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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Understanding Your Mounjaro Journey: What to Expect
Starting Mounjaro (tirzepatide) is a significant step in your weight management, and understanding what your journey will look like helps you feel confident and prepared. This guide walks you through the entire process—from eligibility and first injection through dose escalation, side effects, and expected outcomes. Everyone's Mounjaro journey is different, but knowing the typical timeline and what each stage involves helps you manage expectations and stay motivated.
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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.
The Mounjaro Timeline: What Happens Week by Week
Your Mounjaro journey follows a structured 20-week dose escalation schedule, starting with a low dose to allow your body to adjust. Week 1–4: You'll inject 2.5 mg once weekly—this is the starting dose designed to minimise side effects as your body adapts. Week 5–8: The dose increases to 5 mg, which is the first active maintenance dose. Week 9–12: You move to 7.5 mg if tolerated. Week 13–16: 10 mg. Week 17–20: 12.5 mg. Week 21+: 15 mg is the maximum dose. Each dose increase happens only if you've tolerated the previous dose for a full 4 weeks. Your prescriber may keep you on a lower dose if that's working well for you—there's no requirement to reach the maximum. Our GLP-1 weight loss calculator can help you explore potential outcomes at different dose levels based on clinical trial data.
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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.
Side Effects and Managing Your First Weeks
Most people experience some gastrointestinal side effects in the early weeks—nausea, diarrhoea, and stomach pain are very common. These typically improve within the first few days to a week as your body adjusts. Eating smaller, lower-fat meals and staying hydrated helps significantly. Injection site reactions (redness, bruising) are common but mild and usually resolve quickly. If you experience severe, persistent stomach pain spreading to your back, or any signs of a serious allergic reaction, contact your GP immediately—these require urgent medical attention. Hair loss has been reported by some users; if this occurs, discuss it with your prescriber as it may resolve over time. Most side effects are temporary and diminish as your body adapts to each dose level.
- 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 weight will I lose on my Mounjaro journey?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight over 72 weeks on Mounjaro, depending on dose. However, individual results vary significantly. Weight loss is achieved alongside a reduced-calorie diet and increased physical activity. Your personal outcome depends on your starting weight, how strictly you follow dietary changes, activity level, and how your body responds to the medication.
Can I switch from Mounjaro to Wegovy if Mounjaro isn't working?
Both Mounjaro (tirzepatide—a dual GIP/GLP-1 agonist) and Wegovy (semaglutide—a GLP-1 agonist only) are prescription-only and work differently. Switching between them is possible but must be managed by your prescriber. There's no official conversion chart; your prescriber will assess whether a switch is appropriate and what dose to start. Approximate equivalence exists (Mounjaro 5 mg ≈ Wegovy 0.5 mg), but this isn't a clinical standard.
How do I inject Mounjaro correctly?
Mounjaro comes in a pre-filled KwikPen. Inject once weekly into your stomach (abdomen), upper thigh, or upper arm—rotate injection sites weekly to prevent irritation. Your prescriber or pharmacist will demonstrate the correct injection technique. You can inject at the same time each week for consistency. Store unopened pens in the fridge (2–8°C); after first use, store at room temperature (below 30°C) for up to 30 days.
What happens if I miss a dose during my Mounjaro journey?
If you remember within 4 days of your missed injection, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your normally scheduled day—never take a double dose. The minimum time between injections is 72 hours (3 days). Contact your pharmacist if you're unsure.
Can I get Mounjaro on the NHS or do I need to go private?
NHS access to Mounjaro is through specialist tier 3/4 weight management services, typically via referral from your GP. NHS availability and waiting times vary significantly by region. Private access is available through a clinical assessment with a UK-licensed prescriber via online pharmacies. NHS eligibility requires BMI ≥30 (or ≥27 with weight-related health conditions); South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower thresholds due to higher health risks.
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.