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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Mounjaro Pen Instructions vs Wegovy — How They Compare
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly prescription weight loss injections available in the UK, but their pen devices and injection protocols differ. Understanding how each works helps you prepare for whichever medication your prescriber assesses as suitable for you. This guide compares the KwikPen instructions for Mounjaro with Wegovy's auto-injection pen, covering storage, dosing, injection sites, and the clinical evidence behind each. Both medications require a prescription and clinical assessment before being prescribed.
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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.
How Mounjaro and Wegovy Pens Differ — Clinical Trial Context
Mounjaro (tirzepatide) uses a KwikPen device — a pre-filled pen that delivers a fixed dose once weekly. The SURMOUNT-1 clinical trial showed participants using Mounjaro at 15 mg achieved average weight loss of approximately 20.9% of body weight. Wegovy (semaglutide) uses an auto-injection pen with a similar once-weekly schedule. The STEP-1 trial showed approximately 14.9% average weight loss at the maximum 2.4 mg dose. Key differences include dose escalation timelines: Mounjaro starts at 2.5 mg with 4-week intervals between increases, while Wegovy starts at 0.25 mg with similar spacing. Both slow gastric emptying, but Mounjaro uses a dual GIP + GLP-1 mechanism, whereas Wegovy activates GLP-1 only. Individual results vary significantly. Your GP or prescriber will assess which mechanism suits your health profile following a clinical consultation.
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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.
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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Storage, Injection Technique, and Individual Suitability
Storage: Mounjaro pens must be refrigerated at 2–8°C unopened and can be kept at room temperature (below 30°C) for up to 30 days after first use. Wegovy has similar refrigeration requirements. Both require a new pen every 4 weeks (four doses). Injection sites for both are the stomach, upper thigh, or upper arm — rotate each week. Eligibility is assessed individually: adults with BMI 30+ (or 27+ with weight-related conditions) may qualify, with lower thresholds for South Asian, Chinese, Black African, or Black Caribbean patients. Neither medication is suitable during pregnancy or if breastfeeding. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, and consult your prescriber about which medication aligns with your medical history.
- 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 pens?
Mounjaro uses a KwikPen with higher doses (up to 15 mg) and a dual GIP + GLP-1 mechanism, while Wegovy uses an auto-injection pen with a maximum 2.4 mg dose and GLP-1-only activation. Both are once-weekly injections. Clinical trials showed Mounjaro achieved slightly higher average weight loss, but individual results vary. Your prescriber will recommend the most suitable option after assessing your health.
How do I store Mounjaro and Wegovy pens?
Both pens must be stored in a refrigerator at 2–8°C before first use. After opening, both can be kept at room temperature (below 30°C) for up to 30 days. Do not freeze either pen. Always check the expiry date and keep pens away from direct sunlight and heat.
What injection sites can I use for Mounjaro and Wegovy?
Both medications can be injected into the stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to prevent skin irritation. Your prescriber or nurse will demonstrate the correct injection technique before you start treatment.
Am I eligible for Mounjaro or Wegovy on the NHS?
NHS access to both medications is through specialist tier 3 weight management services — not standard GP prescribing. Eligibility requires BMI 30+ (or 27+ with weight-related conditions like type 2 diabetes or hypertension). Lower thresholds apply for South Asian and Black African heritage patients. Your GP can refer you to specialist services if appropriate, though waiting lists vary significantly by region.
How do Mounjaro and Wegovy doses compare?
Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg once weekly. Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg once weekly. Both follow 4-week dose escalation intervals. In clinical trials, both supported weight loss when used alongside reduced-calorie diet and physical activity — results vary by individual.
Ready to take control of your weight loss journey?
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