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
Weight Loss Without Trying: What Causes It?
Unintentional weight loss—losing weight without trying or dieting—is not normal and may indicate an underlying health condition. While intentional weight loss through diet and exercise is positive, losing weight without effort can be a sign your body needs medical attention. This guide explains the common causes of unintentional weight loss and when you should contact your GP. If you're losing more than a few pounds per month without changing your diet or increasing physical activity, it's important to get checked.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Unintentional Weight Loss
Several health conditions can cause weight loss without trying, some more serious than others. Thyroid disorders, particularly hyperthyroidism (overactive thyroid), speed up metabolism and burn calories rapidly—even at rest. Type 2 diabetes and uncontrolled blood sugar can prevent your body from absorbing nutrients properly, leading to weight loss despite normal or increased eating. Cancer, including bowel, lung, and stomach cancers, often causes unintentional weight loss as the body uses energy to fight the disease. Digestive conditions like coeliac disease, Crohn's disease, and ulcerative colitis impair nutrient absorption. Chronic infections such as tuberculosis or hepatitis can increase calorie burn. Depression and anxiety may reduce appetite or interfere with eating habits. Medication side effects from blood pressure drugs, chemotherapy, or antidepressants can suppress appetite. Heart failure and chronic lung disease also trigger unintentional weight loss as the body works harder to function.
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 Contact Your GP About Weight Loss
If you've lost more than a few pounds (1-2 kg) unintentionally in a month, or if weight loss continues without explanation, contact your GP promptly. Seek urgent attention if weight loss is accompanied by: persistent fatigue, fever lasting more than a few days, persistent cough, blood in your stool or urine, difficulty swallowing, or severe abdominal pain. Your GP will ask about your symptoms, medications, diet changes, stress levels, and recent illnesses. They may check your BMI using our free BMI calculator to establish a baseline, recommend blood tests to check thyroid function and blood sugar, or refer you to a specialist if needed. Early investigation of unintentional weight loss can catch serious conditions earlier.
- 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 ever normal?
Minor fluctuations of 1-2 pounds are normal, but consistent weight loss without trying—especially more than 5% of your body weight over 6-12 months—is not normal and warrants GP investigation. Stress, poor sleep, or seasonal changes may cause temporary weight loss, but if it continues, see your doctor.
Can medication cause weight loss without trying?
Yes. Many medications suppress appetite or increase metabolism as side effects, including some antidepressants, blood pressure medications, stimulants, and chemotherapy drugs. If you suspect your medication is causing weight loss, discuss this with your GP—never stop medication without advice, but alternatives may be available.
What's the difference between intentional and unintentional weight loss?
Intentional weight loss results from deliberate dietary changes and increased physical activity—this is healthy and sustainable. Unintentional weight loss happens without effort, diet changes, or increased exercise, and may indicate illness or malabsorption requiring medical investigation.
Can hyperthyroidism cause gradual weight management?
Yes, hyperthyroidism (overactive thyroid) causes the body to burn calories much faster than normal, even at rest. This leads to gradual weight management despite normal eating or increased appetite. A thyroid function test (TSH, Free T4) ordered by your GP will confirm diagnosis.
Should I use a weight tracker if I'm losing weight unintentionally?
Yes—tracking weight loss patterns helps your GP understand the rate and timing of loss, which aids diagnosis. Our free progress tracker helps you log weight changes over weeks and months. Bring this data to your GP appointment for a clearer picture of your health.
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.