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 Quickly Does Mounjaro Work?
Mounjaro (tirzepatide) is a prescription-only weight loss injection that works gradually over weeks and months, not overnight. Most people see meaningful weight loss between 8 to 12 weeks of starting treatment, though individual timelines vary significantly. In the UK, access is typically through NHS specialist tier 3 weight management services or private online prescribing following a clinical assessment. Understanding the realistic timeline helps set expectations and supports treatment success.
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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.
Timeline and Expected Results
Mounjaro works through a gradual dose escalation schedule that typically takes 16–20 weeks to reach the maximum therapeutic dose (15 mg). Here's the realistic timeline:
- Weeks 1–4: Starting dose (2.5 mg). Many people experience nausea and appetite changes but minimal weight loss yet.
- Weeks 5–8: Dose increases to 5 mg. Modest weight loss may begin — average 2–4 kg in early stages.
- Weeks 9–16: Further escalations to 7.5 mg then 10 mg. Weight loss typically accelerates as the dose increases.
- Week 17 onwards: At maintenance doses (10–15 mg), most weight loss occurs. Clinical trials showed average weight loss of 15–21% body weight over 72 weeks.
Importantly, weight loss is not linear — some weeks show more progress than others. Using our free GLP-1 weight loss calculator can help you estimate projected outcomes at different doses based on trial data.
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 vs Private Timeline
Access route affects how quickly you can start Mounjaro in the UK. Through the NHS, you'll need a GP referral to a tier 3 specialist weight management service — waiting lists vary by region from weeks to months. Once approved, you follow the standard dose escalation schedule outlined above. Via private online prescribing, the assessment and first prescription can be arranged within days, meaning you could start the same week. Either route requires a clinical assessment to confirm eligibility based on your BMI and health conditions. Regardless of route, the timeline to see meaningful results remains the same: typically 8–12 weeks minimum, with best results after 16+ weeks on higher doses.
- 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 the first week on Mounjaro?
Unlikely. The first week uses a low starting dose (2.5 mg) mainly to assess tolerance. Most people experience appetite suppression and nausea but not significant weight loss in week one. Meaningful weight loss typically begins in weeks 4–8 as doses increase.
How much weight can I expect to lose on Mounjaro?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% body weight over 72 weeks depending on dose — roughly 10–20 kg for many people. However, individual results vary based on diet, activity, and metabolism. Your own results may differ from averages.
Is Mounjaro faster than Wegovy for weight loss?
Clinical data suggests Mounjaro may produce slightly greater weight loss on average than Wegovy (a single GLP-1 agonist), but differences are modest. Both require the same gradual dose escalation and similar timelines to results. The best medication is whichever your prescriber deems most suitable for you.
Can I speed up the Mounjaro dose schedule?
No. The dose titration schedule is fixed at one increase every 4 weeks minimum. Faster escalation increases side effect risk without improving outcomes. Your prescriber may slow the schedule if you experience severe nausea or other issues.
How long before I should expect to see changes on a scale?
Most people notice measurable weight loss (1–2 kg) by week 8–12, assuming you've also made dietary and lifestyle changes. The NHS recommends aiming for 0.5–1 kg per week loss. Track progress with our free progress tracker for a realistic view of your journey.
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.