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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Fastest Weight Loss — UK Prescription Medication Options
When it comes to achieving the fastest weight loss, prescription medications are among the most effective options available in the UK. GLP-1 and GIP/GLP-1 receptor agonist injections have demonstrated significant results in clinical trials, helping eligible adults lose weight alongside lifestyle changes. These medicines work by reducing appetite, increasing fullness, and supporting metabolic health. However, they are prescription-only and require clinical assessment. This guide explains what's available, who's eligible, and how to find out if prescription weight loss medication is right for you.
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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.
UK-Licensed Prescription Weight Loss Medications
Three prescription medications are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, injected once weekly. Clinical trials showed participants lost an average of 15–21% of their body weight over 72 weeks. Maximum dose is 15 mg weekly.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, injected once weekly. STEP trials demonstrated approximately 15% average body weight loss. Maximum dose is 2.4 mg weekly.
- Saxenda (liraglutide) — a GLP-1 receptor agonist, injected daily (not weekly). Offers a daily injection option for those who prefer it.
All three are prescription-only and available through NHS weight management services or private prescribing following clinical assessment. Individual results vary significantly, and these medications work best when combined with reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to estimate potential results at different doses, and calculate your BMI with our free tool to check your starting point.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Eligibility
In the UK, weight management UK services are available through both NHS and private routes. NHS access requires referral to a tier 3 specialist weight management service — typically from your GP — but availability and waiting times vary significantly by region. Private prescribing is faster and doesn't require NHS referral; it's delivered through online consultations with UK-licensed prescribers, with prescriptions dispensed by GPhC-registered pharmacies.
Eligibility criteria are the same regardless of route: you may qualify if your BMI is 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Those of South Asian, Chinese, Black African, or Black Caribbean heritage may have lower thresholds. Start by talking to your GP about your weight loss goals and whether prescription weight loss medication might be suitable for you.
- 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.
What's the fastest prescription weight loss medication available in the UK?
Mounjaro (tirzepatide) showed the highest average weight loss in clinical trials — approximately 20.9% of body weight at the highest dose over 72 weeks. However, "fastest" varies by individual. All prescription weight loss medications require clinical assessment to confirm suitability. Results depend on dose, adherence, diet, and activity level.
Can I get prescription weight loss medication through the NHS?
Yes. Weight loss medication is available through NHS specialist tier 3 weight management services. You'll need a GP referral. Availability and waiting times vary significantly by region (integrated care board). Some areas have shorter waits than others. Ask your GP about local services and referral pathways.
How quickly will I lose weight on Mounjaro or Wegovy?
Clinical trials showed average weight loss of 0.5–1 kg per week, though individual results vary. Initial weeks may show slower loss as doses are increased gradually. Sustainable weight loss requires combining medication with a reduced-calorie diet and increased physical activity. Track your progress using our free progress tracker.
Do I need to be on a diet with prescription weight loss medication?
Yes. These medications are most effective when combined with lifestyle changes. You'll need to follow a reduced-calorie diet and increase physical activity. The medication reduces appetite and supports fullness, making it easier to stick to healthier eating patterns — but diet changes are essential for best results.
What happens if I stop taking prescription weight loss medication?
Weight regain is common if lifestyle changes aren't maintained. These medications work while you're taking them; they don't permanently change appetite or metabolism. Most healthcare professionals recommend continuing the medication long-term if it's working, alongside ongoing diet and exercise habits, to maintain weight loss.
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.