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
Free tracker included with every download this week
Extra Dose in Mounjaro Pen vs Wegovy — UK Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used in the UK, but they work differently and come in different dose formats. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Understanding how their dosing differs — including what you actually get in each KwikPen or pen — helps clarify which might be suitable for you. Both require clinical assessment and prescription before use. Our free GLP-1 weight loss calculator can help you understand potential outcomes with either medication.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
Mounjaro vs Wegovy: Dose Formats and Strengths
Mounjaro (tirzepatide) comes as a pre-filled KwikPen containing four doses. Each pen holds 4 weeks' supply at a single strength:
- 2.5 mg KwikPen (starter/adjustment doses)
- 5 mg KwikPen (maintenance dose)
- 7.5 mg KwikPen (escalation)
- 10 mg KwikPen (escalation)
- 12.5 mg KwikPen (escalation)
- 15 mg KwikPen (maximum dose)
Wegovy (semaglutide) comes as a pre-filled auto-injection pen with a single dose per pen:
- 0.25 mg pen (starting dose, weeks 1–4)
- 0.5 mg pen (weeks 5–8)
- 1 mg pen (weeks 9–12)
- 1.7 mg pen (weeks 13–16)
- 2.4 mg pen (maintenance/maximum dose, week 17+)
The key difference: you get four injections per Mounjaro KwikPen (one per week), but one injection per Wegovy pen. This affects cost, storage, and prescription frequency.
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
STEP vs SURMOUNT Clinical Trial Results
Weight loss outcomes differ between medications. In the SURMOUNT-1 clinical trial, Mounjaro at 15 mg showed average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 clinical trial, Wegovy at 2.4 mg showed average weight loss of approximately 14.9% of body weight over 68 weeks. Both trials used participants on a reduced-calorie diet with increased physical activity. Individual results vary significantly — these are trial averages, not assured outcomes. Your GP or prescriber will assess which medication (if any) is appropriate for you based on your health profile, not on trial data alone. Both require ongoing clinical assessment and are prescribed through specialist weight management services or private online prescribing in the UK.
- 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 is the difference between a Mounjaro KwikPen and a Wegovy pen?
Mounjaro comes as a KwikPen containing four pre-filled doses (one per week for four weeks). Wegovy comes as individual pre-filled pens, one per injection. Mounjaro uses a dial mechanism to select your weekly dose; Wegovy has a fixed dose per pen. Both are once-weekly injections, but the pen formats and refill schedules differ.
Can you get an extra dose from a Mounjaro KwikPen?
A Mounjaro KwikPen contains exactly four doses, calibrated to deliver your prescribed strength once weekly. Each pen should not be used beyond 30 days after first use. You cannot extract or store an extra dose — if you miss a dose or delay your injection, your prescriber can advise on the next step. Never attempt to extract additional medication from a pen.
Which medication shows better weight loss results, Mounjaro or Wegovy?
Clinical trials show different average results: SURMOUNT-1 showed Mounjaro 15 mg resulted in approximately clinically studied weight loss, whilst STEP-1 showed Wegovy 2.4 mg resulted in approximately clinically studied weight loss. However, individual results vary significantly, and these are trial averages under controlled conditions. Your prescriber will recommend based on your individual health, not trial comparisons.
How long does a Mounjaro KwikPen last compared to Wegovy?
A Mounjaro KwikPen lasts exactly four weeks (four weekly injections). Wegovy pens last one week each. If you're on Mounjaro 10 mg, one KwikPen covers four weeks. On Wegovy 2.4 mg, you need a new pen each week. This affects prescription frequency and how you manage your medication supply.
Do I need a GP prescription for both Mounjaro and Wegovy in the UK?
Yes, both are prescription-only medicines in the UK. Access is through NHS specialist tier 3/4 weight management services (referral from your GP) or via private online prescribing (following a clinical assessment with a UK-licensed prescriber). Neither is available without a prescription, and both require clinical suitability assessment.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.