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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Mounjaro Lethargy — Cost & Access in the UK
Lethargy and fatigue are common side effects reported by people taking Mounjaro (tirzepatide) in the UK, particularly during the initial weeks of treatment. If you're considering Mounjaro and concerned about tiredness, understanding the cost, NHS access routes, and what to expect can help you make an informed decision. This guide covers UK prescription options, eligibility, and how fatigue typically presents during treatment. Use our free BMI calculator above to check your eligibility for weight management treatment, and our GLP-1 weight loss calculator to estimate potential outcomes with Mounjaro.
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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.
Mounjaro UK Costs — Private Prescription Route
Private prescription costs for Mounjaro in the UK typically range from £130–£320 per month, depending on the dose and provider. Starting doses (2.5 mg) are usually at the lower end of this range, whilst higher maintenance doses (15 mg) cost more. Each KwikPen device contains 4 weekly doses, meaning most private prescriptions cover one month of treatment.
To access Mounjaro privately, you'll need an online consultation with a UK-licensed prescriber through a GPhC-registered pharmacy. The consultation fee (usually £30–£50) is separate from the medication cost. Prescribers will assess your BMI, medical history, and weight-related conditions to determine eligibility. Unlike NHS access, you don't need a GP referral — you can proceed directly to a private provider. Most private providers offer flexible payment options, including monthly subscriptions or pay-per-pen arrangements. Costs are significantly higher than NHS routes but offer faster, more flexible access without waiting lists. Storage costs (refrigeration) are your responsibility with private prescriptions.
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 — Which Route is Right for You?
NHS access to Mounjaro is free at the point of prescription but requires referral to a specialist tier 3 weight management service through your GP. NICE has approved tirzepatide (Mounjaro) for NHS use, but availability varies significantly by region and ICB. Waiting times can range from weeks to many months in some areas. Once prescribed on the NHS, there are no additional costs — the NHS covers all medication expenses and monitoring.
Private access is immediate and doesn't require NHS involvement, but you pay the full cost. Many people choose private access when NHS waiting times are lengthy or when they prefer faster access and continuity with a single prescriber. Consider your budget, timeline, and preference for ongoing support. Whichever route you choose, Mounjaro requires regular monitoring — GPs will monitor your progress and manage any side effects like lethargy, which often improve as your body adjusts to the medication.
- 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.
How much does Mounjaro cost on prescription in the UK?
Private prescription costs range from £130–£320 per month depending on dose, plus a £30–£50 consultation fee. NHS prescriptions are free but require GP referral to specialist services with potential waiting lists of weeks to months. Costs vary by region and provider availability.
What causes lethargy when taking Mounjaro?
Lethargy is thought to result from rapid appetite suppression, reduced calorie intake, and metabolic changes as your body adjusts to tirzepatide. It's usually temporary — most people report improvement within 4–8 weeks. Staying hydrated, eating nutrient-dense foods, and getting adequate sleep can help manage fatigue during initial treatment.
Can I access Mounjaro through the NHS if I have a high BMI?
Yes, if you meet NICE criteria (BMI ≥30, or ≥27.5 with weight-related health conditions), you may be eligible. Ask your GP for referral to tier 3 specialist weight management services. However, NHS access is limited by regional availability and waiting lists — private access is an alternative.
How long does lethargy last when starting Mounjaro?
Fatigue typically peaks during weeks 1–4 and often improves by weeks 5–8 as your body adjusts. Dose titration (gradual increase) helps minimise side effects. If lethargy persists beyond 8 weeks, discuss this with your prescriber — they may adjust your dose or offer additional support.
Should I choose NHS or private Mounjaro prescription?
Choose NHS if you can wait and want free ongoing care. Choose private if you prioritise speed, want to avoid waiting lists, or prefer direct access to a licensed prescriber. Both routes require eligibility assessment and medical monitoring — use our GLP-1 weight loss calculator to explore potential outcomes regardless of route chosen.
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.