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 showing weight loss effects within 2–4 weeks, though most people notice more significant changes after 8–12 weeks of consistent use. The timeline depends on your starting dose, how your body responds, and whether you're following a reduced-calorie diet and increased physical activity alongside the injection. Unlike some medications, Mounjaro works gradually—this is actually beneficial for sustainable weight loss. If you're comparing Mounjaro with Wegovy (semaglutide), there are important differences in how quickly each medication takes effect. Our GLP-1 weight loss calculator can help you understand realistic timelines based on your individual 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 vs Wegovy: Onset Timeline and Clinical Evidence
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that begins affecting appetite within days, but visible weight loss typically emerges after 2–4 weeks. In the SURMOUNT-1 clinical trial, participants on the maximum 15mg dose showed an average weight loss of 20.9% of body weight at 72 weeks. Early weeks often focus on appetite reduction rather than rapid scale changes.
Wegovy (semaglutide) is a GLP-1-only agonist with a similar timeline: appetite suppression within days, noticeable weight loss after 2–4 weeks. The STEP-1 trial showed approximately 14.9% average weight loss at 68 weeks on the maximum 2.4mg dose. Both medications require the same prescription-only clinical assessment in the UK.
- Weeks 1–2: Appetite reduction, possible nausea (manageable)
- Weeks 3–8: Initial weight loss becomes visible, dose escalation continues
- Weeks 9–16: Cumulative effects, more consistent weekly loss
- Weeks 17+: Stabilisation at maintenance dose, continued gradual loss
The key difference is mechanism: Mounjaro's dual action on GIP and GLP-1 may produce slightly faster early results for some people, though individual variation is significant.
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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.
Suitability and Next Steps in the UK
Whether Mounjaro or Wegovy suits you better is a decision for your prescriber following clinical assessment. Both require the same eligibility criteria: BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes or hypertension. Both are available through NHS specialist weight management services (with variable waiting times by region) or via private online prescribing following UK consultation.
Realistic expectations are essential: weight loss is not assured, results vary significantly between individuals, and both medications work best alongside reduced-calorie diet and increased physical activity. Your GP or licensed prescriber will assess your medical history, current medications, and weight loss goals to recommend the most appropriate option. Use our BMI calculator above to check your current position and understand your eligibility context.
- 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 quickly does Mounjaro work compared to Wegovy?
Both typically show initial appetite reduction within days but visible weight loss within 2–4 weeks. Mounjaro's dual GIP+GLP-1 mechanism may offer slightly faster early effects for some people, though clinical trial data shows similar average weight loss at 68–72 weeks. Individual results vary considerably regardless of which medication you use.
Will I see weight loss results in the first week of Mounjaro?
Most people experience appetite suppression in the first week, which may lead to naturally eating less. However, significant scale changes are unlikely in week one. Sustainable weight loss typically becomes visible after 2–4 weeks, once your body has adjusted to the injection and you've consistently maintained a calorie deficit.
Why does Mounjaro take time to show results?
Mounjaro works by gradually regulating appetite, insulin release, and how your body processes food. The dose titration schedule (starting at 2.5mg and increasing every 4 weeks) allows your body to adjust and minimises side effects. Faster weight loss would require higher calorie deficits, which increases risks and muscle loss.
Can I switch from Wegovy to Mounjaro if Wegovy isn't working?
Switching between medications should only be done under prescriber guidance. There is no official established conversion chart between Wegovy and Mounjaro doses—your prescriber will assess whether a switch is appropriate and manage any transition carefully. Both require ongoing clinical oversight.
How does the NHS decide between Mounjaro and Wegovy prescriptions?
NHS tier 3 weight management services assess your individual health profile, medical history, and previous weight loss attempts. Both medications are NICE-approved for NHS use where available, though regional variation and waiting times exist. Private online prescribers follow similar assessment protocols to recommend the most suitable option.
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.