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 Administer Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they use different delivery devices. Mounjaro comes in a pre-filled KwikPen that you inject once weekly, while Wegovy uses a pre-filled auto-injection pen. Both are subcutaneous injections — meaning you inject under the skin rather than into muscle or vein. This guide explains how to administer each medication, the injection sites, and key differences to help you understand the practical side of treatment if prescribed by your healthcare provider.
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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 Administration — KwikPen Injections
Mounjaro comes in a pre-filled KwikPen device, a reusable pen-shaped injector that contains a four-week supply (four individual doses). Each pen delivers a single 0.5 mL injection once per week. You can inject Mounjaro into your stomach (abdomen), upper thigh, or upper arm — rotating injection sites each week helps prevent skin irritation. Before injecting, allow the pen to sit at room temperature for 15 minutes if refrigerated. Check the liquid is clear and colourless. Pinch the skin, insert the needle at a 90-degree angle, and press the injection button until you hear a click. Hold for 3 seconds, then withdraw. The KwikPen has a dial to select your current dose, making it straightforward to track which week of your dose titration you're on. After four weeks, dispose of the used pen and start a new one.
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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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Wegovy Administration — Auto-Injection Pens vs Mounjaro
Wegovy uses a pre-filled auto-injection pen that differs from Mounjaro's KwikPen design. Each Wegovy pen contains a single dose and is single-use — you dispose of it after one injection. Wegovy is injected once weekly into the same sites: stomach, upper thigh, or upper arm. Like Mounjaro, you rotate sites weekly. The auto-injection feature means the needle retracts automatically after injection, which some people find safer or easier than Mounjaro's manual button press. Storage is similar — keep refrigerated at 2–8°C unopened, and after first use store at room temperature for up to 30 days. Both medications have straightforward UK prescriptions issued by licensed prescribers, and both require clinical assessment before prescribing. If prescribed through NHS specialist services or private providers, your healthcare team will provide detailed injection training and written guidance.
- 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.
What's the main difference between Mounjaro and Wegovy injection devices?
Mounjaro uses a reusable KwikPen with four weekly doses in one pen; Wegovy uses single-use auto-injection pens, one per week. Both are once-weekly subcutaneous injections into the same sites (stomach, thigh, or arm). The auto-injection feature of Wegovy differs from Mounjaro's manual button press.
Can I inject Mounjaro and Wegovy in the same body sites?
Yes. Both use identical injection sites: stomach, upper thigh, or upper arm. You should rotate your injection site each week to prevent skin irritation or lipohypertrophy (thickened skin). Your prescriber will provide guidance on rotation patterns.
Do I need training to use the Mounjaro KwikPen?
Yes, your prescriber or pharmacist should provide injection training before your first dose. Both Mounjaro and Wegovy require proper technique — your healthcare provider will demonstrate how to load the pen, select your dose, inject safely, and dispose of needles. Written instructions accompany each prescription.
What's the difference between Mounjaro dose titration and Wegovy dose escalation?
Both medications use gradual dose increases over weeks. Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg weekly; Wegovy starts at 0.25 mg and reaches 2.4 mg. Each dose increment lasts a minimum of 4 weeks. Your prescriber determines the titration schedule based on your response and side effects, following NHS and NICE guidelines.
Are Mounjaro and Wegovy available on the NHS in the UK?
Mounjaro and Wegovy are available through NHS specialist tier 3/4 weight management services following assessment of eligibility criteria (BMI ≥30 or ≥27 with comorbidities). Waiting times and regional availability vary significantly. Both are also accessible through private prescribing with a UK-licensed healthcare provider following clinical assessment.
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