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
When Will Mounjaro Start Working?
Most people notice appetite changes within the first 1–2 weeks of starting Mounjaro, though some may take slightly longer. However, significant weight loss typically takes 8–12 weeks of consistent use to become noticeable. In clinical trials, the full effects of tirzepatide (the active ingredient in Mounjaro) became clearer around 16–20 weeks as doses were gradually increased. Individual timelines vary considerably depending on your starting dose, how quickly your dose increases, diet, and activity level. This guide explains what to expect at each stage of your Mounjaro journey, from week 1 through to your first significant results.
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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.
How Mounjaro Works and When Changes Begin
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two natural hormone pathways that regulate appetite, blood sugar, and how your body uses food energy. Unlike some medications that take weeks to build up in your system, tirzepatide often produces noticeable effects quite quickly because it directly signals your brain to reduce hunger.
First signs typically appear:
- Week 1–2: Reduced appetite, less interest in food, possible nausea (most common initial side effect)
- Week 3–4: Appetite suppression usually stabilises; nausea often improves as your body adjusts
- Week 5–8: Clearer appetite reduction; you may start noticing clothes fitting differently
- Week 9–12: More visible weight loss emerges for most people; dose increases may intensify effects
- Week 16+: Cumulative weight loss becomes more significant, especially as you reach higher maintenance doses (10–15 mg)
Starting on the lowest dose (2.5 mg) means changes are often subtle initially. Use our free GLP-1 weight loss calculator above to estimate potential outcomes as your dose increases over the titration period.
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.
What the Clinical Evidence Shows
The SURMOUNT-1 clinical trial (the primary UK and global trial for tirzepatide) tracked weight loss across 72 weeks. Participants taking 15 mg Mounjaro lost an average of 20.9% of their body weight — but this was the final result after 20+ weeks of dose increases. Early weight loss was much more modest: most participants saw their first 2–3 kg loss between weeks 8–12. Weight loss typically accelerated as doses increased toward 10 mg and 15 mg.
It's important to understand that individual results vary significantly. Some people lose weight steadily from week 4 onwards; others see minimal changes until week 12–16. Factors affecting your timeline include your starting weight, metabolism, how strictly you follow a reduced-calorie diet, physical activity levels, and whether you have type 2 diabetes. Always track progress using our free progress tracker to identify your personal pattern rather than comparing to others.
- 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 immediately on Mounjaro?
No — immediate weight loss (first 1–2 weeks) is usually water loss or reduced food intake, not fat loss. Most people see measurable fat loss starting around week 8–12. Appetite suppression often begins in week 1, but weight loss takes longer to become visible on the scales.
What if I don't feel Mounjaro working after 4 weeks?
This is normal. You're still on the starting dose (2.5 mg). At week 5, your dose increases to 5 mg, which typically produces stronger appetite suppression. Give the medication time — the full effects emerge as you titrate through doses 5 mg → 7.5 mg → 10 mg → 15 mg over 16–20 weeks.
Can Mounjaro work faster on a lower-calorie diet?
Yes. Mounjaro works alongside diet and exercise — it's not meant to work alone. Maintaining a reduced-calorie diet (typically 500 kcal below your normal intake) plus Mounjaro produces faster results than Mounjaro alone. Your prescriber will advise on realistic targets for your situation.
How does Mounjaro's timeline compare to Wegovy?
Both are similar — appetite changes appear within 1–2 weeks, but significant weight loss takes 8–12 weeks. Mounjaro's dual mechanism (GIP + GLP-1) may produce slightly faster results in clinical trials compared to Wegovy (GLP-1 only), but individual variation is large. Both require consistent use and lifestyle changes.
Is Mounjaro a UK prescription-only medication?
Yes. Mounjaro is a prescription-only medicine requiring clinical assessment before dispensing. Access in the UK is through NHS specialist weight management services (with waiting lists) or private online prescribing following an online consultation with a UK-licensed prescriber.
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.