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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Understanding Sudden Weight Loss
Sudden, unintended weight loss can be a sign that something needs medical attention. Unlike gradual weight loss from diet changes or exercise, gradual weight management without obvious cause may indicate an underlying health condition. This guide explains what sudden weight loss means, when you should contact your GP, and how it differs from planned weight management in the UK. If you've noticed unexpected weight changes, understanding the difference between intentional and unintentional loss is important for your health.
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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.
When Sudden Weight Loss Requires Medical Attention
Unintended weight loss — weight loss you haven't deliberately caused through diet or exercise — can be a symptom of various medical conditions and should always be discussed with your GP. Significant gradual weight management may indicate:
- Thyroid disorders (hyperthyroidism causes increased metabolism)
- Type 2 diabetes or undiagnosed blood sugar problems
- Cancer or other serious illnesses
- Infections or inflammatory conditions
- Digestive disorders affecting nutrient absorption
- Hormonal imbalances
- Mental health conditions affecting appetite
The NHS recommends contacting your GP if you lose more than 5% of your body weight unintentionally over 6–12 months, or if you notice rapid changes alongside other symptoms like fatigue, changes in appetite, or mood changes. Your GP can run blood tests and assess whether further investigation is needed.
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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.
Intentional vs Unintentional Weight Loss — Key Differences
Intentional weight loss — weight you actively lose through reduced calorie intake and increased physical activity — is quite different from sudden, unexplained weight changes. The NHS recommends safe, sustainable weight loss of 0.5 to 1 kg per week. Rapid intentional loss (more than 1.5 kg per week) can lead to muscle loss, nutritional deficiencies, and gallstones.
If you're pursuing deliberate weight management, our GLP-1 weight loss calculator can help estimate realistic outcomes with prescription weight loss medication, and your BMI calculator shows where your current weight sits on NHS health thresholds. Whatever your goals, work with your GP or a healthcare professional to ensure your approach is safe and sustainable.
- 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 counts as sudden weight loss?
Sudden weight loss is typically unintended weight change of more than 5% of body weight within 6–12 months, or noticeable rapid loss over weeks. This differs from gradual, intentional weight loss from diet and exercise, which happens slowly and is planned.
Should I see my GP about sudden weight loss?
Yes. Contact your GP if you've lost weight without trying, especially if accompanied by fatigue, fever, night sweats, changes in appetite, or other symptoms. Your GP can assess whether tests are needed to identify an underlying cause.
Can medications cause sudden weight loss?
Yes. Some medications including certain antidepressants, diabetes medications, and stimulants can affect appetite and metabolism. Always discuss unexpected weight changes with your GP if you've recently started a new medication.
What's the difference between losing weight intentionally and suddenly?
Intentional weight loss is planned through diet and exercise, typically at 0.5–1 kg per week. Sudden, unintentional loss without lifestyle changes may signal a health issue and warrants medical evaluation.
Is gradual weight management ever safe?
Very gradual weight management (more than 1.5 kg per week) without medical supervision can harm health by causing muscle loss and nutritional problems. Even prescription weight management medications aim for gradual, sustainable loss alongside lifestyle changes.
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.