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?
Sudden weight loss — losing weight without trying to diet or exercise — can happen for many reasons. Whilst some causes are benign, unexplained weight loss can sometimes indicate an underlying health condition that needs investigation. If you've noticed a significant drop in your weight over a short period, it's important to understand what might be causing it and when to seek medical advice from your GP. This guide covers the common causes of sudden weight loss and what you should do about it.
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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.
Common Medical Causes of Sudden Weight Loss
Several health conditions can cause sudden weight loss without dietary changes. Thyroid problems, particularly hyperthyroidism (overactive thyroid), cause rapid metabolism and unintended weight loss. Type 2 diabetes and uncontrolled blood sugar can lead to sudden weight loss as the body struggles to use glucose effectively. Depression and anxiety often reduce appetite and affect eating patterns, resulting in weight loss over weeks or months.
Cancer, gastrointestinal conditions (such as inflammatory bowel disease, coeliac disease, or irritable bowel syndrome), and chronic infections like tuberculosis can all trigger sudden weight loss. Dental problems, difficulty swallowing, or mouth ulcers may reduce food intake. Hormonal changes — including menopause in women — can affect weight. Medication side effects, particularly from certain blood pressure tablets, antidepressants, or chemotherapy, frequently cause unintended weight loss.
Your GP can investigate these causes through blood tests, physical examination, and relevant medical history. Use our free BMI calculator to establish your current weight status as a baseline for discussing changes with your healthcare provider.
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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.
When to See Your GP About Weight Loss
You should contact your GP if you experience unexplained weight loss of more than 5% of your body weight over 6–12 months, or sudden loss over a shorter timeframe without intentional dieting or increased exercise. Other warning signs include weight loss combined with persistent fatigue, night sweats, loss of appetite, abdominal pain, or changes in bowel habits.
Your GP will assess your overall health, medical history, and any other symptoms. They may recommend blood tests to check thyroid function, blood sugar, and nutritional status. Early investigation of sudden weight loss can identify treatable conditions quickly. If you're starting any new medication or managing a health condition, inform your GP of any weight changes — they may need to adjust your treatment or investigate further causes.
- 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.
Is weight loss always a cause for concern?
Not always. Weight loss from intentional diet and exercise is healthy. However, sudden weight loss without trying — especially more than 5% of body weight over several months — warrants investigation by your GP. Many causes are treatable once identified.
Can stress or anxiety cause sudden weight loss?
Yes. Mental health conditions like anxiety, depression, and chronic stress commonly cause appetite loss and reduced food intake, leading to unintended weight loss. This is treatable, and your GP can help. Addressing the underlying mental health issue often resolves weight changes.
What weight loss is considered "sudden"?
Generally, losing more than 5% of your body weight over 6–12 months, or noticeable loss over weeks, is considered significant. Your baseline BMI helps establish normal weight range — use our free BMI calculator to track changes and discuss with your GP.
Should I try weight loss medication if I'm losing weight unintentionally?
No. Prescription weight loss medication like Wegovy or Mounjaro is only for people who intentionally want to manage their weight as part of a planned programme. Unexplained weight loss requires medical investigation first, not medication.
How does my GP investigate sudden weight loss?
Your GP will take a detailed medical history, perform a physical examination, and typically order blood tests to check thyroid function, blood sugar levels, nutritional status, and other markers. Further investigations like imaging may be recommended depending on findings.
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.