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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When Will Mounjaro Start Working — Mounjaro vs Wegovy Timeline
Mounjaro (tirzepatide) typically begins affecting appetite and food intake within the first 1–2 weeks of starting treatment, though weight loss itself usually becomes noticeable after 4–6 weeks. However, like Wegovy (semaglutide), individual timelines vary significantly depending on starting dose, how your body responds, and lifestyle factors. Both medications work through different mechanisms — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Understanding how each works can help set realistic expectations for your weight loss journey.
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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.
How Mounjaro and Wegovy Work — Onset Comparison
Mounjaro and Wegovy have different mechanisms, which can affect how quickly they work. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously — this dual action may contribute to faster appetite suppression in some patients. Wegovy (semaglutide) is a GLP-1-only agonist, activating a single pathway.
Clinical trial data shows differences in timeline: Mounjaro users in the SURMOUNT trial reported reduced appetite within 1–2 weeks of the starting dose (2.5 mg), with weight loss acceleration typically visible by weeks 4–8. Wegovy users in STEP trials showed similar early appetite changes but slightly more gradual initial weight loss. Both medications require dose titration — your dose increases every 4 weeks — and full effects are typically not seen until reaching maintenance doses (15 mg for Mounjaro, 2.4 mg for Wegovy after 4–5 months).
Important: Your UK prescriber will start you at the lowest dose to minimise side effects. Weight loss acceleration often happens after dose increases, not immediately at starting dose.
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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.
Individual Timeline and What to Expect
Your personal timeline depends on several factors: starting dose, how quickly your dose increases, your adherence to a reduced-calorie diet, physical activity level, metabolism, and baseline health conditions. Some patients on Mounjaro report noticeable appetite reduction by week 2; others may take 6–8 weeks to feel significant effects.
For both medications, weeks 1–4 typically show nausea and appetite changes (side effects); weeks 4–12 show emerging weight loss; weeks 12–24 show accelerated loss as doses increase. Try our GLP-1 weight loss calculator to estimate potential weight loss timelines at different doses, and use our progress tracker to monitor your actual results week by week. Always discuss realistic expectations with your prescriber during your initial assessment.
- 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.
Does Mounjaro work faster than Wegovy?
Mounjaro's dual mechanism (GIP and GLP-1) may cause slightly faster appetite suppression in some patients, but clinical weight loss timelines are similar — both average 4–6 weeks for noticeable loss. Individual variation is significant. Your prescriber can advise what to expect for your specific situation.
When should I expect to see weight loss on Mounjaro?
Most patients on Mounjaro see measurable weight loss by 4–6 weeks, though appetite changes often occur within 1–2 weeks. Full effects typically emerge after 8–12 weeks as your dose increases. Faster results are not assured — results vary based on diet, activity, and individual metabolism.
Why does Mounjaro require dose titration if it starts working quickly?
Dose titration (increasing every 4 weeks) is used to minimise nausea and side effects while allowing your body to adjust. Each dose increase typically produces additional weight loss acceleration. Starting low reduces stomach upset while maximising long-term adherence and results.
Can I compare Mounjaro onset time with Wegovy before choosing?
Both require NHS specialist referral or private clinical assessment — your prescriber will recommend based on your health profile, not onset speed. Clinical differences are modest. Your GP or licensed prescriber can discuss which suits you better after reviewing your individual circumstances.
What if Mounjaro doesn't start working for me after 8 weeks?
If you're not seeing appetite suppression or weight changes after 8 weeks on an appropriate dose, discuss with your prescriber — they may adjust your dose, review your diet and activity, or (rarely) switch medications. Patience is important; some patients need 12+ weeks to respond fully.
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.