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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What If I Inject Mounjaro In The Wrong Spot?
Mounjaro is a once-weekly injection that must be administered into specific areas of your body to work safely and effectively. If you've accidentally injected your KwikPen in the wrong location, understanding what happens next and knowing when to contact your GP is important. This guide explains approved Mounjaro injection sites, what to do if you've missed the correct area, and how this compares to using Wegovy, which has similar injection requirements.
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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 And What Happens If You Miss
Mounjaro is designed to be injected into fatty tissue (subcutaneously) in three approved locations: your stomach (abdomen), upper thigh, or upper arm. Each area has a network of small blood vessels and fatty tissue that allows the medication to absorb properly into your bloodstream.
If you accidentally inject Mounjaro into a muscle instead of fatty tissue, the medication may still absorb but potentially at a different rate, which could affect how quickly you notice effects. Injecting into a vein (which is unlikely with proper technique) could cause the tirzepatide to enter your bloodstream too rapidly.
- Stomach area: Most common site — easily accessible, good fatty tissue layer
- Upper thigh: Reliable site with adequate subcutaneous space
- Upper arm: Requires assistance or careful technique to ensure correct depth
- Site rotation: Rotate injection location each week to reduce irritation and lipohypertrophy (fat tissue thickening)
If you believe you've injected into muscle or are experiencing severe pain, swelling, or bruising that doesn't improve within 24 hours, contact your GP or pharmacist for reassurance and guidance.
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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.
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Mounjaro Vs Wegovy: Injection Site Safety Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) use identical injection site guidance — they're both subcutaneous injections into the same three approved areas. The mechanism of action differs (Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only), but injection safety is essentially the same for both medications.
Proper injection technique matters equally for both: never inject into muscle, vein, or scar tissue. Both medications are supplied in pre-filled pens (KwikPen for Mounjaro, auto-injector for Wegovy) designed to deliver the correct dose at the right depth into subcutaneous fat. If you're unsure about your injection technique with either medication, ask your prescriber or pharmacist for a demonstration before your next dose.
- 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 anywhere on my body?
No. Mounjaro must be injected into fatty tissue only in these three approved areas: stomach, upper thigh, or upper arm. Never inject into muscle, vein, or areas with scarring or bruising. Always rotate the site each week to prevent tissue damage and irritation.
What happens if I accidentally inject Mounjaro into a muscle?
The medication may still work, but absorption could be faster or slower than intended, potentially affecting how quickly effects are noticed. If you experience severe pain, significant swelling, or bruising lasting over 24 hours, contact your GP for reassurance and advice.
Is Wegovy injection safer than Mounjaro if I'm worried about technique?
Both medications use identical injection site safety rules. Neither is inherently "safer" — safety depends on correct technique. Both come in pre-filled pens designed to deliver medication at the right depth. If you're nervous about injections, ask your prescriber for a practical demonstration.
What should I do if I'm unsure whether I injected in the right spot?
If you felt normal resistance, heard a click, and have no unusual pain or swelling, you likely injected correctly. Minor bruising or redness at the site is normal. If you're genuinely concerned, contact your pharmacist or GP — they can reassure you and check for any issues.
Do I need to wait longer between doses if I inject in the wrong spot?
No. If you've injected your dose (even if slightly off-site), continue your normal schedule the following week. Your body will still receive the tirzepatide — timing and consistency matter more than having a perfect injection site every time. ```
Ready to take control of your weight loss journey?
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