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
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Side effects of drastic weight loss and safer UK alternatives
Losing weight too quickly can cause serious health problems, including muscle loss, nutritional deficiencies, gallstones, and dehydration. The NHS recommends losing 0.5–1 kg per week for sustainable results. However, prescription weight management medications like Mounjaro and Wegovy, available through NHS services or private prescribing, can help achieve meaningful weight loss safely when combined with lifestyle changes. This guide explains the risks of gradual weight management and how to access professional support in the UK.
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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.
Risks of drastic weight loss and prescription weight management options in the UK
gradual weight management (more than 1.5 kg per week) significantly increases health risks. Losing weight too quickly causes your body to break down muscle tissue alongside fat, reducing metabolic rate and making weight regain more likely. Extreme calorie restriction (under 800 kcal daily) can trigger nutritional deficiencies, fatigue, hormonal imbalances, and weakened immunity.
UK-licensed prescription medications for weight management work differently. Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 and GIP/GLP-1 receptor agonist injections that help regulate appetite and support feeling fuller after smaller meals. Clinical trials showed Mounjaro users lost an average of 15–21% body weight, and Wegovy users lost approximately 15%, over 68–72 weeks—sustainable results achieved alongside reduced-calorie diet and increased physical activity.
- Mounjaro: Dual mechanism (GIP + GLP-1), doses 2.5–15 mg weekly
- Wegovy: GLP-1 agonist, doses 0.25–2.4 mg weekly
- Saxenda: GLP-1 agonist, daily injection (0.6–3 mg)
All three medications require a prescription and clinical assessment. They are most effective when combined with sustainable lifestyle changes, not as replacements for diet and exercise.
Clinically BackedWeight Loss
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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.
NHS access versus private prescribing in the UK
NHS access: Prescription weight loss medications are available through tier 3 specialist weight management services, typically following referral from your GP. Eligibility generally requires a BMI of 30 or above, or 27–29.9 with weight-related conditions (type 2 diabetes, high blood pressure, cardiovascular disease). Regional availability varies significantly—some areas have long waiting lists.
Private access: UK-licensed prescribers can assess and prescribe Mounjaro or Wegovy following an online consultation. This route bypasses NHS waiting lists but involves direct costs. Your GP can provide personalised advice about whether medication is appropriate for you and may support your treatment plan regardless of route.
Before considering any weight loss approach, use our free BMI calculator to understand your personal health context, and speak with your GP about safe options tailored to your circumstances.
- 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 are the most serious side effects of very gradual weight management?
Extreme gradual weight management can cause muscle loss, nutritional deficiencies (iron, B12, calcium), gallstones, dehydration, hormonal imbalance, hair loss, and weakened immune function. These risks increase significantly when weight loss exceeds 1.5 kg per week without medical supervision. The NHS recommends 0.5–1 kg per week as a safer rate.
How do prescription weight loss medications help avoid drastic weight loss risks?
Medications like Mounjaro and Wegovy help regulate appetite naturally, supporting moderate calorie deficit without extreme restriction. They work alongside lifestyle changes rather than replacing them. Clinical evidence shows they help achieve sustainable weight loss of 15–21% over 12–18 months, reducing risks associated with rapid or unsustainable approaches.
Am I eligible for prescription weight loss medication on the NHS in the UK?
Generally, you may be eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease). Lower thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults. Your GP can assess your individual circumstances and refer you to specialist services if appropriate.
What's the difference between NHS and private access to weight loss medications in the UK?
NHS access is through specialist tier 3 weight management services (often with waiting lists), while private online prescribers can assess and dispense medication more quickly without NHS referral. Both require a proper clinical assessment. Ask your GP about your options—they can advise on suitability and may support private treatment.
Should I try drastic weight loss or wait for medication?
Drastic weight loss carries significant health risks. If you're considering gradual weight management, speak with your GP first. Prescription medications combined with sustainable diet and exercise offer safer, evidence-based results. Try our GLP-1 weight loss calculator to explore potential outcomes, and use our BMI calculator to assess your personal health context.
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.