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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Phentermine Weight Loss Pills — What UK Adults Need to Know
Phentermine is a prescription weight loss medication that has been used for decades, but it is no longer commonly available for weight management in the UK. If you're exploring weight loss medication options, modern alternatives like GLP-1 injections (such as Wegovy or Mounjaro) may be more relevant to UK prescription weight loss. This guide explains phentermine's history, why it's rarely prescribed in the UK now, and what current weight management options are available to UK adults with a BMI indicating they might benefit from prescription support.
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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.
Why Phentermine Isn't Standard in Modern UK Weight Management
Phentermine is an appetite suppressant that works by stimulating the central nervous system and reducing hunger signals. It was widely used from the 1960s onwards but has largely fallen out of favour in the UK and Europe due to safety concerns and the development of newer, more effective medications.
- Phentermine carries risks of increased heart rate and blood pressure — concerns that limit its use in modern practice
- It is approved by the MHRA in the US but has extremely limited availability in the UK
- UK medical guidance increasingly favours GLP-1 receptor agonists like Wegovy (semaglutide) and dual GIP/GLP-1 agonists like Mounjaro (tirzepatide) — medications with stronger clinical trial data for weight loss and lower cardiovascular risk profiles
- Modern weight loss injections address appetite in a more physiological way, mimicking natural gut hormones rather than stimulating the nervous system
If you've been searching for phentermine in the UK, your GP can discuss more current prescription weight loss options suitable for UK adults.
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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.
Current UK Weight Loss Medication Options and How to Access Them
For UK adults meeting eligibility criteria (BMI 30 or above, or 27+ with weight-related health conditions), prescription weight loss medication is available through two routes.
- NHS specialist services: Referral through your GP to a tier 3 weight management service — availability varies significantly by region and waiting times can be lengthy
- Private prescription: Online consultation with a UK-licensed prescriber, followed by dispensing through a GPhC-registered pharmacy — no NHS referral needed
Try our free BMI calculator above to check your personal BMI and explore your eligibility. Our GLP-1 weight loss calculator can help you understand potential outcomes with modern prescription options like Wegovy or Mounjaro, which align with current UK medical evidence and NHS guidance.
- 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 on the NHS in the UK?
No, phentermine is not routinely prescribed on the NHS for weight management. It is rarely used in UK clinical practice due to cardiovascular safety concerns and the availability of newer alternatives like GLP-1 medications. Modern weight loss injections are now the standard prescription option in the UK.
Why was phentermine used for weight loss if it's not safe?
Phentermine was used for decades because it does suppress appetite. However, it increases heart rate and blood pressure, which poses risks particularly for people with cardiovascular conditions. Modern medications like Wegovy and Mounjaro have been proven safer and more effective in clinical trials with fewer cardiovascular side effects.
What should I do if I want prescription weight loss medication in the UK?
Speak to your GP about specialist weight management services. They can refer you to a tier 3 NHS service (though waiting lists vary), or discuss private prescription options. Your GP can assess your BMI and any weight-related health conditions to determine if you're eligible for prescription treatment.
How do modern weight loss injections compare to older medications like phentermine?
GLP-1 medications like Wegovy work differently — they mimic a natural gut hormone that regulates appetite and blood sugar, rather than stimulating the nervous system. Clinical trials show they produce greater average weight loss (around 15% of body weight) with a better safety profile than phentermine-era medications.
What's the difference between Wegovy and Mounjaro for UK weight loss?
Wegovy (semaglutide) is a GLP-1 agonist; Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist. Both are prescription-only and available through NHS specialist services or private prescribing. Mounjaro may produce slightly greater average weight loss in trials, but both require clinical assessment and are used alongside dietary and lifestyle changes.
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.