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 on SlimFast and Prescription Medication Options in the UK
SlimFast is a meal replacement product that can support calorie reduction, but for many people managing their weight, prescription weight management medication offers a different approach. If you're considering weight loss in the UK, you may be eligible for clinically-proven prescription options alongside lifestyle changes. This guide explains how SlimFast fits into the broader weight management picture and introduces the licensed prescription alternatives available through the NHS and private prescribing. Understanding your options helps you make informed decisions with your GP.
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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
If you have a BMI of 30 or above, or 27 or above with weight-related health conditions, you may be eligible for prescription weight management medication in the UK. Three GLP-1 and GIP/GLP-1 receptor agonist medications are currently licensed for weight management:
- Mounjaro (tirzepatide) — dual GIP and GLP-1 receptor agonist; clinical trials showed average weight loss of 15–21% of body weight over 72 weeks at the highest dose
- Wegovy (semaglutide) — GLP-1 receptor agonist; average weight loss of approximately 15% in clinical trials over 68 weeks
- Saxenda (liraglutide) — once-daily GLP-1 injection (rather than weekly); average weight loss of 5–10% in clinical trials
All three are prescription-only medicines and work by helping reduce appetite and support feeling fuller after smaller meals. Individual results vary. Use our free GLP-1 weight loss calculator to explore potential weight loss projections at different doses.
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 Access vs Private Prescribing
NHS access: Prescription weight loss medication through the NHS is typically available via specialist tier 3 weight management services after GP referral. Availability and waiting times vary significantly by region. Your GP can advise whether you are eligible and refer you to your local service.
Private access: You can obtain a prescription through a UK-licensed online prescriber after a clinical assessment, without needing an NHS referral. This route is available to eligible adults with a BMI of 30 or above (or 27+ with comorbidities). Check your BMI using our free BMI calculator above to see whether you fall into the eligible range, and always consult your GP before starting any weight management treatment.
- 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 SlimFast suitable for everyone trying to lose weight?
SlimFast is a meal replacement product that can help reduce calories if used consistently alongside a balanced diet. However, it's not personalised to individual health conditions. If you have type 2 diabetes, high blood pressure, or other health conditions, speak to your GP about whether SlimFast is suitable for you, or discuss prescription weight loss options that may be more appropriate.
What's the difference between meal replacement products and prescription weight loss medication?
Meal replacements like SlimFast rely on calorie reduction alone. Prescription GLP-1 medications like Mounjaro and Wegovy work by signalling to your brain to reduce appetite and help you feel fuller after smaller meals — alongside lifestyle changes. Prescription options are clinically-tested and monitored by a healthcare provider, whereas meal replacements are self-managed over-the-counter products.
Can I use SlimFast alongside prescription weight loss medication?
Both use calorie reduction as the foundation. It's important to discuss any combination of approaches with your prescriber or GP. They can advise whether combining SlimFast with prescription medication is safe and effective for your specific situation, as some combinations may not be necessary or ideal.
How do I find out if I'm eligible for prescription weight loss medication on the NHS?
Speak to your GP. Eligibility depends on BMI (30 or above, or 27+ with health conditions like diabetes or high blood pressure) and your individual health profile. Your GP can assess you, discuss your weight management goals, and refer you to specialist services if appropriate. Use our free BMI calculator to check your current BMI before your appointment.
What's the difference between losing weight with SlimFast versus GLP-1 medication?
SlimFast requires disciplined calorie tracking and meal replacement adherence — results depend entirely on your consistency. GLP-1 medications reduce appetite at a physiological level, which many people find easier to sustain. Clinical trials show GLP-1 medications produce greater average weight loss (15–21%) than most meal replacement programmes alone, though individual results vary significantly.
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.