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
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How Long Does Mounjaro Take To Start Working?
Mounjaro typically begins to show effects within the first 2–4 weeks of starting treatment, though the most noticeable changes often happen after 4–8 weeks as your dose increases. Unlike some medications, Mounjaro works gradually — you're unlikely to see dramatic results after a single injection. This page compares Mounjaro's onset timeline with Wegovy and explains what to expect during dose titration so you can set realistic expectations for your weight management journey in the UK.
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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 Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work through appetite regulation, but they have different onset profiles due to their mechanisms and dose schedules. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 receptors only. In clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy), weight loss was measurable by week 4–6, but most significant losses occurred between weeks 8–16 as doses increased.
- Mounjaro: Starts at 2.5 mg with 4-week intervals between dose increases (dose titration). Peak effects typically seen at weeks 12–20 when most people reach their therapeutic dose.
- Wegovy: Starts at 0.25 mg with weekly escalation. Therapeutic dose (2.4 mg) reached by week 16. Some users report appetite changes earlier than Mounjaro users, around week 2–3.
- Timing: Individual variation is significant — some people notice appetite suppression after one injection, others take 6–8 weeks. Your prescriber will monitor your response and adjust accordingly.
Your GLP-1 weight loss calculator can help you estimate potential progress at different stages of treatment.
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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 During Your First Weeks
Week 1–2: You'll receive your first Mounjaro KwikPen injection. Many people experience nausea or mild appetite reduction immediately, though this may ease as your body adjusts. You may not see weight changes yet — the medication is starting to work internally.
Week 4–8: As you increase to the 5 mg dose during your second and third injections, appetite suppression becomes more noticeable. Modest weight loss (1–3 kg) often begins here. Gastrointestinal side effects like nausea or loose stools are most common at this stage.
Week 12+: By your fourth injection at 7.5 mg or higher, most people report meaningful changes in hunger levels and food preferences. Cumulative weight loss becomes clearer. Side effects typically diminish as your body adapts. Your GP or prescriber will assess your progress and response to determine whether to continue dose escalation or maintain your current level. Using our progress tracker can help you log changes and share them with your prescriber for informed decisions.
- 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 after my first Mounjaro injection?
Unlikely immediately. Most people see modest appetite changes within 2–4 weeks, but noticeable weight loss typically takes 6–8 weeks as your dose increases during titration. This is normal — Mounjaro works gradually. Your body is responding even if the scales haven't shifted yet.
How does Mounjaro's onset compare to Wegovy?
Wegovy users often report appetite suppression slightly earlier (weeks 2–3) because escalation is weekly, reaching therapeutic dose by week 16. Mounjaro's dose increases happen every 4 weeks, so peak effects may take longer (weeks 12–20). Both show measurable weight loss by week 6–8 in clinical trials.
What if I don't feel Mounjaro working after 4 weeks?
This is common. Appetite changes can be subtle initially, and weight loss lags behind hormonal effects. Continue your dose schedule — most people notice significant changes by weeks 8–12. If you have concerns, contact your prescriber; they may adjust your approach or assess whether this medication suits you.
Does dose titration affect how quickly Mounjaro works?
Yes. Your Mounjaro UK prescription schedule involves four-week intervals between doses to allow your body to adjust and monitor response. Faster escalation isn't recommended as it increases nausea risk without speeding weight loss. The titration schedule is designed for safety and effectiveness combined.
Can I use a progress tracker to monitor Mounjaro's effects?
Absolutely. Our free progress tracker lets you log weight, appetite changes, energy levels, and side effects weekly. Sharing this data with your GP helps them assess whether Mounjaro is working for you and whether dose adjustments are needed. Consistent tracking also keeps you motivated during the gradual onset phase.
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.