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
Unexpected Weight Loss And Fatigue — When To Seek Help
Unexpected weight loss and fatigue together can be concerning and warrant a GP consultation to rule out underlying health conditions. However, if you're actively managing your weight through prescription medication, these symptoms may be expected side effects that typically improve over time. This guide explains what might cause these symptoms, when prescription weight management options in the UK are available, and how to discuss your health 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.
UK Prescription Weight Management Options
If you're experiencing unexpected weight loss as part of a deliberate weight management plan, several prescription-only medications are licensed in the UK for weight management in eligible adults. The three main options are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These GLP-1 and GIP/GLP-1 receptor agonists can cause weight loss and may trigger side effects including fatigue, nausea, and reduced appetite. Clinical trials show average weight loss of 15–21% body weight with these medications. Eligibility typically requires a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes or high blood pressure. All three medications are prescription-only and require a clinical assessment before being prescribed. Individual results vary, and these medications work alongside reduced-calorie diet and 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.
NHS Vs Private Access And Next Steps
In the UK, prescription weight loss medication is available through two routes: NHS specialist tier 3 weight management services (via GP referral, with regional variation and waiting lists) and private online prescribing through GPhC-registered pharmacies (faster access, costs vary by dose). If you're experiencing unexplained weight loss alongside fatigue without actively taking weight management medication, speak to your GP urgently—this could indicate thyroid problems, diabetes, infections, or other medical conditions requiring investigation. Your GP can assess your eligibility for NHS weight management support, refer you to specialist services, or discuss private prescription options. Use our free BMI calculator to understand your current weight category, and our GLP-1 weight loss calculator can help estimate potential outcomes if medication is appropriate for you.
- 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 causes fatigue when taking weight loss medication?
GLP-1 medications slow stomach emptying and reduce appetite, which can cause nausea, reduced food intake, and fatigue. These side effects typically improve as your body adjusts over 2–4 weeks. Ensure you're eating enough protein and staying hydrated. If fatigue is severe or persists beyond 4 weeks, contact your prescriber.
When should I see my GP about unexpected weight loss and fatigue?
See your GP immediately if weight loss is unexplained (not deliberate), rapid (over 1.5kg per week), or accompanied by other symptoms like fever, night sweats, or persistent cough. These could indicate thyroid disease, diabetes, infection, or cancer. Urgent investigation is needed.
Am I eligible for prescription weight loss medication in the UK?
You may be eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, cardiovascular disease). Eligibility is assessed individually by a prescriber following a clinical consultation. Your GP can discuss referral to NHS specialist services.
How quickly will I lose weight on Mounjaro or Wegovy?
Most people see gradual weight loss over weeks 4–8 as doses increase. Average weight loss is 15–21% body weight over 68–72 weeks in clinical trials. Rate varies by individual, starting dose, and lifestyle changes. Your prescriber will monitor progress and adjust doses as needed.
Can I get weight loss medication through the NHS quickly?
NHS access is through specialist tier 3 weight management services. Waiting lists vary by region and ICB (integrated care board). Some areas have wait times of 6–12 months or longer. Private online prescribing offers faster access but involves out-of-pocket costs. Your GP can advise on local NHS options.
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.