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 Without Trying?
Unintended weight loss can be a sign that something needs attention. While some weight loss happens naturally as we age or change our routines, losing weight without making deliberate dietary or lifestyle changes may indicate an underlying health condition, medication side effect, or metabolic change. This guide explains the common reasons for unexplained weight loss and when to speak with your GP. If you've noticed a sudden change in your weight management, it's important to understand what might be causing it.
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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 Causes of Unexplained Weight Loss
Several conditions and factors can cause unintended weight loss. Thyroid problems like hyperthyroidism speed up your metabolism, causing gradual weight management even when eating normally. Type 2 diabetes and high blood sugar can trigger weight loss because your body struggles to use glucose effectively. Stress, anxiety, and depression often suppress appetite and disrupt eating patterns. Cancer, chronic infections (including tuberculosis), and gastrointestinal conditions like Crohn's disease or coeliac disease can all cause progressive weight loss.
Medication side effects are another common cause — some antidepressants, blood pressure medications, and chemotherapy drugs reduce appetite. Hypercalcaemia (high calcium levels), hormonal changes, and conditions affecting nutrient absorption can also lead to unintended weight loss. Age-related muscle loss (sarcopenia) sometimes shows on the scales even when fat loss is minimal.
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 See Your GP — Weight Management & Medical Assessment
Contact your GP if you've lost more than 5% of your body weight unintentionally over 6–12 months, or if weight loss happens rapidly without obvious cause. Seek urgent medical attention if weight loss is accompanied by blood in stools or vomit, persistent abdominal pain, difficulty swallowing, or significant changes in mood or energy.
Your GP will take a full medical history, examine you, and may order blood tests to rule out thyroid problems, diabetes, infections, and nutritional deficiencies. Once any underlying condition is identified and a UK pharmacy, weight loss often stabilises. Use your free BMI calculator to track your current weight range, and keep a record of any weight changes to discuss with your healthcare team — this helps your GP identify patterns and investigate properly.
- 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 weight loss of 1–2kg per month without dieting concerning?
Not necessarily. If you feel well and weight loss is gradual, it may reflect natural changes in your metabolism or activity. However, keep track of your weight using our free progress tracker and mention it to your GP at your next appointment if the trend continues or you feel unwell.
Can medications cause weight loss?
Yes. Some antidepressants, stimulant medications, chemotherapy drugs, and certain blood pressure medicines reduce appetite or increase metabolism. If you've recently started a new medication and noticed weight loss, inform your GP — they may adjust your dose or offer an alternative.
What's the difference between intentional and unintentional weight loss?
Intentional weight loss follows a calorie deficit through diet and exercise. Unintentional weight loss happens without deliberate changes. Our GLP-1 weight loss calculator is designed for planned weight management with medication support; unexplained weight loss requires medical investigation.
When does weight loss suggest a serious condition?
Rapid, significant weight loss (more than 5% body weight in under 6 months) alongside fatigue, pain, or persistent symptoms warrants urgent GP assessment. Conditions like cancer, infections, and thyroid disease often present with unintended weight loss as an early sign.
Should I use weight loss medication if I'm losing weight unintentionally?
No. Prescription weight loss medications like Mounjaro or Wegovy are only appropriate for adults with a BMI of 30 or above (or 27+ with weight-related health conditions) who are actively trying to lose weight. Unintentional weight loss requires diagnosis and treatment of the underlying cause first.
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.