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
Free tracker included with every download this week
What Causes Unexplained Weight Loss?
Unexplained weight loss — losing weight without trying or making intentional changes to diet and exercise — can be a sign that something needs medical attention. While gradual, intentional weight loss through lifestyle changes or prescription weight loss medication is common, losing weight unintentionally warrants investigation by your GP. Understanding potential causes helps you recognise when to seek advice and what to expect from a medical assessment.
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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.
Common Medical Causes of Unexplained Weight Loss
Several serious health conditions can cause unexplained weight loss, and your GP will investigate thoroughly:
- Type 2 diabetes and prediabetes — high blood sugar can trigger weight loss despite increased appetite
- Thyroid disorders (hyperthyroidism) — an overactive thyroid dramatically increases metabolism, leading to gradual weight management
- Cancer — weight loss is sometimes an early symptom of various cancers and requires urgent investigation
- Chronic infections — conditions like tuberculosis, HIV, and persistent bacterial infections cause metabolic stress and weight loss
- Gastrointestinal disorders — coeliac disease, Crohn's disease, and inflammatory bowel disease reduce nutrient absorption
- Depression and mental health conditions — anxiety, depression, and stress can suppress appetite and lead to unintentional weight loss
- Hormonal imbalances — conditions affecting cortisol, oestrogen, or other hormones may trigger weight changes
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 and What Tests May Help
Contact your GP if you have lost more than 5% of your body weight in the past 6 months without intentional diet or exercise changes. Use our free BMI calculator and weight converter to document your baseline, then track changes over time with our progress tracker — this information helps your GP assess the pattern. Your GP will ask about other symptoms (fatigue, appetite changes, mood, digestion), your medical history, and may arrange blood tests, thyroid function tests, or imaging. Early investigation of unexplained weight loss often leads to faster diagnosis and treatment of underlying conditions.
- 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 unexplained weight loss ever normal?
Some weight fluctuation is normal, but losing more than 5% of body weight without dietary changes or increased exercise is unusual and worth investigating. Rapid or progressive weight loss particularly warrants GP assessment.
Could unexplained weight loss be a side effect of medication?
Yes. Some medications (certain antidepressants, stimulants, diabetes drugs, and cancer treatments) can cause unintended weight loss. Always discuss medication side effects with your GP or pharmacist.
How much weight loss is considered unexplained?
Generally, losing more than 2–3 kg (5 lbs) unintentionally, or 5% of your body weight within 6 months, warrants medical review. Sudden losses should be assessed promptly.
Can thyroid problems cause unexplained weight loss?
Yes. Hyperthyroidism (overactive thyroid) significantly increases metabolic rate, causing gradual weight management even with normal or increased food intake. A simple blood test can check thyroid function.
Should I be worried about unexplained weight loss?
Unexplained weight loss should always be evaluated by your GP to rule out serious conditions. Early assessment is reassuring and may lead to prompt diagnosis if an underlying cause exists.
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.