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 Injection Sites
Mounjaro (tirzepatide) is injected once weekly under the skin. The stomach (abdomen) is the most common injection site, though you can also inject into your upper thigh or upper arm. Proper injection technique and rotating your injection sites helps prevent skin problems and ensures consistent medication absorption. This guide explains exactly where to inject Mounjaro safely, following NHS and MHRA guidance for UK users.
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 and Technique
Primary injection sites for Mounjaro:
- Stomach (abdomen) — most common and easiest to self-inject. Inject into the fatty tissue around your waist, avoiding the area 2 inches around your belly button.
- Upper thigh — front and outer area of the thigh, into fatty tissue below the skin.
- Upper arm — outer surface between shoulder and elbow. This site typically requires assistance from another person.
Your KwikPen is pre-filled and ready to use. Always clean the injection site with an alcohol wipe and let it dry before injecting. Push the pen firmly against your skin at a 90-degree angle when injecting. Hold in place for 3 seconds after the needle retracts to ensure the full dose is delivered. The needle is very thin (29-gauge) and most users report minimal discomfort.
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.
Rotating Injection Sites and Skin Health
Rotating your injection site each week is essential to prevent lipohypertrophy (thickened fatty tissue) and lipoatrophy (loss of fatty tissue), which can affect how well Mounjaro is absorbed. Never inject into the same spot twice in a row. If you prefer stomach injections, choose a different area of your abdomen each week—left side one week, right side the next, or above and below the belly button on rotation.
If you notice redness, bruising, pain, or lumps at injection sites, inform your GP or prescriber. These are usually minor and resolve with proper rotation, but medical advice ensures safe continued use. Always keep your KwikPen refrigerated (2–8°C) until use, and never reuse needles.
- 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 must rotate your injection site each week to prevent skin damage and ensure consistent absorption. Choose a different area of your abdomen, or alternate between abdomen, thigh, and arm. Repeating the same site can cause lipohypertrophy or lipoatrophy, affecting how the medication works.
How do I know if my Mounjaro injection technique is correct?
The injection should be quick, at a 90-degree angle to your skin, into fatty tissue. Hold the pen in place for 3 seconds after injection. You should hear or feel a click as the needle retracts. If you experience excessive pain or bleeding, or if the pen doesn't click, contact your prescriber or pharmacist for guidance.
Is stomach injection better than thigh injection for Mounjaro?
Both stomach and thigh are effective injection sites. Stomach (abdomen) is most common because it's easier to self-inject, but thigh injections work equally well. The key is rotating sites each week. Choose whichever site you find most comfortable, then rotate systematically.
What should I do if I accidentally inject into a bruised or sore area?
Avoid injecting into areas with active bruising, pain, or lumps. Wait until the area heals before using it again. If you have limited injection sites due to skin reactions, contact your GP or prescriber immediately. They can assess whether you should continue treatment or if an alternative approach is needed.
Does Mounjaro absorption differ between stomach, thigh, and arm injections?
Clinical data shows no significant difference in absorption between these three sites when proper injection technique is used. The stomach is slightly faster-absorbing than thigh, which is slightly faster than arm, but these differences are minor. Consistent rotation and correct technique matter more than site choice.
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.