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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Weight Loss Fatigue — UK Prescription Options
Fatigue during weight loss is common, especially when making significant dietary changes or losing weight quickly. The good news is that prescription weight management options available in the UK may help you lose weight more effectively, potentially reducing the need for extreme calorie restriction that often causes tiredness. This guide explains the licensed medications available, eligibility criteria, and how to access them through the NHS or privately.
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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.
UK-Licensed Weight Loss Medications
Three prescription medications are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All three are GLP-1 receptor agonists (Mounjaro is a dual GIP/GLP-1 agonist), which work by reducing appetite and helping you feel fuller after smaller meals. This can reduce the need for severe calorie restriction.
- Mounjaro: Once-weekly injection, doses 2.5–15 mg. Clinical trials showed average weight loss of 15–21% of body weight alongside diet and exercise.
- Wegovy: Once-weekly injection, doses 0.25–2.4 mg. Average weight loss approximately 15% of body weight in clinical trials.
- Saxenda: Daily injection, doses 0.6–3 mg. Longer-established option with well-documented safety profile.
All three require a prescription and clinical assessment. Individual results vary significantly. These medications are not a substitute for lifestyle changes but are used alongside a reduced-calorie diet and increased physical activity.
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 and Private Access in the UK
NHS access: Weight loss medications are available through specialist tier 3 weight management services. Your GP can refer you if you have a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, or cardiovascular disease). However, NHS waiting lists are often long, with some regions taking 12+ months for specialist assessment.
Private access: Available through online consultations with UK-licensed prescribers. You can access prescription weight loss medication without an NHS referral. A prescriber will assess your eligibility based on your BMI, health history, and individual circumstances.
To find out if you're eligible, calculate your BMI using our free BMI calculator above, then discuss options with your GP or arrange a private consultation.
- 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.
Will prescription weight loss medication stop me feeling fatigued during weight loss?
These medications may help by reducing the need for extreme calorie restriction, which is often the cause of fatigue. By helping you feel fuller on fewer calories, you can achieve sustainable weight management without drastic dietary changes. However, some people experience fatigue as a side effect initially. This typically improves as your body adjusts to the medication and you settle into a sustainable eating pattern.
What is the difference between Mounjaro and Wegovy for weight loss UK?
Both are once-weekly injections licensed for weight management. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (15–21% vs 15%), though individual results vary. Both require prescription and clinical assessment. Your prescriber will recommend the most appropriate option for you.
Am I eligible for prescription weight loss treatment on the NHS?
You may be eligible if you have a BMI of 30 or above, or BMI of 27–29.9 with at least one weight-related health condition such as type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease. Lower BMI thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. Speak to your GP about referral to a specialist weight management service in your area.
How much does prescription weight loss medication cost privately in the UK?
Private costs range from approximately £129–£320 per month depending on the medication, dose, and provider. This is typically cheaper than starting on the NHS (if available) as you avoid waiting times. Costs vary between online prescribing services, so compare options. Some providers offer subscription discounts for ongoing treatment.
Can I switch from one weight loss injection to another?
Switching between medications should only be done under medical supervision. Your prescriber may recommend switching if one medication isn't working well or causes unacceptable side effects. There is no official established conversion chart between medications — your prescriber will guide the transition. Never stop one medication and start another without clinical guidance, as this could affect your results and safety.
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.