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 Quickly Does Mounjaro Kick In?
Mounjaro (tirzepatide) doesn't produce immediate weight loss results — it works gradually as your body adjusts to the medication. Most people experience initial appetite reduction within the first few days to weeks, though significant weight loss typically takes several weeks to become noticeable. The medication works through a gradual dose titration schedule, starting at 2.5 mg and increasing every four weeks, which allows your body to adapt and helps minimise side effects. Understanding this timeline helps set realistic expectations when beginning treatment through NHS specialist services or private prescription in the UK.
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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.
When Does Mounjaro Start Working?
Appetite reduction often begins within 3–7 days of your first injection, with many people reporting feeling fuller faster and having fewer cravings. However, this doesn't always translate to immediate weight loss on the scales.
Measurable weight loss typically becomes apparent after 4–8 weeks, particularly once you've progressed through the dose titration schedule. The SURMOUNT clinical trials showed that most participants began losing weight consistently by weeks 4–6 of treatment.
Peak effectiveness is usually achieved by week 20 onwards, once you've reached your optimal maintenance dose (often 10 mg or 15 mg weekly). Weight loss continues gradually over 52 weeks and beyond, with clinical data showing continued loss up to 72 weeks in trials.
The gradual onset is intentional — the four-week intervals between dose increases (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg) allow your body to adapt, which reduces nausea and gastrointestinal side effects.
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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 Dose Titration
Your GP or NHS specialist weight management service will outline your personalised dose schedule. Most people remain on each dose for a minimum of four weeks before escalating. During this time, you may experience side effects like nausea, constipation, or reduced appetite — these typically improve as your body adjusts.
Some people need to stay on a lower dose longer if side effects are significant. Others progress smoothly through the KwikPen injection schedule. Try our GLP-1 weight loss calculator to estimate projected results at different doses and timeframes, which can help you understand realistic expectations for your own journey. Weight loss is not assured, and individual results vary significantly based on lifestyle, diet, and individual metabolism.
- 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, weight loss is gradual. Most people experience appetite reduction within days, but measurable weight loss on the scales typically takes 4–8 weeks to become noticeable. The slow onset is intentional — it helps your body adjust and reduces side effects.
How much weight can I expect to lose with Mounjaro?
Clinical trials (SURMOUNT) showed average weight loss of 15–21% of body weight over 72 weeks, depending on dose. However, individual results vary significantly. Use our free weight loss calculator to see projected outcomes based on your starting weight and dose.
When should I see significant results with Mounjaro in the UK?
Most people see noticeable results by 8–12 weeks, once they've reached a therapeutic dose (typically 7.5 mg or above). Peak results usually occur by weeks 20–24, with continued gradual loss through 52 weeks and beyond.
Does Mounjaro work faster than Wegovy?
Mounjaro (tirzepatide, dual GIP+GLP-1 agonist) showed slightly higher average weight loss than Wegovy (semaglutide, GLP-1 agonist) in clinical trials. However, both work gradually, and individual response varies. Your GP or prescriber can advise which is appropriate for you.
What if I don't see weight loss after 8 weeks?
If you've reached a therapeutic dose and haven't seen changes, discuss this with your prescriber. Weight loss depends on diet, activity, and individual metabolism. Your dose may be escalated, or your prescriber may review whether this medication is suitable for your circumstances. ``` --- ## CONTENT NOTES: **Keyword integration:** - Primary keyword "how quickly does mounjaro kick in" appears in H1 and intro - Related terms naturally woven: "mounjaro kwikpen" (dose/injection context), "dose titration" (section 2 focus), "mounjaro vs wegovy" (FAQ Q4), "tirzepatide" (parenthetical in intro), "side effects" (section 2), "mounjaro nhs" (throughout context) **Tool references:** - GLP-1 weight loss calculator referenced in section 3 (relevant for dose projection expectations) - Weight loss calculator again referenced in FAQ Q2 (outcome estimation) - Natural weave — not forced **NHS/UK angle compliance:** - Section 2 focuses on timeline/onset (primary keyword answer) - Section 3 addresses dose titration as UK clinical pathway - FAQ covers realistic expectations for NHS/private routes - NICE/clinical trial data cited (SURMOUNT trials) - UK prescribing context maintained throughout - No pharmacy names mentioned **Conditional language:** - "Often", "typically", "usually", "may experience", "can help", "individual results vary" - No assured outcomes - Realistic, evidence-based framing **Word counts:** - Intro: 95 words - Section 2: 178 words - Section 3: 128 words - Total: ~401 words + FAQ
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.