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 Mounjaro — Safe Sites and Technique
Mounjaro (tirzepatide) is a once-weekly weight loss injection that you can administer yourself at home using a pre-filled KwikPen. The injection can be given into three main body areas: your stomach (abdomen), upper thigh, or upper arm. Rotating your injection site each week helps prevent irritation and ensures consistent medication absorption. This guide explains the recommended sites, proper technique, and how Mounjaro compares to other weekly weight loss injection options like Wegovy in the UK.
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.
Mounjaro Injection Sites — Where You Can Inject
Mounjaro can be injected into three safe areas of your body:
- Stomach (abdomen): The most common site. Inject into the fatty tissue at least 5cm away from your belly button. This site is easy to access and typically has good absorption.
- Upper thigh: The outer side of the thigh, in the fatty tissue area. This is a practical option you can do yourself without assistance.
- Upper arm: The back of the upper arm, in the fatty tissue. This site usually requires help from another person to inject safely.
Site rotation is essential. Choose a different injection area each week to reduce skin irritation, lumps, and redness. For example, inject your stomach one week, then your thigh the next, then your arm. This rotation pattern minimises tissue damage and ensures consistent absorption of the medication.
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.
Mounjaro vs Wegovy — Injection Site Differences
Both Mounjaro and Wegovy are once-weekly weight loss injections licensed in the UK, but they use identical injection sites: stomach, upper thigh, and upper arm. The main difference is the medication itself—Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Both require the same rotation approach. Your prescriber will discuss which medication is suitable for you based on your individual health profile, medical history, and any weight-related health conditions. Clinical trials showed both medications can support weight loss when used alongside a reduced-calorie diet and increased physical activity.
- 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 twice?
No. Always rotate your injection site each week to prevent irritation, lumps, and reduced absorption. Choose a different area—stomach, thigh, or arm—each time. If you develop redness or hardness at any site, inform your GP.
Is the upper arm injection site easy to do alone?
The upper arm site is the most difficult to inject yourself because it requires reaching the back of your arm. Most people ask a family member or partner to help. The stomach and thigh are much easier to self-inject.
What should I do if I get pain or redness at the injection site?
Minor redness and slight bruising are common and usually fade within a few days. Use a clean, cool cloth if needed. If you develop severe pain, persistent swelling, or signs of infection (warmth, pus), contact your GP or prescriber immediately.
How deep should I insert the Mounjaro needle?
The needle on the KwikPen is designed to inject into fatty tissue just under the skin (subcutaneously). You do not need to inject deeply into muscle. Push the needle straight in at a 90-degree angle until you hear or feel a click, then hold for 3 seconds.
Can I use the same injection sites as other weekly weight loss injections?
Yes. All once-weekly weight loss injections licensed in the UK—including Mounjaro and Wegovy—use the same three injection sites: stomach, thigh, and arm. The rotation principle applies regardless of which medication you use.
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.