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 Before Mounjaro Kicks In?
Mounjaro (tirzepatide) doesn't work immediately — most people notice changes within 2 to 4 weeks of starting, though some take longer. The timeline depends on your starting dose, individual metabolism, and how your body responds. Unlike some medications, Mounjaro requires a gradual dose titration schedule, which means your prescriber will increase your dose every 4 weeks until reaching your target maintenance level. This article explains the realistic timeline, what to expect at each stage, and how to track progress through your weight loss journey using the NHS and UK prescribing context.
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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 for Mounjaro to Start Working
Mounjaro begins acting in your body from the first injection, but noticeable appetite suppression and weight loss effects typically emerge within 2 to 4 weeks. The initial 2.5 mg starting dose is relatively mild — many people experience minimal side effects and subtle appetite changes. By weeks 5–8, when most people reach the 5 mg dose, effects become more pronounced. Full appetite suppression and consistent weight loss usually occur by weeks 9–12 when doses reach 7.5 mg or higher. Individual timelines vary significantly based on your starting BMI, metabolic rate, and diet adherence. Some people notice effects within 7–10 days; others take 6–8 weeks. Your GP or prescriber will monitor progress at each dose escalation point — this is why the dose titration schedule matters. Patience is essential: Mounjaro is a gradual tool, not an immediate solution. Most UK specialists expect to see meaningful weight loss by week 12–16 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 Week by Week
Weeks 1–4 (2.5 mg): Mild nausea or stomach changes common; appetite may decrease slightly. Weeks 5–8 (5 mg): Appetite suppression becomes noticeable; some weight loss begins. Weeks 9–12 (7.5 mg): More significant hunger reduction; consistent weekly weight loss typically evident. Weeks 13–16 (10 mg+): Full therapeutic effect usually reached; weight loss stabilises at a predictable rate. Remember that your individual response depends on calorie intake, physical activity, and adherence. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, and track your progress weekly to see real changes develop over these crucial early weeks.
- 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.
Does Mounjaro work immediately after the first injection?
No. The first 2.5 mg dose is a starting dose designed to help your body adjust. Most people don't notice significant appetite suppression until the second or third injection (weeks 5–8). Full effects typically take 8–12 weeks to become apparent. This gradual approach reduces side effects and allows your prescriber to find your optimal dose.
How quickly will I see weight loss on Mounjaro?
Early weight loss (0.5–1 kg per week) often appears by weeks 3–4 as appetite decreases. However, meaningful, consistent weight loss typically becomes clear by weeks 8–12 when your dose reaches therapeutic levels. NHS guidance acknowledges that sustainable loss is gradual — rapid early loss is often water weight, not fat loss.
Why does Mounjaro require dose titration if I could go straight to 15 mg?
Dose titration allows your body to adapt and minimises side effects like nausea and stomach discomfort. Starting at 2.5 mg and escalating every 4 weeks helps your prescriber identify the lowest effective dose for you. This approach also improves long-term adherence because side effects are more manageable.
What if I don't notice any difference after 4 weeks on Mounjaro?
Early doses (2.5 mg and 5 mg) are intentionally mild. If you notice no change by week 8, discuss this with your prescriber — some people respond better at higher doses. Ensure you're also maintaining a reduced-calorie diet and increasing physical activity, as Mounjaro works alongside lifestyle changes, not instead of them.
Can I switch from Wegovy to Mounjaro to see effects faster?
Switching medications won't speed up the timeline. Mounjaro also requires dose titration starting at 2.5 mg. The main difference between Mounjaro (tirzepatide) and Wegovy (semaglutide) is the mechanism — Mounjaro is a dual GIP and GLP-1 agonist, but both medications take similar time to reach full effect.
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.