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 Medication in the UK
Weight loss medications are prescription-only treatments that can support weight management alongside diet and exercise. In the UK, several medications are licensed for weight loss, including GLP-1 receptor agonists like Wegovy and tirzepatide (Mounjaro). These medications work by helping to reduce appetite and support feeling fuller after smaller meals. Access to prescription weight loss treatment is available through both NHS specialist services and private prescribing, depending on your individual circumstances and eligibility. Understanding how these medications work and what to expect can help you make informed decisions about your weight management journey.
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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 Loss Medications Work
Weight loss medications available in the UK include GLP-1 receptor agonists and dual GIP/GLP-1 agonists. These work by mimicking natural hormones your body produces after eating. Wegovy (semaglutide) is a GLP-1 injection licensed for weight management, administered once weekly. Mounjaro (tirzepatide) is a dual GIP/GLP-1 injection, also once weekly, which may activate two hormone pathways simultaneously.
These medications may help by:
- Reducing appetite and food cravings
- Supporting a feeling of fullness after eating less food
- Slowing how quickly food leaves your stomach
- Helping regulate blood sugar levels
Clinical trials for GLP-1 weight loss injections showed average weight loss of 14–21% of body weight over 68 weeks, though individual results vary significantly. Weight loss medications are most effective when combined with a reduced-calorie diet and increased physical activity. They are prescription-only medicines requiring assessment by a qualified prescriber.
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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.
Eligibility and Access in the UK
To be eligible for prescription weight loss medication in the UK, you typically need a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or heart disease). Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds due to increased health risks at these levels.
In the NHS, access is through specialist tier 3 weight management services — your GP can refer you, though waiting lists vary by region. Privately, you can access these medications through an online consultation with a UK-licensed prescriber, followed by prescription and dispensing by a GPhC-registered pharmacy. Use our free BMI calculator above to check your BMI, and our GLP-1 weight loss calculator to explore 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.
What is the difference between Wegovy and Mounjaro?
Wegovy (semaglutide) is a GLP-1 receptor agonist. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, activating two hormone pathways. Both are once-weekly injections licensed for weight management in the UK. Clinical trial data suggests similar average weight loss outcomes (14–21%), though individual results vary. Your prescriber can discuss which may be most appropriate for you.
Can I access weight loss medication through the NHS?
Yes, weight loss medications are available through NHS specialist tier 3 weight management services. You need a GP referral. Access and waiting times vary significantly by region and availability. Many areas have long waiting lists. Private access is also available through online prescribing without an NHS referral, though this involves a private cost.
What are the common side effects of weight loss injections?
The most common side effects include nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. These often improve over time. Less common side effects include headache, fatigue, and injection site reactions. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or gallbladder issues. Always discuss side effects with your prescriber.
How much weight can I expect to lose with weight loss medication?
Clinical trials showed average weight loss of approximately 14–21% of body weight over 68 weeks with GLP-1 medications, alongside reduced-calorie diet and exercise. However, individual results vary significantly — some people lose more, some less. Your personal outcome depends on genetics, adherence, diet, activity level, and other health factors. Track your progress using our free progress tracker.
Do I need a prescription for weight loss medication?
Yes, all weight loss medications available in the UK are prescription-only. You cannot buy them over-the-counter. A qualified prescriber must assess your medical history, current medications, and eligibility before issuing a prescription. This assessment can be done through NHS specialist services or private online consultations, but a prescription is always required.
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.