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? A UK Guide
Mounjaro (tirzepatide) is injected under the skin once weekly, and the location you choose doesn't significantly affect how well the medication works — all approved injection sites are equally effective. However, proper injection technique, site rotation, and comfort are important for consistent treatment and reducing side effects like bruising or injection site reactions. This guide explains the recommended injection sites, how to rotate them, and practical tips for safe administration.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 main areas of the body, all of which deliver the medication equally effectively:
- Stomach (abdomen) — the most commonly used site; inject into the fatty tissue on the sides or front of your stomach, avoiding the area 5 cm around your belly button
- Upper thigh — a convenient self-injection site; use the outer area of the thigh, avoiding the inner thigh where there are more nerves and blood vessels
- Upper arm (back of the arm) — usually requires assistance from another person; pinch the skin and fatty tissue and inject into the back of the upper arm between shoulder and elbow
Research shows absorption is similar across all three sites, meaning your Mounjaro KwikPen dose will work effectively regardless of location. The choice comes down to comfort, accessibility, and your personal preference.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Tips
Rotate your injection sites each week to reduce bruising, hardened skin (lipohypertrophy), and injection site reactions. Keep sites at least 2.5 cm (about 1 inch) apart. Using the same exact spot repeatedly can cause skin irritation and may affect medication absorption over time.
Safety considerations: Always use a new needle and clean the skin with an alcohol wipe before injecting. Check that your KwikPen has not been frozen — if it has, discard it. Allow the pen to reach room temperature if it's been refrigerated. Inject slowly and leave the needle under the skin for 6 seconds after pressing the injection button to ensure the full dose is delivered. Our free weight loss calculator can help you track your progress at each dose level.
- 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 well Mounjaro works?
No. Clinical data shows that Mounjaro (tirzepatide) is absorbed equally well from the stomach, thigh, and upper arm. All three sites are equally effective. Your choice should be based on comfort and convenience, not effectiveness.
Can I inject Mounjaro into the same spot every week?
Not recommended. Repeated injections into the same spot can cause lipohypertrophy (hardened fatty lumps), bruising, and reduced absorption over time. Rotate injection sites at least 2.5 cm apart each week to minimise these risks.
Which injection site is easiest for self-injection?
The stomach and upper thigh are easiest for self-injection. The upper arm is possible but usually requires assistance. Many people alternate between stomach and thigh for comfort and convenience during their weekly weight loss injection routine.
What should I do if I get bruising or pain at the injection site?
Mild bruising is common and usually resolves within days. Rotate sites, allow the skin to warm to room temperature before injecting, and use a fresh needle each time. If you develop persistent pain, redness, warmth, or swelling lasting more than a few days, contact your GP or the prescriber who issued your Mounjaro prescription.
How long should the needle stay under the skin when injecting Mounjaro?
Leave the needle under the skin for approximately 6 seconds after pressing the injection button on your KwikPen. This ensures the full dose is delivered. Check the patient information leaflet in your pack for specific timing guidance.
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.