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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Where to Inject Mounjaro vs Wegovy — Key Differences
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections used by UK adults, but they differ in their injection sites and delivery systems. Understanding where and how to inject each medication helps you use them safely and effectively. This guide compares the practical aspects of both injections, including approved sites, pen design, and what to expect during administration.
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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.
Injection Sites: Mounjaro vs Wegovy
Mounjaro (tirzepatide) can be injected into three approved areas:
- Stomach (abdomen) — the most common site, typically 5cm away from the navel
- Upper thigh (front or outer area)
- Upper arm (back of the arm) — requires assistance from another person
Wegovy (semaglutide) uses the same three injection sites:
- Stomach (abdomen) — preferred location
- Upper thigh
- Upper arm — requires help from another person
Both medications should be rotated to a different site each week to reduce injection site reactions like redness or bruising. The key difference is in the delivery device: Mounjaro comes in a KwikPen (pre-filled pen with 4 doses), while Wegovy uses an auto-injection pen. Both are simple to use once you receive training from your prescriber.
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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.
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Clinical Differences and Individual Suitability
Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only — this affects how they work but not where you inject them. In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight, whilst Wegovy showed around 15% average loss. However, individual results vary significantly.
A prescriber will assess which medication is suitable for you based on your health history, weight, and any existing conditions. Use our free GLP-1 weight loss calculator to explore potential outcomes with different doses. Both require a clinical assessment before prescription, whether through NHS specialist services or a private prescriber. Your GP can advise on NHS access in your area.
Injection technique and site care are identical for both medications. Wash your hands before injecting, pinch the skin gently, insert the needle at a 90-degree angle, and press the injection button until you hear or feel a click. After injecting, you can apply pressure with a clean tissue but avoid rubbing. Keep a record of where you inject each week to ensure proper rotation. If you experience persistent pain, swelling, or infection at any injection site, contact your GP or healthcare provider immediately.
- 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 you use the same injection site for both Mounjaro and Wegovy?
No. If switching between medications, continue rotating sites as normal. Each injection should be in a different location from the previous week, regardless of which medication you're using. This reduces the risk of injection site reactions and lipodystrophy (thickened or indented skin).
Does injecting into the stomach hurt more than the thigh?
Pain varies between individuals and depends on needle sensitivity and technique. The stomach has more fatty tissue, which some people find less painful. The thigh is equally effective. Try different sites to find what works best for you. Most users report minimal discomfort once they've done a few injections.
What should I do if I get an injection site reaction?
Mild redness or small bruises are common and usually resolve within a few days. Ice the area for 10–15 minutes if swollen. Rotate to a completely different site for your next injection. If you develop severe pain, significant swelling, warmth, or signs of infection, contact your GP immediately.
Do Mounjaro and Wegovy require different injection techniques?
Both use the same injection technique. The main difference is the delivery device — Mounjaro's KwikPen and Wegovy's auto-injection pen operate slightly differently, but your prescriber will train you on whichever you're prescribed. Both are designed for self-administration.
Can I inject Mounjaro or Wegovy whilst travelling?
Yes. Keep unopened pens refrigerated (2–8°C) and use an insulated travel case with ice packs. Once opened, Mounjaro can be stored at room temperature (below 30°C) for up to 30 days. Wegovy has similar storage rules. Always carry your medication in your hand luggage on flights and inform security it's a prescription medicine.
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