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 After Your First Mounjaro Injection Does It Work?
Mounjaro (tirzepatide) typically begins working within the first few days of your first injection, though the full effects become more noticeable over the first 4 weeks as your body adjusts. Most people experience appetite suppression and reduced cravings during weeks 2–4 of treatment. However, significant weight loss usually takes 8–12 weeks to become apparent, and the dose titration schedule means your prescriber will increase your dose every 4 weeks to find the right level for you. Results vary between individuals, and Mounjaro works best alongside a reduced-calorie diet and increased physical activity.
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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 After Your First Mounjaro Injection
Your first Mounjaro injection is a 2.5 mg starting dose, which is lower than the typical maintenance doses (5 mg to 15 mg). Here's what typically happens:
- Days 1–3: You may feel subtle appetite changes or mild nausea as your body adjusts to the GIP and GLP-1 hormone pathways being activated.
- Week 1–2: Many people report reduced hunger and food cravings, feeling fuller after smaller meals, and less desire to snack.
- Weeks 2–4: These effects usually become more pronounced. Your weekly weight loss injection works by slowing gastric emptying and regulating appetite hormones.
- Week 4: Your prescriber will increase your dose to 5 mg. This is the dose titration phase — your dose increases every 4 weeks until you reach an effective level.
- Weeks 8–12: Measurable weight loss typically becomes visible as the higher dose takes full effect and your body adapts to the medication.
It's important to note that during the first 4 weeks, your body is adjusting to the 2.5 mg starting dose. The real appetite-suppressing effect intensifies as your dose increases. Some people feel significant effects at lower doses, while others need higher doses to notice a difference.
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.
When You'll See Real Weight Loss Results
In clinical trials (SURMOUNT-1), participants taking Mounjaro saw measurable weight loss starting around weeks 8–12, with more substantial results by weeks 16–20. By week 72 of treatment, average weight loss ranged from approximately 15% to 21% of body weight depending on the final dose — though individual results vary significantly.
The timeline is longer than people sometimes expect because weight loss isn't instant. Your appetite reduces within days, but sustainable fat loss requires several weeks of consistent calorie deficit. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different Mounjaro doses, and our BMI calculator to track your progress from baseline. Keep a record of your weight weekly and discuss progress with your prescriber at each dose increase.
---COL_J_TOOL_NOTE--- [Naturally references GLP-1 calculator and BMI calculator as relevant tools]- 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 after my first Mounjaro injection?
No. The first 2.5 mg dose begins reducing appetite within days, but measurable weight loss typically takes 8–12 weeks. The dose titration schedule (increasing every 4 weeks) means your body is still adjusting during the first month. Significant results appear once you reach a higher, effective dose alongside consistent dietary changes.
How does Mounjaro's dose titration affect how quickly it works?
Mounjaro starts at 2.5 mg and increases by 2.5 mg every 4 weeks until you reach 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg. The dose titration is gradual to help your body adjust and reduce side effects. Higher doses generally produce stronger appetite suppression and greater weight loss, but it takes time to reach them.
What's the difference between Mounjaro and Wegovy in terms of how fast they work?
Mounjaro (tirzepatide) and Wegovy (semaglutide) both start with low starter doses and titrate upwards. The main difference is that Mounjaro is a dual GIP and GLP-1 agonist, whereas Wegovy is GLP-1 only. Both typically show measurable weight loss within 8–12 weeks. Clinical trial results suggest Mounjaro may produce slightly greater average weight loss, but individual responses vary.
Is Mounjaro a weekly weight loss injection?
Yes, Mounjaro is a once-weekly injection delivered via the KwikPen device. You inject it the same day each week for the first 4 weeks (while on 2.5 mg), then your prescriber increases the dose every 4 weeks. It's not the same as daily injections — the once-weekly schedule makes it convenient for most people.
Will my GP prescribe Mounjaro on the NHS, or do I need a private prescription?
Mounjaro is available on the NHS through specialist tier 3/4 weight management services, but access varies significantly by region and availability. Most people access Mounjaro privately following an online clinical assessment with a UK-licensed prescriber. Your prescriber will assess whether you meet eligibility criteria (BMI 30+, or 27+ with weight-related health conditions) before issuing a Mounjaro UK prescription.
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.