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) doesn't produce instant weight loss, but many people notice changes within the first few weeks of starting treatment. Weight loss typically becomes more noticeable after 4–8 weeks as your body adjusts to the medication and your appetite naturally decreases. The medication works gradually by activating two hormone pathways (GIP and GLP-1), which help regulate appetite and support feeling fuller after smaller meals. Results vary significantly between individuals, and sustainable weight loss continues alongside reduced-calorie diet and increased physical activity. Understanding the realistic timeline helps set appropriate expectations for your weight management journey.
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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: When Mounjaro Effects Typically Appear
Weeks 1–2: You may notice reduced appetite or changes in food preferences within the first injection. Some people experience nausea, which typically decreases as your body adjusts.
Weeks 3–4: More noticeable appetite suppression. Your reduced calorie intake becomes easier to maintain. Weight loss may start to appear on the scales.
Weeks 5–8: Most people see measurable weight loss by week 8. Your Mounjaro dose remains at 2.5 mg during this phase (the starting dose).
Weeks 9–12 onwards: Dose escalation to 5 mg typically occurs here. Weight loss often accelerates as the dose increases, though your body adjusts for 4 weeks at each new level before further escalation.
Clinical trials showed that participants achieved their most significant weight loss between weeks 8 and 20 as doses reached therapeutic levels. Individual timelines vary — some people see faster results, others progress more gradually.
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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.
Dose Escalation and Weight Loss Progression
Mounjaro requires a dose titration schedule — you start at 2.5 mg and increase every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg maximum). Weight loss typically accelerates each time your dose increases, as the medication's effects become stronger. However, your body needs approximately 4 weeks at each dose level to reach its full effect before advancing. This gradual approach minimises side effects and allows your digestive system to adapt. The SURMOUNT clinical trials showed that participants on the 15 mg maintenance dose achieved average weight loss of approximately 20.9% of body weight over 72 weeks — but this includes the full 5-month dose escalation period. Most weight loss occurs in months 3–6 of treatment once you've reached 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.
When will I see weight loss on the scales with Mounjaro?
Most people notice measurable weight loss within 4–8 weeks of starting. However, this varies — some see changes by week 2–3, others take 8–12 weeks. Consistent diet and exercise changes are essential. Use our free progress tracker to log weekly weight changes and spot patterns beyond daily fluctuations.
Does Mounjaro work faster than Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) have similar timelines, though Mounjaro's dual GIP/GLP-1 mechanism may produce slightly faster results in clinical trials. Average weight loss with Mounjaro is approximately 15–21% of body weight; Wegovy averages approximately 15%. Individual results vary significantly, and the timeline is comparable — both require 4–8 weeks to show initial effects.
Why does Mounjaro have a dose escalation schedule?
Gradual dose escalation (4 weeks per dose level) allows your body to adjust and minimises side effects like nausea and stomach upset. It also helps your prescriber identify your optimal therapeutic dose. Faster escalation increases the risk of gastrointestinal side effects and does not produce faster weight loss — sustained results require the full schedule.
Can I speed up Mounjaro's effects by skipping dose levels?
No. Skipping doses or escalating faster does not produce faster weight loss and significantly increases side effect risk. Your prescriber will manage your dose schedule according to clinical guidelines. The 4-week intervals are evidence-based and designed for safety and sustained results.
What should I expect if Mounjaro isn't working after 8 weeks?
After 8 weeks (still on 2.5 mg), weight loss is often just beginning. If you've made genuine diet and activity changes but see no change after 12 weeks on a higher dose (5 mg+), discuss with your prescriber — they may adjust your dose, review adherence, or consider alternative approaches. Calculate your current BMI using our free BMI calculator to track changes beyond the scales.
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.