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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How Long Does It Take To See Weight Loss With UK Medication?
If you're considering prescription weight management in the UK, timing matters. Most people using GLP-1 medications like Mounjaro or Wegovy notice initial weight loss between 2 to 4 weeks, though individual results vary significantly. The rate depends on your starting weight, the medication type, dose level, and how closely you follow recommended lifestyle changes. This guide explains realistic timelines for UK-licensed weight loss options and helps you understand what to expect.
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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.
UK-Licensed Weight Loss Medications and Expected Timelines
Three prescription weight loss medications are currently licensed in the UK: Mounjaro (tirzepatide), a dual GIP and GLP-1 receptor agonist; Wegovy (semaglutide), a GLP-1 receptor agonist; and Saxenda (liraglutide), a daily GLP-1 injection. Clinical trials show that most people using these medications see measurable weight loss within 4 weeks, with more significant results typically visible after 8 to 12 weeks. At week 16, average weight loss ranges from 5–8% of body weight depending on the medication and dose. By week 52, clinical trial data shows average losses of 15–21% body weight with Mounjaro and around 15% with Wegovy—though results vary considerably between individuals.
- Weeks 1–4: Initial nausea common, minimal weight change (adjustment phase)
- Weeks 5–12: Appetite reduction increases, noticeable weight loss begins
- Weeks 13–26: Steady weekly losses of 0.5–1 kg possible
- Weeks 26+: Weight loss plateaus if dose is stable; further loss may require dose escalation
These timelines assume you're also following a reduced-calorie diet and staying physically active—medication alone is not sufficient for sustained weight 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.
NHS vs Private Access and Starting Your Weight Loss Journey
In the UK, you can access prescription weight loss medication through two routes. NHS access is through specialist tier 3 weight management services, typically requiring a GP referral and assessment by a multidisciplinary team. NHS waiting lists vary significantly by region and can be lengthy. Private access is available through online consultations with UK-licensed prescribers, without NHS referral. Your GP can advise on NHS eligibility based on your BMI and health conditions. Eligibility generally requires a BMI of 30 or above, or 27 or above with weight-related conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Use our free BMI calculator to check your current BMI, and discuss weight loss options with your GP—they can help determine the best route for you and whether prescription medication is appropriate.
- 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.
When will I start losing weight on Mounjaro or Wegovy?
Most people notice weight loss within 2–4 weeks, though this varies. Initial weeks may involve nausea and minimal weight change. Noticeable losses typically begin weeks 5–12. Remember that results depend on dose level, starting weight, diet, and activity—medication works best alongside lifestyle changes.
How much weight can I expect to lose in the first month?
In the first month, expect minimal to modest weight loss (0–2 kg). Weeks 1–4 are primarily an adjustment phase where your body adapts to the medication. Significant weekly losses typically begin after week 4 as appetite suppression increases and you progress through dose escalation.
What if I don't see weight loss after 8 weeks?
If weight loss hasn't appeared by 8 weeks, consult your prescriber. You may need a dose increase, a review of your diet and activity level, or assessment for whether the medication is right for you. Some individuals respond more slowly; others may need a different approach.
Can I speed up weight loss with these medications?
Weight loss rate depends on your calorie deficit, not medication dose alone. Eating fewer calories and increasing physical activity will support faster losses, but the NHS recommends losing no more than 0.5–1 kg per week for safe, sustainable results. Faster losses increase muscle loss risk and aren't recommended without medical supervision.
What happens to weight loss after I reach my goal weight?
Once you reach your goal weight, your prescriber may suggest dose reduction or stopping the medication. Some people maintain weight loss after stopping; others regain weight. Long-term weight management typically requires ongoing lifestyle changes—diet, activity, and possibly ongoing medication—to prevent regain.
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.