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 it take for Mounjaro to take effect?
Most people notice initial effects from Mounjaro within the first 2 to 4 weeks, though significant weight loss typically takes longer. The medication works gradually as your body adjusts to each dose during the titration schedule. Understanding the timeline, combined with knowing your access options and costs in the UK, helps you make an informed decision about whether Mounjaro is right for you. Our GLP-1 weight loss calculator can help you explore potential outcomes, whilst our BMI calculator shows where you stand now against NHS guidelines.
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.
Timeline: When Mounjaro starts working in the UK
Weeks 1–4 (2.5 mg starting dose): Initial nausea and appetite changes often begin within 3 to 7 days. Some users report subtle appetite suppression and feeling fuller faster, though weight loss may be minimal at this stage.
Weeks 5–8 (5 mg dose): More noticeable appetite reduction occurs. Clinical data shows weight loss typically becomes measurable at this point—averaging 2 to 4 kg over 4 weeks for most users, though individual results vary significantly.
Weeks 9–16+ (7.5 to 10 mg doses): By week 12, most people see cumulative weight loss of 5 to 10 kg. The full effect depends on your starting dose and how your body responds to tirzepatide. Dose escalation continues every 4 weeks until you reach your maintenance dose or the maximum 15 mg weekly injection.
Month 6+ (maintenance phase): Average weight loss reaches 15% to 21% of body weight by 6 months in clinical trials, though this varies. Sustained results depend on continuing the injection, maintaining a reduced-calorie diet, and increasing physical activity.
- Initial effects: 2–4 weeks
- Measurable weight loss: 5–8 weeks
- Significant results: 12+ weeks
- Maximum benefit: 6+ months
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 access, cost, and what to expect on your pathway
In the UK, Mounjaro (tirzepatide) is available through two main routes: NHS specialist weight management services and private prescription via online clinical assessment. NHS access requires referral to a tier 3 or 4 specialist service through your GP, though waiting lists can extend 6 to 18 months depending on your region. Private online route: You can access Mounjaro through UK-licensed prescribers following a brief online consultation (£20–£50). Once approved, the KwikPen is dispensed by a GPhC-registered pharmacy. Costs range from approximately £130–£320 per month depending on your dose, with each pen containing 4 weeks of once-weekly injections. Both routes require clinical assessment to confirm you meet eligibility criteria (BMI ≥30 or BMI ≥27 with weight-related health conditions like type 2 diabetes or high blood pressure). Our GLP-1 weight loss calculator helps project outcomes at different doses, and our progress tracker lets you monitor your weight loss journey once you start.
- 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.
Does Mounjaro work faster than Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both once-weekly GLP-1 medications, but Mounjaro uses a dual GIP+GLP-1 mechanism. Clinical trials suggest Mounjaro may produce slightly faster initial appetite suppression (weeks 1–2), but both medications show similar cumulative weight loss by 6 months when accounting for dose equivalence. Individual response varies significantly.
Will I lose weight in my first month on Mounjaro?
Most people experience appetite suppression within the first 2 to 4 weeks, but actual weight loss is usually minimal in month one—typically 0.5 to 2 kg. The dose titration schedule (starting at 2.5 mg) is designed for safety and tolerability. More substantial weight loss becomes visible from week 5 onwards as you progress to higher doses.
What's the cost difference between NHS and private Mounjaro in the UK?
NHS prescription is free at the point of use once referred to specialist services, but waiting lists can be months to years. Private Mounjaro costs £130–£320 monthly depending on dose, plus a one-time online consultation fee (£20–£50). Private access is faster but requires upfront payment. Both routes require clinical assessment confirming eligibility.
Can I switch from Mounjaro to Wegovy if it's not working fast enough?
Any medication switch must be managed by your prescriber and cannot be done immediately. Switching typically requires at least 4 weeks on your current dose to assess efficacy. There is no official conversion chart between Mounjaro and Wegovy—your prescriber will reassess and prescribe based on your individual response and health profile.
How do I know if Mounjaro is actually working for me?
Track your progress using weight measurements, how your clothes fit, energy levels, and appetite changes. Our free progress tracker helps you log weekly weight and spot trends. Measurable weight loss usually appears by weeks 5–8. If you've seen no appetite change or weight loss by week 12, discuss with your prescriber—dose adjustment or an alternative medication may be appropriate.
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.