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
Why Am I Losing Weight Unintentionally?
Unintentional weight loss — losing weight without trying or dieting — can be concerning and may indicate an underlying health condition. Unlike deliberate weight loss through diet and exercise, unexplained weight loss happens without lifestyle changes and warrants attention from your GP. Several medical conditions, medications, stress, and other factors can trigger this, and understanding the cause is important for your health.
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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 Unintentional Weight Loss
Several health conditions can cause unexplained weight loss. Thyroid disorders — particularly an overactive thyroid (hyperthyroidism) — speed up metabolism and cause gradual weight management despite normal or increased appetite. Type 2 diabetes and undiagnosed diabetes prevent the body from properly using glucose, leading to weight loss alongside increased thirst and fatigue.
Gastrointestinal conditions like coeliac disease, inflammatory bowel disease (Crohn's disease or ulcerative colitis), and chronic diarrhoea reduce nutrient absorption. Cancer, particularly in advanced stages, causes significant weight loss due to altered metabolism and reduced appetite. Depression and mental health conditions often reduce appetite and motivation to eat. Chronic infections including tuberculosis and HIV also trigger weight loss.
Other causes include malabsorption syndromes, hypercalcaemia (excess calcium), COPD, heart failure, and liver disease. If you've lost more than 5% of your body weight unintentionally over several weeks, contact your GP for investigation.
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.
Medications and Lifestyle Factors
Certain medications cause weight loss as a side effect, including some antidepressants, stimulants (like methylphenidate), and chemotherapy drugs. Stress, major life changes, and anxiety can reduce appetite and trigger weight loss. Excessive exercise combined with inadequate calorie intake also leads to unintentional loss.
If you've recently started a new medication and noticed weight changes, discuss this with your GP — they may adjust your dose or suggest alternatives. Use our free BMI calculator to check your current weight status and track changes over time. Keeping a record of your weight, appetite changes, and any other symptoms helps your GP diagnose the cause more effectively.
- 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.
When should I see my GP about unintentional weight loss?
Contact your GP if you've lost more than 5% of your body weight unintentionally, or if loss occurs alongside other symptoms like fatigue, fever, night sweats, or changes in appetite. Unexplained weight loss lasting more than a few weeks always warrants medical evaluation to rule out underlying conditions.
Could my weight loss be linked to stress or anxiety?
Yes, stress and anxiety commonly reduce appetite and cause weight loss. However, stress alone shouldn't cause significant, gradual weight management. If weight loss persists despite normal eating habits, see your GP to investigate other potential causes such as thyroid problems or malabsorption.
What if I'm taking medication — could that be causing weight loss?
Some medications including antidepressants, stimulants, and thyroid medications can cause weight loss as a side effect. Always discuss medication side effects with your GP. Never stop taking prescribed medication without medical advice — your GP can adjust doses or suggest alternatives if needed.
Is unintentional weight loss always serious?
Not always, but it should never be ignored. Causes range from minor (stress, medication side effects) to serious (thyroid disease, diabetes, cancer). Only your GP can properly investigate and diagnose the cause. Early detection of conditions like diabetes or thyroid disease improves treatment outcomes significantly.
How can I track my weight loss to show my GP?
Weigh yourself weekly at the same time of day (ideally morning before eating) and record results. Our free progress tracker helps log your weight over time and identify patterns. Note any accompanying symptoms — fatigue, appetite changes, night sweats — as these details help your GP pinpoint the cause more accurately.
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.