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
Drastic Weight Loss Diet — What You Need to Know
A drastic weight loss diet typically aims for rapid results through severe calorie restriction, but the NHS recommends a more measured approach for sustainable health outcomes. While the appeal of quick weight loss is understandable, very restrictive diets often lead to muscle loss, nutritional deficiencies, and rebound weight gain. This guide explains why the NHS recommends losing 0.5–1 kg per week, what happens to your body during drastic dieting, and the safer alternatives available — including prescription weight loss medication and structured weight management UK programmes designed to support lasting change.
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 Drastic Diets Fail (And What Happens to Your Body)
Drastic weight loss diets — typically defined as losing more than 1.5 kg per week — trigger several physiological problems. Your body enters a state of metabolic stress, breaking down muscle tissue alongside fat to meet energy demands. This reduces your metabolic rate, meaning you burn fewer calories at rest, which is why rebound weight gain is common once normal eating resumes.
Additional risks include:
- Nutrient deficiencies (iron, vitamin B12, calcium, protein)
- Gallstones (risk increases significantly with gradual weight management)
- Fatigue, dizziness, and brain fog
- Hair loss and weakened immune function
- Unsustainable restriction leading to binge eating
- Muscle loss that makes future weight management harder
The NHS and NICE both advise against very low calorie diets (under 800 kcal/day) without medical supervision. For sustainable weight management UK residents should aim for 0.5–1 kg loss per week, achieved through a moderate calorie deficit of approximately 500 kcal per day combined with increased physical activity.
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.
Evidence-Based Alternatives to Drastic Dieting
Instead of drastic restriction, the NHS recommends a structured approach combining modest calorie reduction with lifestyle change. Try our free BMI calculator above to understand your current weight status — this context helps set realistic goals.
Effective approaches include:
- Balanced diet with 500 kcal daily deficit (typically 1,500–2,000 kcal/day for women; 1,800–2,500 kcal/day for men)
- Mediterranean diet — strong evidence for sustained weight loss and cardiovascular health
- Intermittent fasting (16:8 or 5:2) — effective for some, though long-term outcomes match standard calorie restriction
- Prescription weight loss medication — GLP-1 weight loss injections (such as Wegovy) for those meeting NHS criteria
- Structured NHS weight loss programmes available through your GP
Weight loss injections may help if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses. All require prescription and clinical assessment — they work best alongside diet and exercise, not instead of them.
- 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 a 1,200 calorie diet safe?
A 1,200 kcal/day diet is below the NHS recommended minimum (1,500 kcal for women; 1,800 kcal for men) and should only be attempted under medical supervision. It can lead to nutrient deficiencies and muscle loss. Speak to your GP before trying very restrictive diets. A moderate deficit of 500 kcal/day (resulting in ~0.5 kg/week loss) is safer and more sustainable.
Can I lose 5 kg in a week safely?
No. Losing 5 kg in a week would require a deficit of approximately 7,000 kcal/day — physiologically impossible and extremely dangerous. Safe weight loss is 0.5–1 kg per week. gradual weight management carries risks including gallstones, muscle loss, and nutritional deficiencies. If you need faster results, discuss prescription weight loss medication options with your GP.
What is a very low calorie diet (VLCD)?
A VLCD is under 800 kcal/day and should only be followed under medical supervision in specialist settings. It may be considered for severe obesity before bariatric surgery, but carries significant risks. NHS weight management services can assess whether VLCD is appropriate for you.
Does GLP-1 weight loss medication work without dieting?
No. Prescription weight loss injections like Wegovy work best alongside a reduced-calorie diet and increased physical activity. Clinical trials (STEP-1) showed average weight loss of ~15% of body weight over 68 weeks when combined with lifestyle changes. They are tools to support change, not substitutes for diet and exercise.
How do I access NHS weight loss support?
Start by speaking to your GP, who can refer you to your local tier 3 specialist weight management service. Alternatively, you can access free tools like our free BMI calculator and progress tracker to understand your starting point. NHS Weight Loss Plan resources are also available online. Availability varies by region — some areas have long waiting lists.
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.