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
Does Mounjaro Work Immediately?
Mounjaro (tirzepatide) does not work immediately. Most people do not see noticeable weight loss in the first week or even the first month. The medication requires a gradual dose titration schedule, starting at 2.5 mg and increasing every 4 weeks up to a maintenance dose. Weight loss typically becomes apparent after 4–8 weeks at a stable dose. Understanding the realistic timeline helps set proper expectations if you're considering this NHS or privately prescribed injection for weight management in the UK.
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.
How Mounjaro Works and the Dose Titration Timeline
Mounjaro is a dual GIP and GLP-1 receptor agonist that helps regulate appetite and support feeling fuller after meals. However, it does not produce gradual improvement. The medication works by gradually influencing hunger hormones, and your body needs time to respond and adapt.
Dose titration schedule:
- Weeks 1–4: 2.5 mg (starting dose)
- Weeks 5–8: 5 mg (first escalation)
- Weeks 9–12: 7.5 mg (optional escalation)
- Weeks 13–16: 10 mg (optional escalation)
- Weeks 17–20: 12.5 mg (optional escalation)
- Week 21+: 15 mg (maximum dose)
Each dose must be held for a minimum of 4 weeks before increasing. This gradual titration allows your body to adjust and reduces side effects. Most people notice modest appetite changes within 2–3 weeks, but meaningful weight loss typically emerges after 4–8 weeks at a stable dose. SURMOUNT trial data showed average weight loss of approximately 15–21% body weight over 72 weeks — not weeks.
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 to Expect: Realistic Weight Loss Timeline in the UK
If you're prescribed Mounjaro through NHS tier 3 services or privately, realistic expectations matter. Week 1–2: You may feel less hungry or experience nausea as your body adjusts. Weeks 3–4: Subtle appetite reduction may be noticeable. Weeks 5–8: As your dose increases and stabilises, weight loss typically becomes measurable — many people see their first 2–4 kg loss. Weeks 9–12 and beyond: Weight loss accelerates as higher doses take effect. Individual results vary significantly based on adherence, diet, physical activity, and metabolism. Using our free GLP-1 weight loss calculator can help you project potential results at different doses, though actual outcomes depend on your personal circumstances.
- 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 Mounjaro help me lose weight in the first week?
No. Weight loss is not expected in week one. You may experience mild appetite changes or nausea as your body adjusts to the 2.5 mg starting dose, but meaningful weight loss typically takes 4–8 weeks at a stable dose to become apparent.
How long does it take for Mounjaro to work properly?
Most people see noticeable effects after 4–8 weeks once they reach a stable maintenance dose. The dose titration process takes 4–5 months to reach maximum dose (15 mg), during which weight loss gradually increases. Full effects are seen over 12–24 weeks.
Can I access Mounjaro through the NHS in the UK?
Yes. NICE has approved tirzepatide (Mounjaro) for NHS use within specialist tier 3 weight management services. You'll need referral from your GP and assessment by a specialist service. Availability varies significantly by region and waiting times can be lengthy in some areas.
What's the difference between Mounjaro and Wegovy timelines?
Both require gradual dose escalation and take similar timeframes to show results. Mounjaro is a dual GIP/GLP-1 agonist, while Wegovy is GLP-1 only. Mounjaro's maximum dose (15 mg) is reached more gradually than Wegovy's (2.4 mg), but the weight loss timeline is comparable — expect 4–8 weeks before seeing measurable results.
What if Mounjaro isn't working after 3 months?
If you're not seeing progress after 3 months at a stable dose, discuss this with your prescriber. They may assess adherence, diet, and activity levels, or consider dose adjustment. Some people respond more slowly or less dramatically than others — individual results vary significantly.
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.