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
Will Cancer Cause Weight Loss?
Yes, cancer can cause unintentional weight loss, and this is one reason why unexplained weight loss is always worth discussing with your GP. Unlike intentional weight loss from diet or exercise, cancer-related weight loss happens without obvious cause and often involves loss of muscle as well as fat. This guide explains why cancer causes weight loss, what to watch for, and when to seek medical advice. If you've experienced unexplained weight changes, use our free BMI calculator and weight converter to track your current health metrics, then speak to your GP who can assess your individual situation.
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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.
How Cancer Causes Weight Loss
Cancer causes weight loss through several mechanisms. Tumours produce substances that change how your body uses energy, increase inflammation, and suppress appetite. Cancer also increases your metabolic rate—meaning your body burns more calories at rest. Additionally, cancer treatments like chemotherapy, radiotherapy, and surgery can cause nausea, difficulty swallowing, or loss of taste, making eating painful or unpleasant. The combination of increased energy demand and reduced food intake leads to rapid, unintentional weight loss.
Cancer cachexia is a specific condition where the body loses muscle and fat tissue despite adequate food intake. This differs from ordinary weight loss because it resists simple nutritional intervention. If you've noticed unexplained weight loss alongside other symptoms like fatigue, persistent cough, or abdominal pain, contact your GP immediately. Early investigation is crucial—whilst many causes of weight loss are not cancer, unexplained weight loss always warrants professional evaluation by the NHS.
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.
Warning Signs and When to See Your GP
Seek medical advice if you've lost more than 5% of your body weight unintentionally over 6–12 months, or if weight loss happens rapidly without lifestyle changes. Other concerning symptoms include persistent fatigue, night sweats, unexplained pain, changes in bowel habits, persistent cough, or blood in urine or stools. You don't need to wait for multiple symptoms—a single unexplained weight loss warrants a GP appointment. Your GP can assess whether investigation is needed and refer you to appropriate NHS services if required. Track your weight regularly using our free progress tracker to identify patterns and bring accurate records to your appointment.
- 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 all unintentional weight loss a sign of cancer?
No. Unintentional weight loss has many causes including thyroid disease, diabetes, depression, medication side effects, infection, and dental problems. However, unexplained weight loss always requires GP evaluation. Your doctor can perform tests to identify the cause.
How quickly does cancer cause weight loss?
Cancer-related weight loss varies widely. Some people lose weight gradually over months; others experience rapid loss. The rate depends on cancer type, stage, and your metabolism. Any unexplained weight loss warrants medical investigation.
Can weight loss injections or GLP-1 weight loss medication cause cancer?
No. GLP-1 medications like Wegovy (semaglutide) used for weight management do not cause cancer. These are prescribed under medical supervision following clinical assessment. If concerned about any medication, discuss with your GP or prescriber.
What should I tell my GP about weight loss?
Note the amount lost, timeframe, whether you changed diet or exercise, and any other symptoms. Track your weight using a progress tracker beforehand. Bring a list of all medications and supplements. This information helps your GP assess the cause.
What happens after my GP refers me for weight loss investigation?
Your GP may request blood tests, imaging scans, or specialist referral. The NHS pathway depends on initial findings. Early investigation means potential causes—cancerous or otherwise—can be identified and a UK pharmacy sooner. Follow your GP's advice about referral timing.
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.