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
What Happens in Your First 4 Weeks on Mounjaro
Starting Mounjaro is a significant step in your weight management journey. Your first 4 weeks involve the starting dose of 2.5mg, delivered as a once-weekly injection. During this period, you'll begin experiencing how tirzepatide works, what side effects might occur, and whether this medication suits your health needs. Whether you're accessing Mounjaro through the NHS or privately in the UK, understanding the timeline, costs, and access routes will help you prepare for this initial phase.
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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.
Mounjaro Cost and UK Private Access Routes
In the UK, Mounjaro is available through two main routes: NHS specialist services and private prescription from GPhC-registered online prescribers. NHS access is free at the point of use but requires referral to a tier 3 or tier 4 specialist weight management service; waiting times vary significantly by region and can extend many months. Private prescription costs typically range from £129 to £199 per month for the 2.5mg starting dose, depending on the prescriber's assessment fee and the specific provider. Each KwikPen contains 4 weekly doses, so a single pen covers your first 4 weeks. After your initial clinical assessment—which usually costs £50 to £100—ongoing prescriptions are typically £120–£180 per month at the starting dose. Private providers handle the clinical assessment and prescription online, dispensing through a regulated pharmacy. The Mounjaro KwikPen must be refrigerated before first use and can be stored at room temperature for up to 30 days after opening, making it practical for home use throughout your first month.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS vs Private: Which Route Is Right for You?
Choosing between NHS and private access depends on your circumstances. The NHS route offers free treatment but involves waiting lists and eligibility assessment through specialist services. The private route is faster—you can typically receive your prescription within days of online consultation—but involves upfront costs. Private prescribers assess your BMI, medical history, and weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease) through a secure online consultation, similar to how you might book a GP appointment online. If you're accessing privately, you'll also benefit from dose titration guidance: your prescriber will advise when to move from 2.5mg to 5mg after your first 4 weeks. Use our GLP-1 weight loss calculator to estimate potential outcomes at different doses, and our BMI calculator to confirm your eligibility based on UK thresholds.
- 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.
How much does Mounjaro cost privately in the UK?
Private Mounjaro typically costs £129–£199 per month for the 2.5mg starting dose, plus an initial assessment fee of £50–£100. Costs vary by provider. Your first KwikPen (containing 4 weekly 2.5mg doses) will usually arrive within 3–5 working days of your online assessment being approved.
Can I get Mounjaro free on the NHS?
Yes, Mounjaro is available free on the NHS through specialist tier 3 weight management services. You'll need a GP referral and eligibility assessment. However, NHS waiting times vary by region—some areas have waiting lists of several months. NICE has approved tirzepatide for use within NHS specialist services for eligible adults.
What dose do I start on for the first 4 weeks?
You start on 2.5mg, injected once weekly. This is the standard starting dose for all patients. After 4 weeks, your prescriber will advise whether to increase to 5mg. The dose titration schedule ensures your body adjusts gradually, reducing the risk of side effects.
Do I need a prescription for Mounjaro?
Yes, Mounjaro is prescription-only in the UK. You cannot purchase it without a valid prescription from a UK-licensed prescriber. Private online prescribers and NHS services both issue prescriptions to a GPhC-registered pharmacy, which then dispenses your KwikPen.
What should I expect during my first 4 weeks?
Many people experience nausea, diarrhoea, or reduced appetite—all common in the first few weeks. Side effects often improve as your body adjusts. You'll inject once weekly on the same day each week. Keep your KwikPen refrigerated before first use, then at room temperature (below 30°C) for up to 30 days after opening.
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.