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
Understanding Unexpected Weight Loss And Fatigue
Unexplained weight loss combined with fatigue can feel worrying, and it's right to take it seriously. If you're losing weight without trying and feeling unusually tired, this needs medical assessment — it can indicate various conditions from thyroid disorders to medication side effects. This guide explains common causes, when to contact your GP, and how weight management options like prescription weight loss medication may affect your energy levels.
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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.
Common Causes Of Sudden Weight Loss And Tiredness
Unintentional weight loss with fatigue can stem from several sources. Thyroid conditions like hyperthyroidism cause rapid metabolism, weight loss, and exhaustion. Type 2 diabetes and prediabetes may trigger weight loss alongside fatigue and increased thirst. Depression and anxiety often reduce appetite and energy simultaneously. Autoimmune conditions such as coeliac disease or lupus can cause both symptoms. Nutrient deficiencies — particularly iron, B12, or vitamin D — lead to tiredness and sometimes weight loss. Medication side effects are common culprits; certain antidepressants, blood pressure medications, and weight loss injections can initially affect appetite and energy. Infections like tuberculosis or long COVID may cause gradual weight loss with persistent fatigue. Cancer and other serious conditions can present this way, though this is less common. The key is that unexplained weight loss is not normal and warrants GP assessment to identify the underlying cause.
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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.
When To See Your GP And What Happens Next
Contact your GP if you've lost more than 5% of your body weight unintentionally over several weeks, or if weight loss is accompanied by fatigue, night sweats, or other symptoms. Your GP will ask about your medical history, medications, diet, and lifestyle changes. They may recommend blood tests to check thyroid function, blood sugar, vitamin levels, and iron status. Use our free BMI calculator to establish your current weight baseline — this helps your GP assess how much weight you've lost and track progress. Be honest about any stress, mood changes, or recent infections. If weight loss is intentional (through diet or prescription weight loss medication), your GP will monitor energy levels and may adjust your approach if fatigue becomes problematic.
- 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.
Can weight loss injections cause fatigue?
Yes, initially. GLP-1 medications like Wegovy and dual GIP/GLP-1 medications like Mounjaro commonly cause nausea, reduced appetite, and tiredness in the first weeks as your body adjusts. This typically improves as you continue treatment. However, persistent fatigue should be reported to your prescriber — it may indicate you need a dose adjustment, or it could signal a separate medical issue requiring investigation.
Is fatigue normal when losing weight?
Some mild tiredness is common during weight loss, especially if you've significantly reduced calorie intake. However, severe or persistent fatigue is not typical and warrants investigation. Ensure you're eating enough protein, staying hydrated, and getting adequate sleep. If fatigue is disproportionate to your weight loss efforts, ask your GP to investigate underlying causes like thyroid problems or nutrient deficiencies.
How can I track whether my weight loss is intentional or unexplained?
Use our free progress tracker to log your weight, diet, and how you feel. This helps you and your GP distinguish between intentional loss (from diet or medication) and unexplained loss. If you're not actively trying to lose weight but losing consistently, that's unexplained and requires medical assessment.
Should I start prescription weight loss medication if I'm already fatigued?
No. If you're experiencing unexplained fatigue, address that first with your GP. Prescription weight loss treatments like Mounjaro or Wegovy can initially worsen tiredness. Starting them while already fatigued may compound the problem. Your GP will assess your overall health and advise whether weight management through prescription medication is appropriate once your fatigue is understood.
What's the difference between tiredness from weight loss and tiredness that needs investigation?
Mild tiredness linked to calorie restriction usually improves with adequate rest and nutrition. Concerning fatigue is sudden, severe, persistent despite good sleep and diet, or accompanied by other symptoms like night sweats, weight loss beyond 5%, or unexplained bruising. That requires GP assessment — don't assume it's just weight loss related.
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.