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?
Unexplained weight loss—losing weight without intentional dieting or exercise—can be concerning and often signals an underlying health condition. In the UK, your GP should assess any significant unintended weight loss to rule out medical causes. While some reasons are temporary, others require investigation. Understanding the potential causes and knowing when to seek medical advice is essential 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 Reasons for Unintended Weight Loss
Several health conditions can cause weight loss without trying. Thyroid disorders, particularly hyperthyroidism, accelerate metabolism and cause gradual weight management. Type 2 diabetes and prediabetes can trigger weight loss as your body struggles to regulate blood sugar. Cancer, chronic infections, and gastrointestinal conditions like coeliac disease or inflammatory bowel disease (IBD) often lead to unintended weight loss. Depression, anxiety, and stress can suppress appetite significantly. Hormonal changes during menopause or from certain medications—including some antidepressants or blood pressure treatments—may also contribute. Less commonly, autoimmune diseases, tuberculosis, or hyperpituitarism cause weight loss. Additionally, some people taking prescription weight management medication for intentional weight loss may experience faster results than expected, especially during dose escalation. Any unexplained weight loss exceeding 5% of your body weight over 6–12 months warrants GP review.
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.
Weight Loss Medication and Prescription Options in the UK
If you're overweight or obese and eligible, UK-licensed weight loss injections like Mounjaro (tirzepatide) and Wegovy (semaglutide) are available through NHS specialist services or private prescription. These GLP-1 medications may help reduce appetite and support sustainable weight management uk alongside lifestyle changes. Eligibility depends on BMI and health conditions. Saxenda (liraglutide) is another prescription option. Use our GLP-1 weight loss calculator to estimate potential results. However, if your weight loss is unintended and unrelated to medication, contact your GP immediately. Our BMI calculator can help you track whether changes are within expected ranges for your height.
- 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.
Should I worry if I'm losing weight without trying?
Yes, unintended weight loss of more than 5% over 6–12 months should be assessed by your GP. While sometimes temporary, it can indicate thyroid problems, diabetes, infections, or other conditions needing investigation. Early diagnosis is important.
What's the difference between unintended weight loss and weight loss from medication?
Unintended weight loss happens without conscious effort or treatment. Weight loss from prescription medication like Mounjaro or Wegovy is planned and monitored by a prescriber. Always inform your GP if you're taking weight management medication to distinguish intentional from unintended loss.
Can thyroid problems cause weight loss without trying?
Yes, hyperthyroidism (overactive thyroid) speeds up metabolism significantly, causing gradual weight management even with normal eating. Your GP can test thyroid function with a simple blood test if this is suspected.
Is unintended weight loss ever normal?
Minor fluctuations are normal, but persistent unexplained weight loss is not. Stress, illness, or medication changes can cause temporary loss. If it continues beyond a few weeks, contact your GP for assessment.
How can I check if my weight is healthy for my height?
Use our free BMI calculator to see your personal BMI and healthy weight range according to NHS guidelines. Your GP can also discuss whether your weight is appropriate for your individual health profile and whether prescription weight management options might help if needed.
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.