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
How Long Does Mounjaro Take To Work?
Mounjaro (tirzepatide) typically begins to show weight loss effects within 2–4 weeks of starting, though the most noticeable results usually appear after 8–12 weeks of consistent use. Weight loss continues to progress as your dose increases over the first 16–20 weeks during the dose titration phase. Results vary significantly between individuals, and Mounjaro works best when combined with a reduced-calorie diet and increased physical activity. If you're eligible for NHS prescription through a specialist tier 3 weight management service, your GP or specialist team will monitor your progress closely and adjust your dose based on your response.
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.
What The Clinical Evidence Shows About Mounjaro Timeline
The SURMOUNT-1 clinical trial tracked Mounjaro's effects over 72 weeks in UK and international participants. Weight loss typically began within the first 2–4 weeks, though the first noticeable difference for most people came between weeks 4–8. The most significant weight loss occurred between weeks 8–16, coinciding with dose escalation from 5 mg to 10 mg. By week 16, participants on the 15 mg dose had already lost an average of 12–15% of their body weight. Maximum weight loss effects were usually seen after 20+ weeks of consistent use, once the final maintenance dose was reached. Individual variation is substantial — some people respond rapidly while others see gradual, steady progress. Your Mounjaro KwikPen dose increases every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg), and your weight loss tends to improve with each escalation.
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 Prescription, Eligibility, And Getting Started With Mounjaro
In the UK, Mounjaro is available through NHS specialist tier 3 weight management services or privately via online clinical assessment. NHS access requires referral from your GP and assessment by a multidisciplinary team. Waiting lists vary significantly by region — some areas have 6–12 month delays. Private prescription is faster but involves cost. Regardless of route, your prescriber will assess your BMI, health history, and weight-related conditions before offering Mounjaro. Once prescribed, you inject once weekly at home using your KwikPen. Regular check-ins with your healthcare team help monitor side effects, assess weight loss progress, and adjust your dose if needed. Use our free BMI calculator and GLP-1 weight loss calculator to track your baseline and projected progress.
- 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 quickly will I see weight loss results on Mounjaro?
Most people notice the first small changes within 2–4 weeks, but meaningful weight loss typically becomes apparent after 8–12 weeks. The most dramatic results usually occur during dose titration (weeks 8–20) as your dose increases to the maintenance level. Individual response varies considerably, so patience is important.
Does Mounjaro work better the longer you take it?
Yes. Mounjaro's effects build over time, especially during the first 16–20 weeks of dose escalation. Weight loss continues to improve as your dose reaches the maintenance level of 15 mg. After reaching the final dose, weight loss typically stabilises, though some people see continued modest improvement up to 6–12 months.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is available through NHS specialist tier 3 weight management services. You'll need referral from your GP and assessment by a specialist team. NICE has approved tirzepatide for NHS use, but regional availability and waiting lists vary significantly across the UK.
What if Mounjaro isn't working for me after 8 weeks?
If you're not seeing expected weight loss by week 8–12, discuss this with your prescriber. You may need a dose increase, a review of your diet and activity levels, or assessment for underlying factors. Never skip doses or increase your dose without guidance from your healthcare team.
How long can I stay on Mounjaro for weight management?
Mounjaro is a long-term weight management treatment. You can remain on it as long as it's working, you're tolerating it well, and your prescriber confirms it's appropriate. Many people use Mounjaro for 12+ months. Your GP or specialist will review your ongoing need and progress regularly.
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.