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 in the UK?
Weight loss medications work by affecting hormones and appetite signals in your body — not by speeding up your metabolism. In the UK, prescription weight management drugs like Mounjaro, Wegovy, and Saxenda are licensed for eligible adults with obesity or overweight with health conditions. These medications help reduce hunger, increase feelings of fullness, and support sustainable weight loss when combined with lifestyle changes. Understanding how they work can help you decide if prescription weight loss treatment is right for you.
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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.
How UK-Licensed Weight Loss Drugs Work
The main weight management UK medications approved by the MHRA are GLP-1 receptor agonists and dual GIP/GLP-1 agonists. These medications mimic natural hormones your gut produces after eating.
GLP-1 medications (Wegovy, Saxenda) activate the GLP-1 hormone pathway, which signals your brain that you are full, reduces appetite, and slows how quickly food leaves your stomach. Mounjaro (tirzepatide) works on two hormone pathways — GLP-1 and GIP — providing dual action that may support greater appetite suppression and improved fat utilisation.
Clinical trials showed that participants using these medications alongside reduced-calorie diets and increased physical activity lost an average of 15–21% of body weight over 68–72 weeks — though individual results vary significantly. These are prescription-only medicines requiring clinical assessment before use. Unlike crash diets, GLP-1 weight loss works gradually and supports sustainable behaviour change.
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 Weight Loss Treatment in the UK
NHS weight loss medication is available through specialist tier 3 weight management services following a GP referral. However, NHS access varies significantly by region, and waiting times can be several months to years in many areas. Private prescription weight loss UK options are available through GPhC-registered online prescribing services following a clinical assessment — offering faster access without NHS referral.
To find out if you are eligible, use our free BMI calculator to check your current BMI. You are likely eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Your GP can discuss both NHS and private options with you. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
- 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.
Do weight loss drugs actually work?
Yes — clinical trials show participants lose an average of 15–21% of body weight when using GLP-1 or dual agonist medications alongside lifestyle changes. Individual results vary. They work best combined with a reduced-calorie diet and increased physical activity, not as a standalone treatment.
How quickly do weight loss drugs work?
Most people experience reduced appetite within days to weeks of starting. Meaningful weight loss typically becomes visible after 4–8 weeks. Maximum benefit is usually seen over 16–20 weeks as doses are gradually increased. Weight loss continues as long as you use the medication alongside healthy habits.
Will I gain weight back after stopping weight loss medication?
Weight regain is possible after stopping — this is why lifestyle changes (diet and exercise) are essential alongside medication. Maintaining new habits and continuing healthy behaviours significantly reduces weight rebound. Discuss long-term management with your prescriber.
What are the side effects of weight loss drugs in the UK?
Common side effects include nausea, diarrhoea, constipation, and reduced appetite — usually mild and temporary. Serious side effects are rare but can include pancreatitis or gallbladder problems. Always report severe stomach pain to your GP. Most people tolerate these medications well with gradual dose increases.
Can I get weight loss drugs on the NHS?
Yes — GLP-1 medications are available through NHS specialist weight management services for eligible adults. Access requires a GP referral to tier 3 services and varies by region. Private options are also available. Speak to your GP about referral or eligibility.
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.