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 Week on Mounjaro — UK Guide
Starting Mounjaro in your first week means beginning with a 2.5 mg starting dose — the lowest dose designed to help your body adjust to this weekly weight loss injection. In the UK, you can access Mounjaro through either NHS specialist weight management services or private prescription routes. Understanding both access pathways, typical costs, and what to expect in those initial days helps you prepare properly for this prescription weight loss journey.
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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.
UK Access Routes and Typical Costs for Mounjaro
NHS route: Mounjaro is available through NHS specialist tier 3 weight management services following NICE approval, but access is limited and varies significantly by region. Your GP must refer you to your local specialist service, and waiting lists can extend several months depending on your area.
Private prescription route: You can obtain Mounjaro privately through a UK-licensed online prescriber following a clinical consultation. A typical first month supply (one 2.5 mg KwikPen containing four weekly injections) costs between £129–£180 depending on the provider and dose. Subsequent months at higher doses (5 mg–15 mg) typically range from £150–£320 per month. These costs are per dose escalation level, not per individual injection.
What you're paying for: The KwikPen device itself, clinical assessment, prescription issuance, and pharmacy dispensing. Storage requirements (refrigeration for unopened pens) should be considered before ordering.
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.
First Week Experience and What to Expect
Your 2.5 mg starting dose is deliberately low — the first weekly weight loss injection is about tolerating the medication rather than expecting dramatic weight loss. Many people experience mild nausea, especially in days 2–4 after injection, which typically settles as your body adjusts. Expect to take your first injection on your chosen day of the week, then the same day each week thereafter using your KwikPen device.
In week one, focus on staying hydrated, eating regular small meals (especially if nausea occurs), and monitoring how you feel. Use our free GLP-1 weight loss calculator to understand realistic weight loss projections across the full treatment course — first-week losses are typically minimal. Track your progress using our progress tracker to establish a baseline before dose escalation in week 5.
- 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 a first Mounjaro prescription cost in the UK?
A 2.5 mg KwikPen (four weekly injections) typically costs £129–£180 from private UK prescribers. Costs increase with higher doses — by month 3 at 7.5 mg, expect £200–£250. NHS access is free but involves referral to specialist services with variable waiting times by region.
Will I lose weight in my first week on Mounjaro?
Significant weight loss in week one is unlikely. The 2.5 mg starting dose focuses on tolerating the medication rather than maximum effect. Most weight loss occurs over weeks 4–12 as your dose escalates. Use our GLP-1 weight loss calculator to see realistic projections based on your dose progression.
Can I get Mounjaro on the NHS as a starting dose?
Yes, but access is through NHS specialist tier 3 weight management services only, not standard GP prescription. Eligibility requires BMI ≥30 or ≥27 with weight-related health conditions. Referral and assessment are required, and waiting lists vary significantly by region.
What happens after my first week on Mounjaro 2.5 mg?
Week 2–4 you continue 2.5 mg weekly. In week 5, your prescriber typically increases to 5 mg if tolerating well. Each dose level lasts minimum 4 weeks before further escalation. Your full progression to maximum dose (15 mg) takes 20+ weeks.
Is there a cost difference between NHS and private Mounjaro?
NHS Mounjaro is free at point of use once prescribed. Private prescription costs £129–£320 monthly depending on dose level. Private access is faster (no referral needed) but involves upfront costs. NHS is free but may involve months of waiting for specialist services.
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.