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
Unexplained Weight Loss and Fatigue — When to Seek Help and Treatment Options
Unexplained weight loss combined with persistent fatigue is not something to ignore — it can indicate an underlying health condition or metabolic imbalance. If you're experiencing unintended weight changes alongside tiredness, your GP should assess you first to rule out medical causes like thyroid disorders, diabetes, or other conditions. Once medical causes are excluded, prescription weight management medication may be an option for eligible adults in the UK seeking structured support for weight loss.
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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 Loss Medication Options
If your GP has ruled out underlying health conditions and you're eligible, three prescription-only weight management medications are available in the UK:
- Mounjaro (tirzepatide) — a dual GIP/GLP-1 injection taken once weekly, with average weight loss of 15–21% of body weight in clinical trials
- Wegovy (semaglutide) — a once-weekly GLP-1 injection, showing approximately 15% average weight loss in clinical trials
- Saxenda (liraglutide) — a daily GLP-1 injection, an established option for weight management
You're eligible if you have a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply. These medications work by helping regulate appetite and supporting a calorie deficit when combined with diet and exercise changes. Try our GLP-1 weight loss calculator to explore potential outcomes at different doses, and use our BMI calculator to check your eligibility range.
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 access is through specialist tier 3 weight management services — your GP can refer you, though waiting times vary significantly by region and can be lengthy. Private access is available through online consultations with UK-licensed prescribers; if clinically appropriate, a prescription is issued and dispensed by a registered pharmacy. Whichever route, a thorough clinical assessment is essential. Start by speaking with your GP about your unexplained weight loss, fatigue, and interest in weight management support. They can rule out medical causes, assess your BMI using our weight converter if needed, and discuss which pathway — NHS referral or private prescribing — suits 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 causes unexplained weight loss and fatigue together?
Unintended weight loss with fatigue can signal thyroid disorders, diabetes, depression, sleep disorders, or nutritional deficiencies. Medical assessment is essential first. Your GP can run blood tests and rule out underlying conditions before considering weight management medication or lifestyle interventions.
Can prescription weight loss medication cause fatigue?
GLP-1 and GIP/GLP-1 medications may cause tiredness as a side effect in some people, particularly during dose increases. However, many users report improved energy as weight decreases and blood sugar stabilises. Always discuss side effects with your prescriber if they're concerning.
How do I know if I'm eligible for Mounjaro or Wegovy in the UK?
You're eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions like diabetes or high blood pressure. Lower thresholds apply for certain ethnic backgrounds. A GP or licensed prescriber will assess your individual eligibility through a clinical consultation.
How long does NHS weight loss medication referral take in the UK?
NHS waiting times vary widely by region and local service capacity. Some areas have waits of several months to over a year for specialist weight management appointments. Private online prescribing is typically faster if you meet clinical criteria, usually within days to weeks.
Will weight loss medication stop my fatigue?
If fatigue is caused by obesity-related conditions like sleep apnoea or metabolic dysfunction, weight loss may improve it significantly. However, if fatigue stems from another cause like thyroid disease or depression, medication alone won't address it. Your GP's initial assessment determines the underlying cause.
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.