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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Unintentional Weight Loss — When It Matters
Unintentional weight loss — losing weight without actively trying to diet or exercise — can sometimes signal an underlying health condition that needs attention. Unlike intentional weight management through diet, exercise, or prescription weight loss medication, unexplained weight loss is your body's way of telling you something may be wrong. This guide explains the common medical causes and when you should contact 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.
Common Medical Causes of Unintentional Weight Loss
Unintentional weight loss can result from various medical conditions. Thyroid disorders, particularly an overactive thyroid (hyperthyroidism), speed up your metabolism, causing gradual weight management despite normal or increased appetite. Type 2 diabetes and undiagnosed diabetes prevent your body from properly using glucose, leading to weight loss as your body breaks down muscle and fat for energy.
Cancer, particularly in the early stages, often causes unexplained weight loss as the body redirects nutrients. Gastrointestinal conditions like coeliac disease, inflammatory bowel disease (Crohn's and ulcerative colitis), and chronic pancreatitis reduce nutrient absorption. Mental health conditions including depression and anxiety can suppress appetite. Infections such as tuberculosis, hepatitis, and untreated HIV also commonly cause gradual weight loss.
- Overactive thyroid (hyperthyroidism)
- Type 1 and type 2 diabetes
- Cancer (various types)
- Coeliac disease and IBS
- Depression and anxiety disorders
- Chronic infections and autoimmune conditions
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.
When to See Your GP — Red Flag Signs
Contact your GP promptly if you've lost more than 5% of your body weight unintentionally over the past 6–12 months, or if weight loss occurs rapidly over a few weeks. Other warning signs include persistent fatigue, fever, night sweats, loss of appetite, digestive problems, or changes in bowel habits.
Your GP will take a medical history, perform blood tests, and may arrange imaging to identify the cause. The NHS weight management pathway begins with your GP, who can assess whether weight loss is intentional or requires investigation. Use our free BMI calculator to track your current weight against healthy ranges — this context helps your GP understand your health picture. Early diagnosis is crucial for conditions like diabetes and thyroid disorders.
Never dismiss unexplained weight loss as positive, even if you've been considering weight loss intentionally. Rapid, unplanned weight loss differs fundamentally from deliberate weight management through NHS programmes or prescription weight loss medication options. If you're losing weight without trying, your priority is medical assessment, not celebrating the number on the scales.
- 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 unintentional weight loss always serious?
Not always, but it warrants investigation. Minor weight loss from stress or temporary illness is usually harmless, but significant unexplained loss — more than 5% body weight over months — often signals an underlying condition requiring GP assessment. Thyroid problems, diabetes, infections, and cancer are common causes.
How much unintentional weight loss should concern me?
Losing more than 5% of your body weight without dieting or increased exercise over 6–12 months is considered significant. Rapid loss (several kilos over weeks) also requires GP attention. Track your weight regularly using a progress tracker to notice patterns and report them accurately to your GP.
Can medication cause unintentional weight loss?
Yes, several medications can cause weight loss as a side effect, including stimulants, antidepressants, blood pressure medications, and cancer treatments. Always inform your GP of all medications you're taking when reporting unexplained weight loss.
Is unintentional weight loss a sign of type 2 diabetes?
Unintentional weight loss can be an early sign of type 2 diabetes, particularly in younger adults. Diabetes prevents proper glucose use, forcing the body to break down muscle and fat. Your GP can order blood tests to check your blood sugar levels and assess diabetes risk.
How does unintentional weight loss differ from intentional weight management?
Intentional weight loss follows a planned approach — calorie reduction, exercise, or prescription medication under medical supervision. Unintentional weight loss occurs without effort and usually signals illness. If you're considering intentional weight management, our GLP-1 weight loss calculator can help you understand medical weight loss options available through the NHS or private prescribing.
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.