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
How Do Weight Loss Drugs Work?
Weight loss drugs approved in the UK work by targeting hormones that control hunger, appetite, and how your body processes food. The most commonly prescribed medications are GLP-1 receptor agonists — injectable medicines that mimic a natural hormone your gut produces after eating. These prescription weight loss medications are designed to be used alongside a reduced-calorie diet and increased physical activity. Understanding how they work can help you discuss options with your GP or prescriber to see if weight loss medication might support your weight management goals.
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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.
How GLP-1 Weight Loss Medications Work
GLP-1 receptor agonists are the primary weight loss drugs available in the UK for weight management. GLP-1 (glucagon-like peptide-1) is a natural hormone your body produces in the gut after you eat. These medications mimic and amplify GLP-1's effects in several ways:
- Reduce appetite: They signal to your brain's appetite control centre that you feel fuller, reducing hunger signals
- Slow stomach emptying: Food moves more slowly through your digestive system, helping you feel satisfied longer after eating
- Regulate blood sugar: They help stabilise blood glucose levels, reducing cravings and energy crashes
- Support food choices: Many users report reduced cravings for high-calorie foods
The main GLP-1 weight loss medications licensed in the UK are Wegovy (semaglutide) and Saxenda (liraglutide). Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it targets two hormone pathways simultaneously for potentially stronger appetite suppression. All are prescription-only medicines requiring a clinical assessment before use.
Clinically BackedWeight Loss
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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.
Weight Loss Results and Individual Variation
Clinical trials show weight loss medication can support significant results when combined with lifestyle changes. In the STEP-1 trial, patients using Wegovy lost an average of approximately 15% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity. However, individual results vary considerably — some people lose more, some less. Weight loss typically occurs gradually over weeks and months, not immediately.
These medications are not a structured approach. They work best as part of a comprehensive weight management approach including dietary changes, increased activity, and behaviour support. Your GP or a licensed prescriber will assess whether prescription weight loss medication is suitable for you based on your health status, BMI, and any weight-related health conditions. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current weight category.
- 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.
Are weight loss drugs safe?
Prescription weight loss medications approved in the UK are licensed and monitored by the MHRA. Like all medicines, they can cause side effects — nausea and digestive changes are most common. Serious side effects are rare but possible. Always discuss risks and benefits with your GP or prescriber before starting any medication.
How quickly do weight loss drugs work?
Most people begin noticing appetite reduction within 1–2 weeks of starting, though weight loss typically becomes visible after 4–6 weeks. Maximum effect is usually seen after 12–16 weeks on a stable dose. Results are gradual and individual — there is no assured timeline.
Do I need to diet if I take weight loss medication?
Yes. Weight loss medication works best alongside a reduced-calorie diet and increased physical activity. These medicines support appetite control, but lifestyle changes are essential for sustained results. Your GP or a weight management service can provide dietary guidance.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) targets two hormone pathways (GIP and GLP-1), whilst Wegovy (semaglutide) targets one (GLP-1 only). Clinical trials suggest Mounjaro may produce slightly higher average weight loss, but both are effective. Individual response varies — a prescriber will advise which is suitable for you.
Can I get weight loss medication on the NHS?
NHS provision varies significantly by region. GLP-1 medications are available through NHS specialist weight management services (tier 3), but access depends on your ICB and waiting lists can be very long. Private online prescribing is an alternative route following a clinical assessment by a UK-licensed prescriber.
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.