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 affect appetite within the first few days to weeks, though noticeable weight loss usually takes longer. Most people on this prescription-only medication start to see meaningful changes around 4–8 weeks, with more substantial results appearing after 12 weeks or more. The timeline varies significantly between individuals and depends on dose, diet, physical activity, and individual metabolism. Your GP or prescriber will monitor your progress and adjust your dose every 4 weeks if needed. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses based on clinical trial data.
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 Effects Typically Appear
Week 1–2: Most people experience reduced appetite and changed appetite signals. Nausea is common and usually mild. Some notice they feel fuller sooner after eating. Weight loss is typically minimal or none at this stage.
Week 3–4: Appetite-suppressing effects usually become more noticeable. Some individuals begin to see 0.5–1kg weight loss. Stomach side effects (nausea, diarrhoea) may persist but often improve with time.
Week 5–12: More noticeable weight loss becomes visible, typically 1–2kg per week. By week 12, clinical trial data shows many people have lost 4–8kg depending on their starting weight and lifestyle changes. Dose may be increased at week 4, 8, and 12 depending on response and tolerance—this is called dose titration.
Week 13+: Continued gradual weight loss as the dose reaches maintenance levels. Peak effects usually emerge after 16–20 weeks on a stable dose.
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.
What Affects How Quickly Mounjaro Works
Individual response to Mounjaro varies based on several factors. Diet quality and calorie deficit are crucial—this medication supports weight loss but does not replace healthy eating. Physical activity level significantly impacts results; people who exercise see faster weight loss. Your starting weight matters: heavier individuals often see faster initial loss. Metabolic factors including age, genetics, and existing medical conditions influence response. Adherence to dose timing ensures consistent drug levels. Your prescriber will assess your individual timeline and may adjust your expectations based on your health history. Track your progress with our free progress tracker to monitor changes week by week alongside lifestyle modifications.
- 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 of Mounjaro?
Unlikely—meaningful weight loss typically begins after 3–4 weeks. Early effects focus on appetite reduction rather than weight change. Most people lose 0.5–1kg in week 1, though some see none. Clinical trials show weight loss accelerates after the first month.
How much weight will I lose in the first month on Mounjaro?
Average first-month weight loss ranges from 1–4kg depending on starting weight, diet adherence, and activity level. Results vary significantly. Clinical trial data shows more substantial loss (4–8kg) by week 12. Use our GLP-1 weight loss calculator for individual projections.
What if Mounjaro is not working after 4 weeks?
If you see no appetite changes after 4 weeks, speak to your prescriber—you may need a dose increase. Mounjaro works by gradual dose titration (increasing every 4 weeks). Your prescriber will assess whether the current dose is right for you.
Can I speed up how quickly Mounjaro works?
Results depend on dose, diet, and exercise—not on how fast you increase doses. Rushing titration increases side effects without improving results. Follow your prescriber's schedule: typically 4 weeks at each dose level before increasing. Calorie deficit and physical activity are equally important.
How does Mounjaro's timeline compare to Wegovy?
Both tirzepatide (Mounjaro) and semaglutide (Wegovy) show appetite effects within 1–2 weeks and measurable weight loss by week 4–8. Clinical trials suggest Mounjaro may achieve slightly faster average weight loss, but individual variation is significant. Both require commitment to diet and exercise. ```
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.