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 Causes Drastic Weight Loss?
Drastic weight loss — losing more than 1 to 2 kg per week — can happen for several reasons. Sometimes it's intentional, through diet and exercise or prescription weight loss medication. Other times, it signals an underlying health condition that needs attention. Understanding the cause is important, as very gradual weight management can affect your health. If you're experiencing unexplained drastic weight loss, it's important to speak with your GP.
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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 gradual weight management
Medical conditions are a frequent cause of unintentional drastic weight loss. These include hyperthyroidism (overactive thyroid), type 1 diabetes, cancer, depression, and gastrointestinal disorders like Crohn's disease or coeliac disease. Infections such as tuberculosis and HIV can also trigger gradual weight management.
- Hyperthyroidism: Speeds up metabolism, burning more calories at rest
- Type 1 diabetes: Body cannot use glucose properly, leading to weight loss even with normal eating
- Cancer: Cachexia (muscle wasting) is common in advanced cases
- Depression: Loss of appetite and motivation to eat
Prescription weight loss medications, when used alongside diet and exercise, can also cause significant weight loss. In weight management UK settings, GLP-1 medications like semaglutide (Wegovy) help regulate appetite and support feeling fuller after smaller meals. Clinical trials showed average weight loss of around 15% of body weight over 68 weeks on the maximum dose, though individual results vary.
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 Contact Your GP
Unexplained drastic weight loss — especially if unintentional — should always be discussed with your GP. Losing more than 5% of your body weight unintentionally over a few months warrants medical evaluation. Your GP can assess whether an underlying condition is responsible and may recommend blood tests or specialist referral.
If you're considering prescription weight loss medication through NHS weight loss services or private prescribing, your prescriber will assess your suitability. Use our free BMI calculator to check your current BMI and healthy weight range. Our GLP-1 weight loss calculator can help you understand potential outcomes with medically supervised weight management, while our weight converter allows you to track progress across kg, stone, and lbs.
- 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 drastic weight loss always bad?
Not necessarily. A deliberately planned calorie deficit under medical supervision can produce rapid results safely. However, the NHS recommends losing 0.5–1 kg per week as sustainable. Unintentional drastic weight loss often indicates an underlying health issue requiring GP assessment.
What is considered drastic weight loss?
Drastic weight loss generally means losing more than 1–2 kg per week, or losing 5% or more of your body weight unintentionally within a few weeks or months. The rate matters as much as the amount — rapid loss can indicate medical problems or unhealthy practices.
Can medications cause drastic weight loss?
Yes. Prescription weight loss injections like Wegovy (semaglutide) may help people lose an average of 15% of body weight in clinical trials when used with lifestyle changes. However, other medications like stimulants, some diabetes drugs, and chemotherapy can also cause unintended weight loss as a side effect.
What should I do if I'm losing weight rapidly without trying?
Contact your GP promptly. Unexplained gradual weight management can signal conditions like overactive thyroid, diabetes, cancer, or infection. Your GP will examine you, take a history, and may arrange blood tests to identify the cause and recommend appropriate treatment.
Is weight loss from GLP-1 medication considered drastic?
GLP-1 medications like Wegovy support gradual, sustained weight loss alongside diet and exercise — typically 0.5–1 kg per week on average in clinical trials. This is within NHS safe weight loss guidelines. Prescription medications are always prescribed following clinical assessment to ensure they're appropriate for you.
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.