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 With Slim Fast — UK Prescription Options
Slim Fast is a well-known meal replacement brand, but if you're looking to lose weight in the UK, prescription weight management medication may be more effective. For adults with a BMI of 30 or above, or 27 or above with weight-related health conditions, prescription options like Mounjaro and Wegovy are licensed in the UK and backed by clinical evidence. These GLP-1 medications work alongside lifestyle changes to support meaningful weight loss. This guide explains your medication options, NHS eligibility, and how to find out if you're suitable for prescription weight loss treatment.
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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 Medication
If you're interested in weight management in the UK beyond meal replacements, prescription weight loss medication is available through NHS specialist services or private prescribing. The three main options licensed in the UK are:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist; once-weekly injection with doses ranging from 2.5 mg to 15 mg. Clinical trials showed average weight loss of 15–21% of body weight alongside a reduced-calorie diet and increased physical activity.
- Wegovy (semaglutide) — a GLP-1 receptor agonist; once-weekly injection with a maximum dose of 2.4 mg. STEP trial data showed average weight loss of around 15% of body weight.
- Saxenda (liraglutide) — a GLP-1 receptor agonist; daily injection up to 3 mg. Approved for weight management but less commonly prescribed than Mounjaro and Wegovy in the UK.
All three are prescription-only medicines requiring clinical assessment. Individual results vary significantly, and these medications work best when combined with dietary changes and increased physical activity, not as a replacement for lifestyle modification.
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 — How to Access Weight Loss Medication
NHS access: Available through tier 3 specialist weight management services following GP referral. Eligibility depends on BMI and health conditions, but NHS availability varies significantly by region. Waiting lists can be several months to over a year in some areas.
Private access: Available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber. No NHS referral needed. Costs range from approximately £129–£320 per month depending on dose. To find out if you're eligible for either route, speak to your GP or complete an online clinical assessment with a registered private prescriber. Use our free GLP-1 weight loss calculator above to estimate potential weight loss at different doses, and check your BMI using our free BMI calculator to determine which eligibility category you fall into.
- 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 is the difference between Slim Fast and prescription weight loss medication?
Slim Fast is a commercial meal replacement product designed to reduce calorie intake through shake or bar substitutes. Prescription weight loss medications like Mounjaro and Wegovy work through biological mechanisms — they activate hormone pathways that reduce appetite and support feeling fuller. Prescription medications are only available through clinical assessment, whereas Slim Fast can be purchased over the counter.
Am I eligible for prescription weight loss medication in the UK?
You may be eligible if you have a BMI of 30 or above, or 27 or above with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or cardiovascular disease). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply. Your GP or a licensed prescriber can assess your individual eligibility.
How much weight can I expect to lose with prescription weight loss medication?
Clinical trials show average weight loss of 15–21% of body weight with Mounjaro and around 15% with Wegovy over several months, but individual results vary significantly. Some people lose more, some less. These medications work best alongside a reduced-calorie diet and increased physical activity — they are not a standalone solution.
Is weight loss medication available on the NHS in the UK?
Yes, Mounjaro and Wegovy are available on the NHS through specialist tier 3 weight management services. However, availability varies significantly by region, and waiting lists can be lengthy. Your GP can refer you to your local specialist service. Private access is also available through online prescribing without an NHS referral.
Are there side effects with prescription weight loss medication?
Yes. Common side effects include nausea, diarrhoea, constipation, and stomach pain — particularly when starting or increasing doses. Most side effects improve over time. Serious side effects are rare but can include pancreatitis (severe stomach pain). Always discuss potential side effects with your prescriber and report any concerning symptoms to your GP.
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.