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
Mounjaro First Week Weight Loss — What to Expect
Starting Mounjaro is a significant step in your weight management journey, but understanding what happens in that first week is essential. Most people don't experience dramatic weight loss in week one — that's normal and expected. Instead, your body is adjusting to the medication, and the real benefits develop over several weeks as your dose increases. This guide explains what to expect, how to access Mounjaro in the UK, and the costs involved whether you're pursuing NHS or private prescription routes.
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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.
Mounjaro First Week: What Happens in Your Body
During your first week on Mounjaro, you'll receive the starter dose of 2.5 mg — a low dose designed to minimise side effects as your body adjusts. This is part of the dose titration schedule, where your dose increases gradually every four weeks until reaching an effective maintenance dose.
- Week 1–4: You're on 2.5 mg — your body is adjusting to the medication
- Minimal weight loss expected in week one — this is normal
- Most side effects (nausea, stomach changes) occur early and often improve over days
- Real weight loss acceleration happens around week 5 onwards as your dose increases
- Clinical trials showed average weight loss builds over 12–16 weeks, not instantly
The weekly weight loss injection works by activating GIP and GLP-1 hormone pathways, which regulate appetite and fullness signals. In week one, these pathways are just beginning to activate. Weight loss typically becomes noticeable by weeks 3–4 of treatment. Track your progress using our free progress tracker to monitor changes beyond just the scales.
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.
UK Cost and Access: NHS vs Private Prescription
NHS route: Mounjaro is available through NHS specialist tier 3/4 weight management services, but access is limited and varies by region. Your GP must refer you, and waiting lists can be months or years depending on your local ICB. When approved by the NHS, the medication is free.
Private prescription route: Available immediately through UK-licensed online prescribers following a clinical assessment. Private Mounjaro costs typically range from £150–£320 per month depending on your dose and provider — the 2.5 mg starter dose costs less than higher maintenance doses like 15 mg. Each Mounjaro KwikPen contains 4 weekly doses. Many people choose private prescription to avoid NHS waiting lists, though costs accumulate quickly over time.
Using our free GLP-1 weight loss calculator can help you understand potential outcomes at different dose levels to inform your decision.
- 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.
Will I lose weight in my first week on Mounjaro?
No significant weight loss is expected in week one. You're on the starter dose (2.5 mg), and your body is still adjusting. Real weight loss acceleration typically begins in weeks 3–4 and becomes more noticeable by weeks 5–8 as your dose increases through the titration schedule.
How much does Mounjaro cost on private prescription in the UK?
Private Mounjaro costs range from approximately £150–£320 per month depending on dose and provider. The starter dose (2.5 mg) costs less than higher maintenance doses. Each KwikPen contains 4 weekly injections. NHS prescription is free but requires a referral and often involves significant waiting times.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is approved by NICE for NHS use in specialist weight management services (tier 3/4). However, access is limited and varies significantly by region. You need a GP referral and may face waiting lists of several months to years depending on your local ICB and capacity.
What is a Mounjaro KwikPen and how often do I inject?
A KwikPen is Mounjaro's pre-filled injection device containing 4 weekly doses. You inject once every 7 days in your stomach, thigh, or upper arm. Each pen lasts exactly 4 weeks, so you'll need to reorder or collect a new pen monthly. Always rotate injection sites to avoid irritation.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only medication. Clinical trials showed Mounjaro achieved slightly higher average weight loss (~21% vs ~15%), but individual results vary. Both are prescription-only, available through NHS specialist services or private prescription, and require clinical 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.