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 intentionally dieting or changing your exercise habits — can be a sign that something needs medical attention. Losing more than a few kilograms unintentionally over weeks or months is not normal and warrants a conversation with your GP. Understanding potential causes helps you know when to seek advice and what to expect during an assessment.
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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
Several medical conditions can cause unexplained weight loss, and your GP will help identify the underlying cause. Type 2 diabetes is one of the most common — when blood sugar is not controlled properly, your body cannot use glucose effectively, leading to weight loss alongside increased thirst and fatigue. Hyperthyroidism (overactive thyroid) increases your metabolism significantly, causing gradual weight management even when eating normally.
Other important causes include:
- Cancer: Unexplained weight loss is sometimes an early sign and warrants investigation
- Depression and anxiety: Mental health conditions often affect appetite and eating patterns
- Digestive problems: Coeliac disease, Crohn's disease, or IBS can reduce nutrient absorption
- Medications: Some drugs suppress appetite or affect metabolism
- Infections: Chronic infections like tuberculosis or HIV can cause weight loss
- Heart disease: Heart failure can lead to unintentional weight loss
Your GP will take a detailed history, perform blood tests, and possibly imaging to identify the cause.
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 Contact Your GP About Weight Loss
You should arrange a GP appointment if you have lost more than 5% of your body weight unintentionally over a period of weeks or months. Contact your GP immediately if weight loss is accompanied by persistent fever, night sweats, blood in stools or vomit, severe fatigue, difficulty swallowing, or changes in bowel habits. Use our free BMI calculator to understand your current weight status and track changes over time — this information helps your GP assess whether weight loss is clinically significant for you.
Documenting your weight loss — how much weight, over what timeframe, and any accompanying symptoms — makes your GP appointment more productive and helps identify patterns.
Do not assume unexplained weight loss will resolve on its own. Early investigation is important because many treatable conditions cause unintentional weight loss. Your GP may refer you to a specialist such as an endocrinologist (for thyroid or diabetes issues), gastroenterologist (for digestive problems), or oncologist if cancer screening is indicated. After diagnosis, treatment often reverses weight loss — for example, treating hyperthyroidism with medication typically restores normal weight within weeks or months.
- 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 losing 1 stone unintentionally in 3 months something to worry about?
Yes — losing approximately 6.4 kg (1 stone) unintentionally over three months is significant and warrants a GP appointment. This represents roughly 5-clinically studied weight loss depending on your starting weight. Your GP will investigate potential causes and arrange appropriate tests. Do not delay — early diagnosis of the underlying cause improves outcomes.
Can stress alone cause unexplained weight loss?
Stress and anxiety can suppress appetite and contribute to weight loss, but significant unintentional weight loss usually has a medical cause. Stress may be one factor, but your GP should still investigate to rule out conditions like thyroid disease, diabetes, or digestive disorders. Mental health support and medical investigation often go together.
What does my GP do when investigating unexplained weight loss?
Your GP will ask detailed questions about your symptoms, medications, diet, and medical history. They will perform blood tests to check thyroid function, blood sugar, and other markers. Depending on findings, you may need imaging (ultrasound or CT scan) or referral to a specialist. The goal is to identify the cause so treatment can begin.
Is unexplained weight loss a sign of cancer?
Unexplained weight loss can be associated with some cancers, but it is far more commonly caused by other conditions like thyroid disease or diabetes. Your GP will assess your full clinical picture and risk factors. If cancer is a possibility, appropriate screening tests will be arranged. Early investigation is important regardless of the cause.
What healthy weight range should I aim for if I have lost weight unintentionally?
Rather than aiming for a specific weight, focus on getting a diagnosis and treatment for the underlying cause. Once the medical issue is addressed, your weight typically stabilises naturally. Use our BMI calculator and weight converter to understand where your current weight falls within NHS healthy weight ranges for your height, and discuss target weight with your GP or a dietitian.
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.