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 to 4 weeks of starting, though most people notice the most significant changes between weeks 8 and 12. The timeline depends on your dose, how your body responds, and how consistently you follow dietary guidance. Unlike some medications, Mounjaro's dual GIP and GLP-1 action means results can vary between individuals. Understanding realistic timelines helps you set appropriate expectations and stay motivated during your weight management journey using this prescription injection.
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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.
Mounjaro vs Wegovy: Timeline and Onset Comparison
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only UK-licensed weight management injections, but they work on different mechanisms. Mounjaro activates both GIP and GLP-1 receptors (dual action), whilst Wegovy targets GLP-1 only. This difference affects how quickly effects appear.
Mounjaro timeline: Initial appetite reduction and nausea often appear within days of the first dose. Noticeable weight loss typically emerges by week 2–4. Maximum effects at each dose level require 4 weeks minimum before escalation. Clinical trial data (SURMOUNT) showed average weight loss of approximately 15–21% over 72 weeks at maintenance doses.
Wegovy timeline: Similarly, appetite suppression can begin within days. Weight loss becomes apparent by week 2–3. Wegovy requires a slower dose escalation schedule (5 doses over 16 weeks to reach 2.4mg). STEP trial data showed average weight loss of approximately 15% over 68 weeks.
Both work fastest when combined with a reduced-calorie diet and increased physical activity—medication alone is not the answer.
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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.
Which Works Faster: Individual Suitability Matters More
Neither medication is objectively "faster" because individual response varies significantly. Some people see noticeable weight loss within 4 weeks; others require 8–12 weeks. Your prescriber will assess which is more suitable based on your health history, other medications, and personal circumstances.
Both Mounjaro and Wegovy require a UK prescription following clinical assessment. Your GP or a licensed prescriber will consider your BMI, any weight-related health conditions, and whether you're eligible under NHS guidance or accessing treatment privately. The KwikPen format means both are easy to self-inject once weekly. Use our free GLP-1 weight loss calculator above to explore potential weight loss timelines at different doses, and track your progress using our progress tracker as you begin your journey.
- 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 soon will I lose weight on Mounjaro?
Most people notice appetite reduction within days, but measurable weight loss typically appears by week 2–4. The most significant changes generally occur between weeks 8–12 as your dose increases. Individual timelines vary—some see faster results than others depending on adherence and starting weight.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials show similar average weight loss rates (Mounjaro ~15–21%, Wegovy ~15% over comparable periods). Mounjaro's dual mechanism may work slightly differently, but neither is definitively "faster." Your prescriber will recommend the most suitable option based on your individual health profile.
Why does Mounjaro require dose escalation if I want results quickly?
Dose titration over 4–5 weeks allows your body to adjust and helps minimise side effects like nausea. Jumping to higher doses quickly increases nausea risk and doesn't produce faster weight loss. Each dose level must be held for 4 weeks before escalation—this is the evidence-based safe approach.
Can I use the BMI calculator to track my weight loss progress on Mounjaro?
Yes. Use our free BMI calculator to check your starting BMI, and recalculate every 4 weeks to monitor how your BMI category changes. Our progress tracker also lets you log weekly weights and visualise your journey over time alongside medication use.
Will I keep the weight off after stopping Mounjaro?
Weight regain often occurs after stopping GLP-1 medications if lifestyle changes aren't maintained. The medication supports reduced appetite, but long-term success depends on sustained dietary changes and physical activity. Discuss your long-term plan with your prescriber. ```
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.