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
Does Mounjaro Make You Tired?
Yes, fatigue is a recognised side effect of Mounjaro (tirzepatide), though it is not among the most common side effects. Some people taking this prescription weight loss injection report feeling more tired than usual, particularly during the initial weeks or when doses are increased. However, fatigue is temporary for most people and typically improves as the body adjusts to the medication. If tiredness persists or significantly affects your daily life, speak to your GP or prescriber — they can assess whether it is related to Mounjaro or another factor.
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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 and Mounjaro — What the Evidence Shows
Fatigue is listed as a common side effect of Mounjaro in UK prescribing information, meaning it may affect up to 1 in 10 people taking the medication. However, clinical trial data (SURMOUNT-1) did not identify tiredness as one of the most frequently reported side effects — nausea, diarrhoea, and vomiting were far more common. This means that while some people do experience fatigue on Mounjaro, many do not.
How Mounjaro works may explain why fatigue occurs in some cases. As a dual GIP and GLP-1 receptor agonist, it affects appetite regulation, blood sugar control, and how quickly food leaves the stomach. These changes require your body to adapt, which can contribute to tiredness during the adjustment period. Additionally, if Mounjaro significantly reduces your appetite, you may consume fewer calories than your body needs, potentially leading to low energy. Our GLP-1 weight loss calculator can help you understand expected weight changes and energy considerations across different doses.
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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.
Managing Fatigue While Taking Mounjaro
If you experience tiredness while taking this prescription medication, several practical steps may help. First, ensure you are eating enough — despite reduced appetite, your body still requires adequate calories and nutrients to function. Include protein, healthy fats, and complex carbohydrates in smaller meals. Stay hydrated throughout the day, as dehydration can worsen fatigue. Monitor your energy levels across different doses using our free progress tracker, which can help you identify patterns and discuss them with your prescriber.
Importantly, fatigue often improves within 2–4 weeks as your body adjusts to Mounjaro. If tiredness persists beyond this timeframe, worsens, or is accompanied by other symptoms like dizziness or chest discomfort, contact your GP immediately. Your prescriber may adjust your dose or investigate whether another factor is causing the fatigue.
- 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 common side effect of Mounjaro?
Fatigue is classified as a common side effect, potentially affecting up to 1 in 10 people. However, clinical trials showed nausea and diarrhoea were more frequently reported. Individual experiences vary — many people taking Mounjaro do not experience tiredness at all.
How long does Mounjaro fatigue last?
For most people, fatigue improves within 2–4 weeks as the body adjusts to the medication. If tiredness persists beyond this period, contact your prescriber. They can assess whether it is related to Mounjaro or another cause, such as inadequate calorie or nutrient intake.
Can I prevent fatigue when starting Mounjaro?
While you cannot always prevent fatigue, eating enough nutrient-dense food, staying hydrated, and getting adequate sleep may help. Despite reduced appetite, ensure you consume sufficient calories and protein. Starting with the lowest dose (2.5 mg) also gives your body time to adjust gradually.
Should I stop taking Mounjaro if I feel tired?
Do not stop without speaking to your prescriber first. Fatigue often improves with time. However, if tiredness is severe, significantly affects your quality of life, or is accompanied by other concerning symptoms, contact your GP. They will advise whether to continue, adjust the dose, or explore other causes.
Can I check my weight loss progress and energy levels together?
Yes — use our free progress tracker to log both your weight changes and energy levels alongside dose changes. This helps identify patterns and provides useful information to discuss with your prescriber about whether fatigue is improving or worsening over time.
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.