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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Weight Loss Pills and Phentermine in the UK
Phentermine is a prescription weight loss medication, but it is not licensed or available in the UK. Instead, UK adults have access to other prescription weight management options through NHS services and private prescribing. This guide explains what weight loss pills are, why phentermine isn't available here, and what alternatives are approved for weight management in the UK. If you're considering prescription weight loss support, your GP can discuss the options suitable for your individual health needs.
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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.
Why Phentermine Isn't Available in the UK
Phentermine is an appetite suppressant approved in the United States but has not been licensed for use in the UK by the MHRA (Medicines and Healthcare products Regulatory Agency). It was available in the UK decades ago but was withdrawn due to safety concerns related to cardiovascular side effects and the potential for dependence when used long-term.
- Phentermine acts as a central nervous system stimulant, increasing heart rate and blood pressure
- Long-term safety data was limited compared to newer alternatives
- UK prescribers now have safer, evidence-based weight management options available
- Any phentermine obtained outside official NHS or UK-licensed prescribing channels would be unregulated and unsafe
Instead of phentermine, UK adults seeking prescription weight loss medication can access GLP-1 receptor agonists like Wegovy (semaglutide) and tirzepatide (Mounjaro), which have strong clinical evidence and MHRA approval for weight management.
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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 Prescription Weight Loss Options Are Available in the UK?
The UK has three main prescription weight management medications licensed for adults seeking medical support for weight loss. These work through different mechanisms than phentermine and have been approved based on modern clinical evidence. Wegovy (semaglutide) and Mounjaro (tirzepatide) are once-weekly injections that activate GLP-1 and GIP/GLP-1 pathways, helping regulate appetite and food cravings. Access is available through NHS specialist weight management services or private online prescribing following a clinical assessment. You can estimate potential weight loss outcomes using our GLP-1 weight loss calculator, and calculate your current BMI with our free BMI calculator to understand your weight management context.
- 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 in the UK and cannot be legally prescribed by UK healthcare providers. It was withdrawn from the UK market due to safety concerns about cardiovascular effects and long-term dependency risk. UK prescribers recommend safer, modern alternatives like Wegovy and Mounjaro instead.
What weight loss medications are available in the UK?
Three prescription weight management medications are licensed in the UK: Wegovy (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide). All are GLP-1 or GLP-1/GIP receptor agonists available through NHS specialist services or private prescribing following clinical assessment.
How do UK weight loss medications work differently from phentermine?
Phentermine is a stimulant that suppresses appetite. UK-approved medications like Wegovy and Mounjaro are GLP-1 agonists that work by mimicking natural hunger-regulating hormones, slowing gastric emptying, and improving glucose control — with a better long-term safety profile.
Can I access weight loss medication through the NHS?
Yes, through tier 3 specialist weight management services. You need a GP referral and an assessment confirming eligibility (BMI 30+ or 27+ with weight-related health conditions). Availability and waiting times vary by region. Private prescription options are also available without NHS referral.
What should I do if I want to lose weight in the UK?
Start by speaking with your GP about your weight management goals. They can assess whether you're eligible for prescription medication, refer you to specialist services, or recommend behaviour change programmes. Calculate your BMI using our free tool to understand your current weight category and discuss options with your healthcare provider.
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.