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 Before Mounjaro Starts Working?
Mounjaro (tirzepatide) doesn't produce weight loss results overnight — most people begin noticing changes within the first 4 weeks, though the full effect develops over several months. As a prescription-only dual GIP and GLP-1 receptor agonist, Mounjaro works by activating two natural hormone pathways that support appetite regulation and food satisfaction. The timeline depends on your individual response, dose level, and how consistently you combine the injection with a reduced-calorie diet and increased physical activity. Understanding when to expect results helps set realistic goals and track progress effectively.
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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 Mechanism Comparison
Mounjaro timeline: Most people report noticing changes by week 3–4, with more significant weight loss typically visible by weeks 8–12. The SURMOUNT-1 clinical trial showed participants lost an average of 15–21% of body weight over 72 weeks at the maximum 15 mg dose — though results vary considerably between individuals. Weight loss accelerates as your dose increases during the titration phase (starting at 2.5 mg and escalating every 4 weeks).
Wegovy timeline: Clinical evidence from the STEP trials shows similar early timeline — changes noticeable within 3–4 weeks — but Wegovy reaches its maximum dose (2.4 mg) faster. Participants in STEP-1 achieved approximately 14.9% average weight loss over 68 weeks. The key difference is mechanism: Mounjaro's dual GIP+GLP-1 action targets appetite and blood sugar through two pathways, whilst Wegovy activates GLP-1 alone. This may explain why some people respond faster to one than the other, though both require consistent lifestyle changes alongside the injection.
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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.
Factors That Affect How Quickly Mounjaro Works
Your results depend on several individual factors: starting weight, metabolism, adherence to dose schedule, diet quality, and physical activity level. People with higher starting BMI sometimes see faster initial results. Consistency matters significantly — missing doses or not maintaining a reduced-calorie diet slows progress. Your GP or prescriber will monitor your response at each 4-week dose adjustment, and may adjust your titration schedule if needed. Some people see measurable changes by week 2–3 (reduced hunger, smaller portion sizes), whilst others need 8–12 weeks before weight loss becomes visible on scales. Use our free BMI calculator and GLP-1 weight loss calculator to track your starting point and project realistic timelines based on clinical trial data.
- 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 do you first feel Mounjaro working?
Most people notice changes within 1–3 weeks — typically reduced appetite, less food cravings, or feeling fuller faster. However, visible weight loss on scales usually takes 4–8 weeks. These early signs indicate the medication is activating your GIP and GLP-1 receptors, even before significant weight loss occurs.
Why does Mounjaro take longer than expected?
Mounjaro requires consistent calorie deficit alongside the injection. If weight loss plateaus, it's often because calorie intake has matched energy expenditure. Your GP can review your diet and activity levels. Individual metabolism varies — some people's bodies respond faster than others to tirzepatide.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials show similar timelines — both show noticeable changes within 3–4 weeks. Mounjaro achieved 15–21% average weight loss (SURMOUNT trial), Wegovy 14.9% (STEP trial). The dual mechanism may work faster for some people, but individual response varies. Your prescriber will recommend based on your health profile.
What if Mounjaro isn't working after 8 weeks?
Contact your GP or prescriber. They'll check your dose level, review your diet and activity, and assess whether your body is responding to tirzepatide. Sometimes increasing to the next dose or adjusting your calorie deficit helps. Not everyone responds to GLP-1 medications — alternatives exist.
Can you switch from Wegovy to Mounjaro if results are slow?
Yes, prescribers can switch between weight loss medications. Mounjaro's dual GIP+GLP-1 action may work better for you. Any switch requires clinical assessment and careful dose planning — you won't simply transfer your current dose. Discuss this with your GP or prescriber, who'll assess your eligibility and response.
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.