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 Do You Inject Mounjaro?
Mounjaro (tirzepatide) is injected under the skin once per week, and you have three main injection site options. The most common site is the stomach (abdomen), but you can also inject into the upper thigh or upper arm — whichever is most comfortable for you. Rotating your injection site each week helps prevent skin irritation and lipodystrophy (changes in fat deposits). Your prescriber will show you the correct injection technique when you start treatment. If you're using the Mounjaro KwikPen device, the injection takes just a few seconds.
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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
Mounjaro can be injected into three areas of your body:
- Stomach (abdomen) — the most commonly used site, usually on either side of the navel or the lower abdomen
- Upper thigh — the front or outer side of the thigh
- Upper arm — the back of the upper arm (usually requires help from another person)
All three sites are equally effective for delivering the medication. You should rotate your injection site each week — for example, inject into your stomach one week, then your thigh the next week, then your arm the following week. This rotation helps prevent lipodystrophy (abnormal fat deposits or indentations) and ensures consistent absorption of the tirzepatide. Never inject into areas that are bruised, tender, scarred, or have stretch marks.
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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.
How to Inject Mounjaro Safely
When using your Mounjaro KwikPen, follow these steps: remove the pen from the fridge and let it sit at room temperature for 15 minutes; check that the liquid is clear and colourless; attach a new needle; select your injection site and clean it with an alcohol wipe; pinch the skin gently; insert the needle at a 90-degree angle; press and hold the injection button until you hear two clicks (taking about 3 seconds); remove the needle and release the skin. Apply gentle pressure with a cotton ball if needed. Never massage the injection site immediately after. Our GLP-1 weight loss calculator can help you track your expected weight loss progression alongside your injections.
- 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 into the same spot every week?
No. You should rotate your injection site each week to prevent lipodystrophy and ensure consistent medication absorption. Rotating between abdomen, thigh, and arm is recommended. You can use the same general area (e.g., abdomen) but inject slightly to the left or right each week to avoid the exact same spot.
Do I need to warm up my Mounjaro pen before injecting?
Yes. Remove your KwikPen from the fridge and leave it at room temperature (below 30°C) for 15 minutes before injection. Never use a warm water bath or microwave. Letting it reach room temperature makes the injection less painful and easier to administer.
Which Mounjaro injection site hurts the least?
This varies between individuals. Most people find the stomach least painful because the skin is thicker and there are fewer nerves. The thigh can also be comfortable. The upper arm may require assistance. Start with your preferred site and rotate as recommended.
What should I do if I accidentally inject Mounjaro into a bruised area?
If you've injected into a bruised or tender area, contact your prescriber or pharmacist for guidance. Avoid that site for future injections. Continue with your normal injection schedule the following week. Do not take a double dose.
Can someone else inject my Mounjaro for me?
Yes. If you're uncomfortable self-injecting, a trained family member or healthcare professional can administer your Mounjaro — particularly if you're using the upper arm site. Your prescriber will demonstrate the correct technique to whoever will be helping you.
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.