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 Fast Does Mounjaro Work?
Mounjaro (tirzepatide) is a prescription-only weight management injection that works as a dual GIP and GLP-1 receptor agonist. Most people begin noticing changes within 2–4 weeks of starting, though significant weight loss typically develops over several weeks and months. Individual results vary considerably, and the medication works gradually rather than providing rapid changes. This guide explains the Mounjaro timeline and what to expect.
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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: When Does Mounjaro Start Working?
Weeks 1–2 (2.5 mg starting dose): Most people begin experiencing appetite suppression and reduced food cravings within the first 1–2 weeks. Some notice nausea or mild stomach changes. Weight changes are typically minimal at this stage.
Weeks 3–4: As the body adjusts to the medication, appetite reduction usually becomes more noticeable. Some people may see initial weight loss of 1–2 kg, though this varies widely. The KwikPen device delivers a consistent dose each week.
Weeks 5–8 (5 mg dose): After the first dose increase, more significant appetite suppression typically occurs. Average weight loss becomes more apparent. Many people report feeling fuller after smaller meals and fewer cravings between meals.
Weeks 9–16+ (7.5 mg–10 mg doses): At higher maintenance doses, weight loss momentum typically accelerates. Clinical evidence suggests meaningful reductions become consistent at this stage, though individual timelines vary significantly.
Clinically BackedWeight Loss
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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 the Clinical Evidence Shows
The SURMOUNT-1 clinical trial tracked weight loss over 72 weeks on tirzepatide. At the maximum dose of 15 mg, participants lost an average of approximately 20.9% of their body weight. However, most of this loss occurred gradually over the full 72-week period—not immediately. Results were achieved alongside a reduced-calorie diet and increased physical activity. Importantly, individual results vary significantly; some people lose more, others less. Your GP or prescriber can discuss realistic expectations based on your personal health profile.
- 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 long before Mounjaro shows real weight loss results?
Most people see noticeable appetite changes within 2–4 weeks, but meaningful weight loss typically develops over 8–12 weeks. The SURMOUNT trial showed weight loss progressed gradually over the full 72-week period. Results depend on dose, adherence, diet, and activity level.
Does Mounjaro work immediately on the first injection?
No. The first 2.5 mg dose is a starting dose to allow your body to adjust. Most people don't experience significant appetite suppression until weeks 2–3. Dose increases (every 4 weeks minimum) gradually build the medication's effect.
Is Mounjaro faster than Wegovy for weight loss?
Mounjaro uses a dual GIP and GLP-1 mechanism versus Wegovy's GLP-1-only mechanism. Clinical trials suggest tirzepatide may produce slightly greater average weight loss, but direct comparisons show similar timelines—both work gradually over weeks and months. Individual response varies.
What if Mounjaro isn't working after 4 weeks?
Four weeks is too early to judge effectiveness. Your prescriber will assess progress at 4-week intervals before increasing the dose. Nausea or appetite changes may indicate the medication is working but require dose adjustment. Always discuss concerns with your GP or prescriber.
Can I speed up how fast Mounjaro works?
Weight loss speed is determined by dose, adherence, calorie deficit, and physical activity—not by rushing doses. Doses increase every 4 weeks minimum to safely reach maintenance levels. Crash dieting or other extreme measures carry health risks and aren't recommended alongside Mounjaro.
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.