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 — When to Worry and What to Do
Losing weight without trying to may seem like a positive outcome, but unexplained weight loss can be a sign that something isn't quite right. While intentional weight management through diet and exercise is common and healthy, unintended weight loss warrants attention. This guide explains what counts as unexplained weight loss, when you should contact your GP, and what medical investigations might be needed to identify the underlying cause.
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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.
When Weight Loss Becomes a Health Concern
Unexplained weight loss typically means losing 5% or more of your body weight unintentionally over a period of weeks to months without changes in diet or activity levels. For example, if you normally weigh 70kg, losing more than 3.5kg without trying could warrant investigation.
Common medical causes include:
- Thyroid disorders (hyperthyroidism speeds metabolism)
- Type 2 diabetes or undiagnosed diabetes
- Depression, anxiety, or stress-related conditions
- Digestive problems (coeliac disease, inflammatory bowel disease, chronic diarrhoea)
- Infections such as tuberculosis or long-term infections
- Certain cancers
- Medication side effects
- Hormonal imbalances
Your GP will help identify which investigations are needed. Use our free BMI calculator to track your current BMI status — this information is useful during your consultation.
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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.
What to Do if You've Lost Weight Unexpectedly
Contact your GP promptly if you've noticed unexplained weight loss, especially if it's accompanied by other symptoms like fatigue, fever, persistent cough, changes in appetite, or digestive issues. Your GP will take your medical history, ask about diet and lifestyle changes, and may order blood tests, thyroid function tests, or imaging depending on your symptoms.
Keep a record of when you first noticed the weight loss and any accompanying symptoms — this helps your GP narrow down possible causes. Don't wait to see if it resolves; unexplained weight loss is one of those health signals worth investigating promptly to rule out underlying conditions and ensure you receive appropriate weight management support if needed.
- 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 2kg in a month unexplained weight loss?
Losing approximately 2kg in one month (about 500g per week) could be normal, especially if you've made dietary changes. However, if you haven't changed your diet or activity and this continues over several months, contact your GP. Losing more than 5% of your body weight unintentionally is more concerning.
Can stress cause unexplained weight loss?
Yes. Stress, anxiety, and depression can reduce appetite, cause digestive issues, or affect metabolism. Many people lose weight during stressful periods. If you're stressed and losing weight, speak to your GP — they can help address both the stress and weight changes.
Should I use weight loss medication if I'm losing weight unintentionally?
No. Prescription weight loss medications like GLP-1 injections (Mounjaro, Wegovy) are only for people who are trying to lose weight for health reasons and meet specific BMI criteria. They're not appropriate for unexplained weight loss. Your GP will investigate the cause first and advise on safe treatment.
When should I definitely see my GP about weight loss?
See your GP immediately if you've lost 5% or more of your body weight unintentionally, especially if accompanied by fever, fatigue, persistent cough, blood in stool, or severe digestive changes. These could indicate serious underlying conditions requiring prompt investigation.
What tests might my GP order for unexplained weight loss?
Your GP may order blood tests (including thyroid function, glucose, and full blood count), imaging scans, or referral to a specialist depending on your symptoms and medical history. They'll explain which tests are needed based on your individual situation.
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.