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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8 Week Weight Loss Timeline with Mounjaro in the UK
When starting Mounjaro (tirzepatide), most UK adults begin on a 2.5 mg dose during weeks 1–4, before dose titration progresses. This guide explains what realistic weight loss looks like in the first 8 weeks, how NHS prescription weight loss injection services work, and what factors affect individual results. Individual outcomes vary significantly — clinical trials showed average weight loss occurs gradually over several months alongside dietary changes and activity increases.
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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.
Mounjaro Dose Titration and 8-Week Timeline
Weeks 1–4: Starting dose (2.5 mg) — This is your adjustment phase. Most people experience minimal weight loss in week 1, though some notice reduced appetite. Nausea is common in the first 2–3 weeks and typically subsides. No dose increase occurs yet; your body is adapting to the medication. Weight loss during this phase is usually modest — 0.5–1.5 kg on average.
Weeks 5–8: First escalation to 5 mg — After 4 weeks on 2.5 mg, your dose increases to 5 mg once weekly. This is when more noticeable appetite suppression typically begins. Clinical trial data suggests participants on 5 mg lose approximately 15% of body weight over 72 weeks when combined with reduced-calorie diet and increased physical activity. In the 8-week window, expect gradual loss rather than rapid change — typically 1–3 kg cumulative over this period.
Important note: These are averages — individual results vary based on diet, activity, metabolism, and adherence. Use our free GLP-1 weight loss calculator to see potential projections at different doses.
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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.
NHS Access vs Private Route for Mounjaro in the UK
NHS route: Mounjaro (tirzepatide) is approved by NICE for weight management through NHS specialist tier 3 weight management services. You'll need a GP referral, and access varies significantly by region — waiting lists can be several months to years. When prescribed, treatment is managed by specialists who monitor progress and manage side effects.
Private route: Available through UK-licensed prescribers via online consultation without NHS referral. Treatment begins immediately after clinical assessment and prescription. Cost varies by dose — typically £150–£250 per month depending on your dose stage. Private prescribers also manage dose titration and side effects. Many patients using the weekly weight loss injection privately track progress with our free progress tracker to monitor results over 8 weeks and beyond.
- 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.
Can I see significant weight loss in just 8 weeks on Mounjaro?
Weight loss in 8 weeks is usually modest — typically 1–3 kg cumulatively. Mounjaro works gradually, especially during dose titration (weeks 1–8 on 2.5 mg and 5 mg). Significant results (10%+ body weight loss) typically emerge over 3–6 months as doses escalate. Individual variation is considerable based on diet, activity, and metabolism. Use our BMI calculator to track your baseline and monitor progress over time.
What is dose titration and why does it happen?
Dose titration means gradually increasing your Mounjaro dose every 4 weeks: 2.5 mg (weeks 1–4), then 5 mg (weeks 5–8), then 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg. This allows your body to adapt, reduces side effects, and lets your prescriber assess your response at each level before increasing further.
What side effects might I experience in the first 8 weeks?
Nausea is most common in weeks 1–3 and usually improves. Diarrhoea, constipation, and stomach discomfort are also common early on. Injection site reactions (redness, bruising) are mild and temporary. Severe side effects (severe abdominal pain, allergic reactions) require immediate medical attention. Your prescriber will advise on managing symptoms.
How does NHS prescription of Mounjaro differ from private prescription?
NHS prescriptions come through tier 3 specialist weight management services (requiring GP referral and waiting) and are fully funded. Private prescriptions follow online clinical assessment, cost £150–£250 monthly, and begin immediately. Both routes involve the same medication and dose titration — the difference is access timing and cost.
Should I diet and exercise while taking Mounjaro in these first 8 weeks?
Yes — clinical trials showed Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Medication alone does not produce significant results. Aim for a moderate calorie deficit (around 500 kcal/day) and regular activity. Your prescriber or dietitian can provide specific guidance tailored to your health.
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.