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
What Is The Reason Of Sudden Weight Loss?
Sudden weight loss can happen for many reasons — some expected, others concerning. While intentional weight loss through diet and exercise is common, unintended weight loss can signal medical issues, medication effects, or stress. If you've noticed unexpected weight change, it's important to understand the cause. For those seeking to manage their weight uk intentionally, prescription medications are now available through the NHS and private services for eligible adults. Your GP can help identify what's behind your weight change and discuss whether treatment options are right for you.
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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 Sudden Weight Loss
Medical conditions are a frequent cause of unintended weight loss. Thyroid disorders, type 2 diabetes, cancer, depression, and digestive problems like coeliac disease or inflammatory bowel disease can all result in weight dropping unexpectedly. Medications used for various health conditions — including some blood pressure tablets, antidepressants, and chemotherapy — commonly cause appetite changes or increased metabolism.
- Stress, anxiety, or major life changes can suppress appetite
- Infections, including viral illnesses, may cause temporary weight loss
- Hormonal changes during menopause or thyroid dysfunction
- Excessive exercise or very restrictive dieting without medical guidance
If your weight loss is unintentional, persistent, or accompanied by other symptoms like fatigue, pain, or fever, your GP should assess you. They can run blood tests and investigations to identify the underlying cause. This is different from intentional weight loss uk managed through lifestyle changes or prescription weight loss medication nhs services.
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.
Prescription Weight Loss Options In The UK
For adults with a BMI weight loss threshold of 30 or above — or 27 or above with weight-related health conditions — several prescription weight loss medication options are now available. Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injectable medications that have shown significant results in clinical trials. Saxenda (liraglutide) is a daily injection option also licensed in the UK.
These glp-1 weight loss medications work by regulating appetite and helping you feel fuller after meals. Access is available through NHS specialist weight management services — though waiting lists vary by region — or through private online prescribing with a UK-licensed prescriber. Try our free GLP-1 weight loss calculator to explore potential outcomes if you think you might be eligible. Your GP can also assess whether you meet eligibility criteria and discuss which option suits your circumstances.
Before considering prescription weight loss treatment, your GP needs to rule out medical causes of any weight changes. Calculate your BMI using our free BMI calculator to understand your current health category according to NHS guidelines. If your weight change is intentional and you're looking to manage weight uk through treatment, a healthcare professional will assess your individual health, medications, and eligibility. Never start any medication without proper clinical assessment. Use our free weight converter if you need to check your weight in different units — understanding your exact weight in stone, lbs, or kg helps your GP assess your health accurately.
- 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 sudden weight loss always a sign of illness?
Not always. Sudden weight loss can be caused by stress, medication side effects, increased exercise, or dietary changes — not just medical conditions. However, unintentional weight loss that continues without explanation warrants a GP appointment for blood tests and assessment. If your weight loss is intentional and part of a planned weight loss uk programme, it's expected and monitored.
Can prescription weight loss medication cause sudden weight loss?
GLP-1 medications like Mounjaro and Wegovy can result in significant weight loss over several months, particularly when combined with dietary changes and activity. Results typically appear gradually rather than suddenly — clinical trials show average losses of 15–21% body weight over 68 weeks. Individual outcomes vary considerably.
How do I know if I'm eligible for prescription weight loss treatment in the UK?
Eligibility generally requires a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Your GP or a private prescriber will assess your individual health during a consultation. Use our free BMI calculator to check your starting point, then speak with your healthcare provider.
What's the difference between unintentional and intentional weight loss?
Unintentional weight loss happens without planned dietary or exercise changes and may indicate an underlying health issue requiring investigation. Intentional weight loss uk is planned through lifestyle modifications or prescription treatment under medical supervision. If unsure which you're experiencing, your GP can help clarify and investigate.
Should I see my GP before trying a weight loss medication?
Yes, absolutely. All weight loss medications in the UK are prescription-only and require clinical assessment. Your GP needs to check for medical causes of any weight changes, review your medications, and confirm you're suitable for treatment. They can discuss NHS options or refer you to specialist services based on your circumstances.
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.