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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What Causes Sudden Weight Loss in the UK?
Sudden weight loss can happen for many reasons — some related to medication, others to underlying health changes. If you've noticed rapid weight changes, it's important to understand the cause. In the UK, prescription weight management medication is now available for eligible adults through both NHS and private routes. This guide explains the common causes of sudden weight loss, including medication effects, and how to discuss weight management options with your GP.
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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.
UK Prescription Weight Loss Medication Options
For adults with a BMI of 30 or above, or 27 or above with weight-related health conditions, UK-licensed prescription weight management medication may be suitable. Three medications are currently approved for weight loss in the UK:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, once-weekly injection. Clinical trials showed an average weight loss of 15–21% of body weight over 72 weeks when combined with diet and exercise.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, once-weekly injection. Research demonstrated approximately clinically studied weight loss on average.
- Saxenda (liraglutide) — a GLP-1 receptor agonist, once-daily injection. These are prescription-only medicines requiring clinical assessment before use.
Each works by helping regulate appetite and reduce food cravings. Results vary significantly between individuals. Check your BMI using our free BMI calculator to see where you fall on the NHS weight scale.
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.
NHS vs Private Access in the UK
NHS route: Prescription weight loss medication is available through specialist tier 3 weight management services. You'll need a referral from your GP. Availability varies significantly by region, and waiting lists can be lengthy in many areas.
Private route: Available through online prescribers following a clinical assessment. No NHS referral needed. Both routes require a prescription-only assessment to determine eligibility based on your BMI, health history, and any weight-related conditions like type 2 diabetes or high blood pressure.
Talk to your GP about which route suits you. They can discuss whether nhs weight loss medication is available in your area or advise on private options. Track any weight changes using our free progress tracker to share with your healthcare provider.
- 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 causes sudden weight loss besides medication?
Sudden weight loss can result from thyroid disorders, diabetes, infections, stress, reduced appetite, or increased physical activity. Some medications unrelated to weight loss (like certain antidepressants or blood pressure drugs) may also cause it. Always discuss unexplained weight changes with your GP to rule out underlying health conditions.
Can Mounjaro and Wegovy cause gradual weight management?
These GLP-1 medications support gradual weight loss — typically 0.5–1 kg per week on average. They work alongside diet and exercise changes. The NHS recommends a sustainable rate of loss rather than rapid loss. Individual results vary, and weight loss isn't assured. Your prescriber monitors your progress.
How do I know if I'm eligible for prescription weight management in the UK?
You may be eligible if your BMI is 30 or above, or 27 or above with weight-related health conditions (diabetes, high blood pressure, heart disease). Use our free BMI calculator to check your current BMI, then discuss results with your GP. They'll assess your individual health profile and eligibility.
Should I be worried about sudden weight loss?
Unexplained sudden weight loss (more than 5% body weight without trying) warrants a GP visit to identify the cause. Weight loss from prescription medication or diet changes is monitored by your prescriber. However, very rapid loss without medical supervision can affect muscle and bone health, so always discuss weight changes with your healthcare provider.
What's the difference between Mounjaro and Wegovy for weight loss?
Both are once-weekly injections for weight management in the UK. Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy is GLP-1 only. Clinical data suggests Mounjaro may support slightly higher average weight loss (15–21% vs ~15%). However, individual results vary greatly. Your GP or prescriber can advise which suits your situation best.
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.