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 Reducing Medication — What You Need to Know
Weight reducing medication refers to prescription-only medicines that can help support weight loss alongside diet and exercise changes. In the UK, the most commonly prescribed weight reducing medications are GLP-1 receptor agonist injections such as Wegovy (semaglutide) and Mounjaro (tirzepatide). These medications work by regulating appetite and helping you feel fuller after smaller meals. They're not suitable for everyone and require a clinical assessment before prescription. This guide explains how they work, who may be eligible, and what the NHS and private options are.
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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.
How Weight Reducing Medications Work
Weight reducing medications approved in the UK work by mimicking or enhancing natural hormones that regulate appetite and blood sugar. GLP-1 receptor agonists like Wegovy activate the GLP-1 hormone pathway, which signals to your brain that you're full, reduces food cravings, and slows how quickly food leaves your stomach. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, meaning it activates two hormone pathways simultaneously for potentially stronger appetite control.
Clinical trials have shown that these medications can support meaningful weight loss when used alongside a reduced-calorie diet and increased physical activity. In the STEP-1 trial, Wegovy helped participants lose an average of approximately 15% of their body weight over 68 weeks. Individual results vary significantly, and weight loss is not assured. These medications are not insulin and do not work in the same way as diabetes treatments, though they do help regulate blood sugar control.
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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.
Who Can Access Weight Reducing Medication in the UK
In the UK, weight reducing medication may be prescribed to adults with a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may have access to treatment at lower BMI thresholds due to higher health risks at those levels.
You can check your personal BMI using our free BMI calculator above to see where you fall. Access is available through NHS specialist weight management services (tier 3) or through private online prescribing with a UK-licensed prescriber. NHS access typically requires a GP referral and waiting lists vary significantly by region. Private prescribing is often faster but involves out-of-pocket costs. A licensed prescriber will assess whether weight reducing medication is appropriate for your individual circumstances.
- 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's the difference between Wegovy and Mounjaro?
Both are prescription weight reducing injections taken once weekly. Wegovy (semaglutide) activates one hormone pathway (GLP-1), while Mounjaro (tirzepatide) activates two (GIP and GLP-1). Clinical trials suggest Mounjaro may support slightly higher average weight loss, but individual results vary. Both require prescription and clinical assessment.
Can I get weight reducing medication on the NHS?
Yes, through NHS specialist weight management services (tier 3). You'll need a GP referral and assessment. Availability varies significantly by region and waiting lists can be long. Private prescribing is an alternative route that doesn't require NHS referral but involves private costs.
What are the common side effects of weight reducing medication?
The most common side effects are nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. These often improve over time. Injection site reactions, headache, and fatigue are also common. Serious side effects are rare but can include pancreatitis. Always discuss side effects with your prescriber.
How much weight will I lose with weight reducing medication?
Clinical trials show average weight loss of 15–21% of body weight depending on the medication and dose. However, individual results vary significantly based on genetics, adherence, diet, and exercise. Weight loss is not assured. These medications work best alongside a reduced-calorie diet and increased physical activity.
How long do I need to take weight reducing medication?
Weight reducing medication is typically used long-term because weight often returns if treatment stops. Your prescriber will discuss the duration with you and monitor your progress. Some people may use it short-term to achieve initial weight loss, then continue with lifestyle changes. Treatment plans are individualised.
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.