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 To Work?
Mounjaro (tirzepatide) typically begins working within 1 to 2 weeks of your first injection, though noticeable weight loss usually takes 4 to 8 weeks to appear. Because Mounjaro is a prescription-only medicine available via the NHS or private prescribers in the UK, the timeline depends on your dose, diet, and individual response. Understanding when to expect results helps you stay motivated throughout your weight management journey and assess whether the medication is right for you.
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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 Differences
Both Mounjaro and Wegovy are GLP-1 or dual-action weight loss injections, but they work at different speeds. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways, which may explain why some patients report appetite suppression within days. Wegovy (semaglutide) is a GLP-1-only agonist, activating a single pathway.
Clinical trial data shows: Mounjaro participants in the SURMOUNT trial experienced measurable appetite changes within 1–2 weeks on the starting dose (2.5 mg). Wegovy participants in the STEP trials typically noticed appetite reduction within a similar timeframe. However, actual weight loss (on a scale) generally becomes visible after 4 weeks for both medications when combined with reduced-calorie diet and physical activity.
The key difference: dual action (Mounjaro) may produce faster or more pronounced early hunger suppression, but sustained weight loss depends on dose escalation, adherence, and lifestyle changes—not the medication alone.
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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.
What to Expect Week by Week on Mounjaro
Weeks 1–2: Initial appetite suppression may begin; nausea or other side effects possible. No weight change yet on scales.
Weeks 3–4: Reduced hunger becomes more consistent. First modest weight changes may appear (typically 0.5–1 kg).
Weeks 5–8: Dose escalation (from 2.5 mg to 5 mg) increases effect. Weight loss accelerates as you settle into lower food intake.
Weeks 9+: Further dose increases (if prescribed) drive sustained loss. After 12 weeks on stable dose, pattern becomes clearer.
Individual timelines vary significantly. Some people see weight change within 2 weeks; others take 8+ weeks. Your prescriber will review progress at each dose escalation to assess whether treatment is working for you and adjust if needed.
- 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 on Mounjaro?
Unlikely. Most people see appetite changes within 1–2 weeks, but actual weight loss (measurable on scales) takes 3–4 weeks minimum. Initial changes are typically hunger suppression and eating smaller portions, not fat loss. Be patient and focus on compliance with reduced-calorie diet.
How does Mounjaro's onset compare to Wegovy?
Both typically begin suppressing appetite within 1–2 weeks. Mounjaro's dual GIP+GLP-1 mechanism may produce slightly faster early hunger signals for some patients, but sustained weight loss timelines are similar. Wegovy is GLP-1-only. Both require 4+ weeks to show measurable scale results.
What if I don't see weight loss after 8 weeks on Mounjaro?
Contact your prescriber. Lack of response after 8 weeks may indicate: incorrect dose for your body, inadequate calorie deficit despite reduced appetite, unmanaged medical conditions, or that the medication isn't suitable for you. Your GP or prescriber may adjust dose or recommend an alternative.
Does the dose affect how quickly Mounjaro starts working?
Yes. Mounjaro dosing is gradual: 2.5 mg weeks 1–4, then 5 mg, 7.5 mg, up to 15 mg. Higher doses produce stronger appetite suppression but also increase side effects. Faster does not mean better—dose titration is structured to find your effective dose while minimising nausea and other effects.
Can I speed up Mounjaro's results with strict dieting?
No. Very restrictive dieting (under 1200 kcal daily) is not recommended and risks muscle loss, fatigue, and nutrient deficiency. Mounjaro works best with a sustainable reduced-calorie diet (500 kcal deficit) plus physical activity. Your prescriber will advise on safe targets for your personal situation.
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.