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
Extreme Weight Loss — Safe, Evidence-Based Approaches
Extreme weight loss — losing more than 1 kg per week — carries significant health risks and is not recommended without medical supervision. The NHS recommends sustainable weight loss of 0.5 to 1 kg per week, which protects muscle mass and long-term health. If you're looking for effective weight management, there are safer, evidence-based options available through the NHS or private prescribing, including GLP-1 weight loss injections and structured dietary programmes. This guide explains what extreme weight loss means, why it's risky, and what alternatives work better for lasting results.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Why gradual weight management Can Be Harmful
Losing weight very quickly — more than 1.5 kg per week — puts your body under significant stress and can cause several serious problems. Muscle loss is the main concern: extreme calorie deficits force your body to break down muscle tissue for energy, not just fat. This reduces your metabolic rate, making it harder to maintain weight loss long-term. gradual weight management also increases risk of nutrient deficiencies, gallstones, dehydration, and hormonal imbalances.
Very low calorie diets (under 800 kcal daily) can cause fatigue, hair loss, mood changes, and weakened immunity. The NHS emphasises that extreme approaches often fail because they are unsustainable — people regain weight quickly once normal eating resumes. Medically supervised programmes exist for specific cases, but gradual weight management without professional oversight carries real health risks and should be avoided.
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.
Effective Alternatives — NHS Weight Management and Prescription Options
If you need significant weight management, the NHS offers structured support through specialist weight management services (tier 3 and 4). These include behaviour change programmes, dietary guidance, and — when appropriate — prescription weight loss medication.
GLP-1 weight loss injections like Wegovy (semaglutide) offer a medically safe route to sustainable weight loss. Clinical trials show adults on GLP-1 medication lose an average of 14–15% of body weight alongside reduced-calorie diet and increased physical activity — without the extreme calorie restriction that causes muscle loss. Both NHS and private prescribing pathways are available. Calculate your BMI using our free tool above to understand your starting point, and use our GLP-1 weight loss calculator to explore potential results at different doses. Your GP can refer you to NHS specialist services, or you can access private prescribing through a licensed online assessment.
- 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.
What counts as extreme weight loss?
Extreme weight loss is typically defined as losing more than 1.5 kg (3.3 lbs) per week. The NHS recommends 0.5–1 kg per week as the safe, sustainable rate. Anything faster increases risk of muscle loss, nutrient deficiencies, gallstones, and metabolic damage.
Can you lose weight very quickly safely?
In clinical settings with medical supervision, slightly faster loss may be managed — but true "extreme" rapid loss carries health risks. GLP-1 medications support steady weight loss without dangerous calorie restriction. Always discuss any weight loss approach with your GP first.
What is a very low calorie diet?
A very low calorie diet (VLCD) is under 800 kcal daily. Only appropriate under medical supervision, typically for 12–16 weeks maximum. The NHS may recommend this in specific cases (before surgery, severe obesity with complications), but it's not suitable for unsupervised use.
How much weight can you safely lose per month?
At 0.5–1 kg per week, you'd safely lose 2–4 kg per month. This rate allows your body to lose fat while preserving muscle and maintaining energy and mood. Faster loss often backfires — muscle loss slows metabolism, making long-term maintenance harder.
Are weight loss injections safer than extreme dieting?
Yes. GLP-1 injections like Wegovy work alongside moderate calorie reduction and exercise, avoiding the extreme deficits that cause muscle loss. Clinical trials show effective weight loss with sustainable results. They require prescription and clinical assessment — your GP or a licensed prescriber will assess suitability.
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.