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 After Your First Mounjaro Injection Does It Work?
Most people begin noticing effects from Mounjaro within the first 2 to 4 weeks, though full effectiveness develops gradually over several months. The medication starts working immediately at the cellular level, but appetite reduction and weight loss become noticeable once you reach your maintenance dose—typically around week 5 onwards. As a weekly weight loss injection, Mounjaro uses a dose titration schedule, meaning you start at 2.5 mg and increase every 4 weeks. Weight loss isn't guaranteed after the first injection; it's a gradual process that builds as your body adjusts and your dose increases.
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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
Weeks 1–4 (2.5 mg starting dose): You receive your first KwikPen injection. At this stage, most people report minimal appetite change—this is the adjustment phase. Some experience mild nausea or digestive changes. Weight loss is typically not significant yet.
Weeks 5–8 (5 mg maintenance dose): After your dose increases, appetite suppression becomes more noticeable. Many people report feeling fuller after smaller meals and reduced food cravings. This is when most people start to see weight changes on the scale.
Weeks 9–20+ (7.5–15 mg escalation): Effects typically strengthen as your dose increases. Clinical trial data showed that participants achieved average weight loss of around 15–21% body weight by week 72, though results vary significantly. Weight loss is not guaranteed—it depends on dietary changes, physical activity, and individual response.
Clinically BackedWeight Loss
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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.
Mounjaro UK Prescription and Next Steps
Mounjaro is a prescription-only medicine available in the UK through NHS specialist weight management services (NICE-approved) or private online prescribing following clinical assessment. Whether you access it on the NHS or privately, a prescriber will assess your eligibility based on BMI and health conditions before issuing a prescription.
Once prescribed, you'll self-inject once weekly using the KwikPen device. Your dose will be increased every 4 weeks as tolerated. Mounjaro vs Wegovy comparison: Mounjaro is a dual GIP/GLP-1 injection, while Wegovy (semaglutide) is a single GLP-1 injection—both are weekly options. Side effects of Mounjaro commonly include nausea and digestive changes, especially when starting or increasing doses. Track your progress with our free progress tracker and use our GLP-1 weight loss calculator to understand what realistic outcomes might look like for your situation.
- 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 immediately after the first injection?
No. Mounjaro begins working at the cellular level immediately, but you won't notice appetite suppression or weight loss right after your first 2.5 mg dose. Most people notice effects developing in weeks 2–4, with more significant changes once you reach your 5 mg maintenance dose around week 5. Full effectiveness typically develops over 8–12 weeks as your dose increases.
When should I expect to lose weight on Mounjaro?
Most people see noticeable weight loss starting around week 5–8, once they've reached the 5 mg dose. However, individual results vary significantly based on diet, activity level, and how your body responds. NHS guidance emphasizes that Mounjaro is most effective alongside reduced-calorie diet and increased physical activity—medication alone does not clinically studied weight loss.
Why does Mounjaro use dose titration instead of starting at full dose?
Dose titration (gradual increase from 2.5 mg to 15 mg) allows your body to adjust and helps minimize side effects like nausea. Starting at a lower dose also allows your prescriber to find the dose that works best for you. Each dose is held for 4 weeks before increasing to allow assessment of tolerance and effectiveness.
Is Mounjaro available on the NHS in the UK?
Yes. NICE has approved tirzepatide (Mounjaro) for use in NHS specialist tier 3 weight management services. Access varies by region and ICB. Waiting lists can be lengthy. Private access is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber, though costs apply (typically £129–£320+ monthly depending on dose).
How does Mounjaro compare to Wegovy in terms of how quickly it works?
Both Mounjaro and Wegovy are weekly weight loss injections that take similar timelines to show effects (weeks 2–4 noticeable changes, weeks 5–8 more significant results). Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is a GLP-1 agonist only. Clinical trials showed Mounjaro achieved slightly higher average weight loss (15–21% vs ~15% for Wegovy), but individual results vary considerably and depend on lifestyle factors.
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.