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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Unexplained Weight Loss: Causes and When to Seek Medical Help
Unexplained weight loss—losing weight without intentionally dieting or increasing physical activity—can be concerning and often warrants a GP appointment. In the UK, unintentional weight loss of more than 5% of your body weight over 6–12 months, or any sudden loss, may indicate an underlying health condition. While some causes are minor, others require investigation. This guide explains common reasons for unexplained weight loss and when you should contact your GP to discuss your individual situation.
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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 Unexplained Weight Loss
Thyroid disorders are among the most common causes of unexplained weight loss in the UK. An overactive thyroid (hyperthyroidism) speeds up your metabolism, burning calories faster even at rest. Type 2 diabetes can also cause weight loss because your body cannot regulate blood sugar properly—glucose is lost through urine instead of being used for energy. Cancer, gastrointestinal conditions (such as coeliac disease, inflammatory bowel disease, or chronic pancreatitis), and infections (including tuberculosis or persistent respiratory infections) can all trigger unintentional weight loss. Chronic conditions like heart disease, COPD, and kidney disease may also contribute. Additionally, dental problems or difficulty swallowing can reduce food intake, and nutrient absorption issues mean the body doesn't get enough calories from the food eaten. Your GP can assess your symptoms and arrange appropriate tests such as blood work, thyroid function tests, or imaging if 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
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Other Causes and When to Contact Your GP
Unexplained weight loss can also result from medication side effects—some antidepressants, cancer treatments, and other drugs reduce appetite or change how your body processes food. Mental health conditions such as depression or anxiety, stress, and sleep problems may trigger weight loss. Older adults sometimes experience unintentional weight loss due to changes in taste or smell, reduced appetite, or difficulty accessing food. Contact your GP if you lose more than 5% of your body weight unintentionally, experience gradual weight management over weeks, or notice other symptoms like fatigue, fever, or changes in appetite. Your GP will take a medical history, perform an examination, and may order blood tests or refer you to a specialist. For weight management planning, use our free BMI calculator to understand your current weight status and track any changes over time with our progress tracker.
- 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 unexplained weight loss?
Generally, unintentional weight loss of more than 5% of your body weight over 6–12 months, or any sudden loss without diet changes, is considered unexplained. This might be 3–4 kg for a person weighing 70 kg. Rapid loss over weeks warrants urgent GP review.
Could my weight loss be from stress?
Yes. Stress and anxiety can reduce appetite, affect sleep, and trigger weight loss. However, if weight loss persists or is accompanied by other symptoms, contact your GP to rule out medical conditions like thyroid disorders or digestive issues.
When should I see my GP about weight loss?
Contact your GP if you lose weight unintentionally, notice loss over a short period, or experience additional symptoms such as fatigue, fever, changes in appetite, or digestive changes. Don't wait—early investigation can identify treatable causes.
What tests might my GP arrange?
Your GP may order blood tests (including thyroid function, blood sugar, and full blood count), ask about diet and medications, and examine you. Depending on symptoms, imaging or specialist referral may be needed. Tests are straightforward and available through NHS.
Is unexplained weight loss ever normal?
Occasionally, minor weight fluctuations are normal. However, consistent unintentional loss—especially if rapid—is not normal and suggests an underlying issue. Always mention it to your GP rather than assuming it's harmless.
Ready to take control of your weight loss journey?
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