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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Does Mounjaro Make You Tired?
Yes, fatigue is a recognised side effect of Mounjaro (tirzepatide), though it's not the most common. Many people experience tiredness when starting the medication or adjusting doses, but this often improves after a few weeks. Understanding what to expect and how it compares to other side effects can help you make an informed decision about whether prescription weight management treatment is right for you.
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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.
Fatigue as a Mounjaro Side Effect — What the Evidence Shows
Fatigue and tiredness are listed as common side effects of Mounjaro, meaning they may affect up to 1 in 10 people taking the medication. However, fatigue is less frequently reported than nausea, diarrhoea, and vomiting, which are the most common side effects. In clinical trials, the exact incidence of tiredness varied, but it was generally reported by a smaller proportion of participants compared to gastrointestinal effects.
Fatigue can occur:
- When first starting Mounjaro at the 2.5 mg dose
- During dose titration (when increasing to the next dose level)
- As your body adjusts to the medication's effects on appetite and metabolism
Most people who experience fatigue report that it improves within 1–2 weeks as their body adjusts. However, if tiredness persists beyond this period or significantly impacts your daily life, it's important to discuss this with your prescriber or GP.
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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.
Accessing Mounjaro in the UK — NHS and Private Routes
In the UK, you can access Mounjaro through NHS specialist weight management services or via private prescription. NHS access is through tier 3/4 specialist services (usually following GP referral), though availability and waiting times vary significantly by region. Private access is faster — you can have an online consultation with a UK-licensed prescriber, and if eligible, receive a prescription within days.
Both routes require a clinical assessment to confirm you meet eligibility criteria: generally, a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Your prescriber will also review side effect risks, including fatigue, and discuss whether Mounjaro suits your circumstances. Use our free BMI calculator above to check your current BMI, and our GLP-1 weight loss calculator to explore potential outcomes alongside realistic expectations about side effects.
- 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 long does Mounjaro fatigue typically last?
Fatigue often improves within 1–2 weeks as your body adjusts to the medication. However, some people experience longer periods of tiredness. If fatigue persists beyond 3–4 weeks or worsens, contact your prescriber — dose adjustment or other support may help.
Is fatigue more common with higher Mounjaro doses?
Fatigue can occur at any dose level, including the starting dose of 2.5 mg. It's often worse during dose increases (titration), as your body adapts to higher tirzepatide levels. Most people find adjustment improves after a few weeks at each new dose.
How does Mounjaro fatigue compare to Wegovy side effects?
Both Mounjaro and Wegovy can cause fatigue, though it's not the most common side effect for either. Nausea and diarrhoea are more frequent. Mounjaro's dual GIP/GLP-1 mechanism may have a different side effect profile than Wegovy's GLP-1-only action, but individual experiences vary significantly.
Can I reduce Mounjaro fatigue by changing my diet or activity?
Yes, supporting your energy levels through adequate nutrition, hydration, and gentle activity may help. Ensure you're eating enough protein and not over-restricting calories — very low calorie intake can worsen fatigue. Your prescriber or a dietitian can advise on nutrition alongside Mounjaro.
Should I stop Mounjaro if I'm experiencing severe tiredness?
No — don't stop without medical guidance. Contact your prescriber or GP instead. Severe fatigue may require dose adjustment, timing changes, or switching medication. Your prescriber can assess whether the tiredness is medication-related or linked to another cause and advise accordingly.
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.