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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Fatigue on Mounjaro — What You Need to Know
Fatigue is a common side effect reported by people taking Mounjaro (tirzepatide) for weight management in the UK. While it's typically mild and tends to improve as your body adjusts to the medication, understanding why it happens, how long it lasts, and your options for managing it is important. This guide covers everything you need to know about fatigue on Mounjaro, including access routes and costs in the UK.
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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.
Understanding Fatigue as a Mounjaro Side Effect
Fatigue and tiredness are listed as common side effects of Mounjaro, affecting up to 1 in 10 people during clinical trials. This happens for several reasons: Mounjaro slows how quickly food leaves your stomach, which can temporarily affect energy levels as your body adjusts. Additionally, as appetite decreases and food intake reduces, some people experience lower calorie intake initially, which may contribute to feeling tired. The fatigue is usually most noticeable in the first 2–4 weeks of starting Mounjaro or after dose increases, then typically improves as your body adapts.
- Fatigue typically resolves within 4–6 weeks as tolerance develops
- Staying hydrated and eating balanced, nutrient-dense meals helps combat tiredness
- Ensuring adequate protein intake supports energy levels during weight loss
- Some people find fatigue improves after a few weeks at the same dose
- If fatigue persists beyond 8 weeks or is severe, discuss with your prescriber
Other common Mounjaro side effects include nausea, diarrhoea, constipation, and stomach pain. Individual experiences vary significantly, and not everyone experiences fatigue.
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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.
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Accessing Mounjaro in the UK — Routes and Costs
In the UK, Mounjaro is available through two main routes: NHS specialist services and private prescription. NHS access is limited — tirzepatide is typically prescribed through tier 3 specialist weight management services following GP referral, though availability varies significantly by region and waiting lists can be very long. Private access is faster: you can have an online consultation with a UK-licensed prescriber, and if approved, receive a prescription dispensed by a GPhC-registered pharmacy. Costs typically range from £129–£320 per month depending on your dose, with starting doses (2.5 mg) at the lower end and maximum doses (15 mg) at the higher end. Both routes require a clinical assessment to confirm eligibility based on your BMI and health profile.
- 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.
Is fatigue a serious side effect of Mounjaro?
Fatigue is a common but usually mild side effect listed in clinical trial data. It's not typically considered serious and generally improves within 4–6 weeks as your body adjusts. However, if fatigue is severe, persistent beyond 8 weeks, or significantly affects daily life, contact your prescriber or GP for guidance. They can assess whether continuing the medication or adjusting your dose is appropriate.
How long does fatigue last on Mounjaro?
Most people experience fatigue for 2–4 weeks after starting Mounjaro or increasing their dose, then notice improvement as their body adapts. Some people feel better within days, while others may take 6–8 weeks. If fatigue hasn't improved after 8 weeks at the same dose, discuss this with your prescriber — it may indicate a need for dietary adjustments or further medical assessment.
What's the difference between Mounjaro and Wegovy for fatigue?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) list fatigue as a common side effect. Clinical trial data suggests similar frequencies, though individual experiences vary. Mounjaro is a dual GIP/GLP-1 agonist while Wegovy is a GLP-1 agonist only. If fatigue on one medication is problematic, a prescriber might consider switching, but this requires clinical assessment and should only be managed by your healthcare provider.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs in the UK typically range from £129–£320 per month depending on your dose. Starting doses (2.5 mg) are usually around £129–£150, while maintenance and higher doses (10–15 mg) range from £250–£320. Prices vary between GPhC-registered private prescribers. Some providers offer discounted rates for multiple months upfront. Always check the full cost before committing to treatment.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is NICE-approved for NHS use in specialist weight management services (tier 3), but availability is extremely limited. You typically need a GP referral to a specialist service in your area. Waiting lists can be many months or years depending on your region. Private prescription is often faster (consultation within days) but requires payment. Check with your local integrated care board (ICB) for NHS availability in your area.
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