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
Phentermine for Weight Loss — What UK Adults Need to Know
Phentermine is not licensed or available for weight loss in the United Kingdom. If you're looking for prescription weight loss support in the UK, there are approved alternatives including GLP-1 medications and access through NHS weight management services. This guide explains what's actually available for weight management in the UK, how to access prescription weight loss medication, and what the evidence shows about different approaches to sustainable weight loss.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
What Weight Loss Medications Are Available in the UK?
In the UK, prescription weight loss medications licensed for weight management include:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, given as a once-weekly injection (2.5–15 mg)
- Wegovy (semaglutide) — a GLP-1 receptor agonist, given as a once-weekly injection (0.25–2.4 mg)
- Saxenda (liraglutide) — a GLP-1 receptor agonist, given as a once-daily injection (0.6–3 mg)
Phentermine, which is used in the United States, is not licensed or approved in the UK for any indication. Phentermine is a sympathomimetic amine approved in the US for short-term appetite suppression but does not have a UK marketing authorisation. Instead, UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions, may be eligible for evidence-based GLP-1 or GIP/GLP-1 medications. Use our free GLP-1 weight loss calculator to explore potential outcomes with approved UK medications, or try our BMI calculator to check your eligibility.
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.
How to Access Prescription Weight Loss in the UK
Prescription weight loss medication in the UK is available through two main routes:
- NHS specialist services — referral from your GP to a tier 3 weight management service; availability varies significantly by region and waiting lists can be lengthy
- Private online prescribing — consultation with a UK-licensed prescriber through a GPhC-registered pharmacy; usually available within days
Both routes require a clinical assessment to confirm eligibility. Your prescriber will review your medical history, BMI, and weight-related health conditions. Weight loss medication works best alongside reduced-calorie diet and increased physical activity. Track your progress with our free progress tracker, and use our weight converter to monitor changes across metric and imperial measurements.
- 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.
Is phentermine available in the UK?
No. Phentermine is not licensed or approved for use in the United Kingdom. The MHRA has not granted marketing authorisation for phentermine for any indication. US-approved medications are not automatically available in the UK — each medication requires separate regulatory approval.
What's better than phentermine for UK weight loss?
Evidence-based alternatives licensed in the UK include Mounjaro, Wegovy, and Saxenda. Clinical trials show these GLP-1 and GIP/GLP-1 medications help people lose 14–21% of body weight on average. Individual results vary, and all require prescription and clinical assessment.
Can I get phentermine privately in the UK?
No. Even privately, prescription medications must be licensed in the UK. A UK-licensed prescriber cannot legally prescribe an unlicensed medication for weight loss. Approved alternatives are available through private online prescribing services.
How do I access weight loss medication on the NHS?
Ask your GP for a referral to your local tier 3 specialist weight management service. Eligibility typically requires BMI 30+ or BMI 27+ with weight-related health conditions. Waiting times vary by region. Private routes are often faster if you don't want to wait for NHS services.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials show Mounjaro may produce slightly greater average weight loss (~15–21% vs ~15%), but individual results vary. Both require prescription and are available through NHS or private routes in the UK.
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.