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) does not produce weight loss overnight. Most people begin to notice appetite changes within the first two weeks, but meaningful weight loss typically takes 4–8 weeks to become apparent. The medication works gradually as your dose increases over several months. This is intentional — the slow titration approach helps your body adjust and reduces side effects. Individual results vary significantly based on dose, diet, physical activity, and metabolism. Your GP or prescriber can discuss realistic timescales for your specific 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.
Timeline: What To Expect Week By Week
Weeks 1–2 (2.5 mg starting dose): Many people notice reduced appetite and food cravings beginning within days, though this is not assured. Some experience nausea or digestive changes. Weight loss is typically minimal at this stage.
Weeks 3–4: Appetite suppression usually becomes more consistent. Some people report feeling fuller after smaller meals. Weight loss may begin but is often slight (0.5–1 kg).
Weeks 5–8 (5 mg dose): This is when noticeable weight loss typically accelerates. Most people see measurable results on scales and in how clothes fit. Energy levels often improve as nausea settles.
Weeks 9–12+ (7.5 mg and higher): Weight loss momentum continues as doses increase. The full effect of each dose takes approximately 4 weeks to stabilise before escalating further. Results become more pronounced at higher tirzepatide doses.
12–20 weeks onwards: Sustained weight loss continues alongside lifestyle changes. Try our GLP-1 weight loss calculator to project potential results at different doses based on clinical trial data.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
Mounjaro's effectiveness is not one-size-fits-all. Calorie intake and physical activity are crucial — the medication supports appetite control, but weight loss requires a sustained calorie deficit alongside diet and exercise. Starting weight and body composition influence timeline; people with higher starting weights often see faster initial loss. Individual metabolism varies — some people respond quickly, others progress more slowly. Dose escalation matters; higher doses (10 mg, 15 mg) typically produce greater weight loss than lower doses. Medical conditions like thyroid issues or PCOS may affect results. Consistency is essential — missing doses or not maintaining lifestyle changes slows progress. Your prescriber can discuss which factors apply to you and adjust your treatment plan accordingly.
- 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?
Probably not significant weight loss, but you may notice appetite changes within 2–7 days. The 2.5 mg starting dose is low; it's designed to let your body adjust. Visible weight loss usually takes 3–4 weeks as your dose increases.
How much weight can I expect to lose on Mounjaro?
Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks on the highest dose (15 mg), alongside diet and exercise. Individual results vary significantly. Use our free GLP-1 weight loss calculator to see projected ranges based on dose.
Is Mounjaro faster than Wegovy for weight loss?
Clinical data suggests Mounjaro (tirzepatide) produces slightly more weight loss on average than Wegovy (semaglutide), but the difference between them is modest and individual variation is huge. Results depend more on dose, consistency, and lifestyle than which medication you take.
What if Mounjaro isn't working after 8 weeks?
Give it more time — most weight loss occurs from week 8 onwards as doses increase. Ensure you're maintaining a calorie deficit and physical activity. If you have concerns, discuss progress with your prescriber; they may adjust your dose or assess whether the medication is right for you.
When should I expect to see results on my weight converter chart?
Use our free weight converter to track progress in stone, kg, or lbs. Most people notice measurable changes (0.5–1 kg loss) by weeks 3–4, with acceleration from week 5 onwards. Consistent tracking helps you recognise progress that might not feel obvious day-to-day.
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.