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 Does Mounjaro Take to Work?
Mounjaro (tirzepatide) typically begins to show weight loss effects within the first 2–4 weeks of starting treatment, though individual results vary significantly. Most people experience noticeable changes in appetite and food cravings during the initial weeks, but meaningful weight loss usually becomes apparent around 8–12 weeks when the dose reaches the first maintenance level. Full effects are typically observed after 16–20 weeks as the dose continues to escalate. Weight loss is not assured and results depend on dose, lifestyle changes, and individual metabolism. Your GP or prescriber can explain realistic timelines for your personal situation.
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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.
Mounjaro Timeline — What Happens When
Mounjaro works through a dual GIP and GLP-1 receptor agonist mechanism, meaning it activates two natural hormone pathways to regulate appetite and support feeling fuller after meals. The tirzepatide dose typically begins at 2.5 mg and escalates every 4 weeks: 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), 10 mg (weeks 13–16), 12.5 mg (weeks 17–20), and up to 15 mg (maximum maintenance dose). During the first 2–4 weeks, many people report reduced hunger and early appetite suppression, even on the starting dose. Meaningful weight loss usually appears around 8–12 weeks once the dose reaches the first maintenance level of 5 mg. Clinical trials showed that by 72 weeks, participants on the 15 mg dose lost an average of approximately 21% of their body weight — though results are highly individual and lifestyle factors (diet and exercise) significantly influence outcomes.
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.
Factors That Affect How Quickly Mounjaro Works
Individual responses to Mounjaro vary based on several factors: starting weight (heavier individuals may see faster initial loss), dose level (higher maintenance doses typically produce greater weight loss), adherence to diet and exercise (medication works best alongside a reduced-calorie diet and increased physical activity), metabolism (individual genetic and metabolic differences), and other medications or health conditions (some conditions affect weight loss response). Side effects like nausea or constipation often improve after 2–4 weeks on each dose. If you're not seeing expected results after 12 weeks, discuss this with your prescriber — they may adjust your dose, review your lifestyle factors, or assess whether the medication remains appropriate for you.
- 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?
Many people notice reduced appetite in the first 1–2 weeks, but actual weight loss usually takes 2–4 weeks to become measurable. Initial appetite suppression doesn't always mean immediate weight loss — your body needs time to adapt to the medication and the reduced calorie intake to register as weight loss.
How much weight can I expect to lose in the first month?
In the first month, weight loss varies widely — some people lose 1–3 kg, others lose minimal weight initially. This is normal. Clinical trials showed the most significant weight loss occurred after 8–12 weeks once doses reached maintenance levels. Use our free GLP-1 weight loss calculator to see projected outcomes at different doses.
Is Mounjaro faster acting than Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications with different mechanisms. Clinical data suggests Mounjaro may produce slightly greater average weight loss (around 21% vs 15%), but individual results vary considerably. Your prescriber can discuss which is more suitable for your circumstances.
What if I don't see weight loss after 8 weeks on Mounjaro?
If you're not losing weight after 8 weeks, speak to your prescriber. Possible reasons include insufficient calorie deficit, underlying medical conditions, other medications interfering, or that Mounjaro may not be the right fit for you. Your dose may need adjusting or an alternative approach explored.
How do I know if Mounjaro is working?
Beyond the scales, signs include reduced appetite, less frequent food cravings, feeling fuller on smaller portions, and improved blood sugar control if you have prediabetes. Use our free progress tracker to log these changes alongside weight. Your GP can also assess improvements in weight-related health conditions.
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.