediately after injection. However, avoid strenuous exercise or hot baths for a few hours if the injection site is red or uncomfortable, as heat may increase irritation.
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.