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
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How to Inject Mounjaro Into Your Thigh
The upper thigh is one of three approved injection sites for Mounjaro (tirzepatide), a prescription-only weight management injection licensed in the UK. Administering your weekly dose into the thigh using the KwikPen device is straightforward once you understand the correct technique. This guide covers everything you need to know about thigh injections, including preparation, needle insertion, and site rotation to minimise discomfort and ensure consistent absorption of this dual GIP and GLP-1 receptor agonist medication.
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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 Thigh Injection Technique
Before injecting, check that your Mounjaro KwikPen has been stored correctly (refrigerated at 2–8°C unopened, or room temperature below 30°C after first use for up to 30 days). Allow the pen to reach room temperature for at least 15 minutes if cold. Clean the injection site on your upper thigh with an alcohol wipe and let it dry completely.
- Pinch the skin on your thigh to create a fold
- Hold the KwikPen perpendicular (straight) to the skin
- Push the needle in fully until you hear or feel a click
- Keep the pen in place for 6 seconds after injection to ensure the full dose is delivered
- Remove the needle and release the pinched skin
- Do not rub the injection site
Your upper thigh offers good accessibility and consistent absorption. Rotate between left and right thigh weekly to allow injection sites to recover and reduce irritation.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Thigh Injection Site Rotation and Safety
Site rotation is essential to prevent lipohypertrophy (thickened tissue) and ensure reliable medication absorption. If you rotate between thigh, stomach, and upper arm over four weeks, each site gets adequate recovery time. Never inject into the same spot within 4 weeks.
- Avoid areas with bruising, hardness, or lumps from previous injections
- If you experience pain, redness, or swelling lasting more than a few days, contact your GP
- Keep a simple rotation log or mark injection sites on a calendar
The thigh is particularly accessible if you're injecting alone and provides steady absorption. Always ensure your Mounjaro KwikPen prescription has been issued following a clinical assessment by a UK-licensed 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.
Is the thigh the best Mounjaro injection site?
All three approved sites (thigh, stomach, upper arm) are equally effective. The thigh is convenient for self-injection and offers stable absorption. Choose based on comfort and accessibility. Always rotate between sites weekly to prevent tissue irritation and maintain consistent absorption of tirzepatide.
How do I know if my thigh injection site is infected?
Seek medical advice if you notice persistent redness, warmth, significant swelling, pus, or severe pain lasting more than a few days. These could indicate infection. Minor bruising or mild soreness usually resolves within 24–48 hours and is normal. Contact your GP if you're concerned.
Can I inject Mounjaro into the same thigh spot twice?
No. You must rotate injection sites, waiting at least 4 weeks before reusing the same spot. This prevents lipohypertrophy (thickened tissue) and ensures reliable absorption. Alternate between left and right thigh, stomach, and upper arm over successive weeks to maintain skin health.
What's the difference between Mounjaro and Wegovy for thigh injections?
Both use once-weekly injections into approved sites including the thigh. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Both require a UK prescription and clinical assessment. Injection technique is identical for both medications.
Should I massage my thigh after injecting Mounjaro?
No, do not rub or massage the injection site. This can affect absorption and increase bruising. Simply clean the area with an alcohol wipe beforehand, inject, and leave the site undisturbed. Light movement of the leg afterwards is fine and may even improve absorption slightly.
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