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
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Mounjaro Weight Loss By Week vs Wegovy
When starting Mounjaro or Wegovy, weight loss doesn't happen immediately—it builds gradually over weeks and months as your dose increases. Both are once-weekly prescription injections licensed for weight management in UK adults, but they work differently and produce different weight loss trajectories. Understanding what clinical trials showed—and how individual results vary—helps you set realistic expectations before beginning treatment. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses.
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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.
Weekly Weight Loss Timelines: Clinical Trial Data
Mounjaro (tirzepatide) uses a dose titration schedule starting at 2.5 mg, increasing every 4 weeks up to 15 mg. In the SURMOUNT-1 clinical trial, participants on the maximum 15 mg dose lost an average of 20.9% of body weight over 72 weeks—approximately 0.3–0.4 kg per week once at maintenance dose, though weight loss is typically faster in the first 8–12 weeks.
- Weeks 1–4: 2.5 mg — minimal weight loss (adjustment phase)
- Weeks 5–8: 5 mg — modest weight loss begins (~0.2–0.3 kg/week)
- Weeks 9–16: 7.5–10 mg — accelerated loss (~0.4–0.6 kg/week)
- Weeks 17+: 12.5–15 mg — sustained weight loss (~0.3–0.5 kg/week at maintenance)
Wegovy (semaglutide) escalates from 0.25 mg to 2.4 mg over 17 weeks. The STEP-1 trial showed an average loss of 14.9% body weight at 68 weeks—slower initial trajectory than Mounjaro, averaging ~0.2 kg/week overall, but consistent adherence produces significant long-term results. This difference reflects Wegovy's single GLP-1 mechanism versus Mounjaro's dual GIP+GLP-1 action.
Clinically BackedWeight Loss
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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.
Why Results Vary: Mechanism, Adherence, and Suitability
Individual weight loss depends on several factors: adherence to reduced-calorie diet, physical activity level, baseline body weight, age, and whether you reach the maintenance dose without side effects stopping you. Mounjaro's dual GIP+GLP-1 mechanism activates two appetite-regulating pathways, which clinical data suggests produces faster average weight loss. Wegovy's single GLP-1 mechanism is effective but produces a slower trajectory. However, "faster" does not mean "better for you"—your GP or prescriber will assess which is appropriate based on your health profile, existing medications, and tolerability. A weekly weight loss injection UK prescription requires clinical assessment. Use our BMI calculator to check your current status before discussing options with a healthcare professional.
- 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 much weight will I lose per week on Mounjaro?
Clinical trials showed average weekly loss of 0.3–0.5 kg once on maintenance dose, though this varies significantly. Early weeks (during dose escalation) may show faster loss; later weeks often slower. Results depend on diet adherence, activity level, and individual metabolism. Always discuss realistic targets with your prescriber.
Is Mounjaro faster than Wegovy for weight loss?
SURMOUNT trial data showed Mounjaro produced ~clinically studied weight loss versus Wegovy's ~14.9% in comparable trials. However, "faster average" doesn't mean it's better for every individual. Your prescriber assesses which is suitable based on your health, comorbidities, and medication interactions. The Mounjaro vs Wegovy decision is personalised.
What happens in the first 4 weeks of Mounjaro?
The first 4 weeks use a 2.5 mg starting dose—this is an adjustment phase with minimal weight loss. Weight loss accelerates once you move to 5 mg in weeks 5–8. Gastrointestinal side effects (nausea, constipation) are most common in the first 2–3 weeks and often improve as your body adjusts.
Can I speed up weight loss by skipping dose escalation?
No. Both Mounjaro and Wegovy require a gradual dose titration schedule—each dose is maintained for a minimum 4 weeks. Accelerating doses increases side effects without improving results and is not medically safe. Your prescriber controls the schedule based on your tolerance and response.
When should I expect to see results on a weekly weight loss injection?
Most people notice modest weight loss by weeks 5–8 once on 5 mg (Mounjaro) or 0.5 mg (Wegovy). Significant results typically emerge by weeks 12–16. However, some people see results slower; others faster. Consistent diet and exercise adherence alongside medication produces best outcomes. Track your progress using our free progress tracker. ```
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.