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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Why Am I Feeling Tired on Mounjaro?
Fatigue is a known side effect that some people experience when taking Mounjaro (tirzepatide), particularly during the early weeks of treatment or when increasing doses. Tiredness is listed as a common side effect in UK clinical data and may be related to gradual weight management, changes in appetite, or how your body adjusts to the medication. If you're feeling unusually fatigued, it's important to understand what's happening and when to speak to your GP or prescriber for personalised advice.
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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.
How Mounjaro Can Affect Energy Levels
Fatigue can develop during Mounjaro treatment for several reasons. When your body experiences gradual weight management, it uses significant energy for metabolic changes, which may temporarily reduce the energy available for daily activities. Additionally, Mounjaro works as a dual GIP and GLP-1 receptor agonist—it slows how quickly food leaves your stomach and reduces appetite, which means you're consuming fewer calories. This calorie reduction, whilst necessary for weight loss, can make some people feel tired, especially in the first few weeks.
Nausea and digestive discomfort—also common when starting the Mounjaro KwikPen or increasing your dose—can affect appetite further and contribute to fatigue. Some people also report that energy naturally returns after the body adjusts to the medication, typically within 2–4 weeks. Individual responses vary considerably; whilst some people experience significant tiredness, others notice minimal impact on energy levels.
When comparing Mounjaro vs Wegovy (semaglutide), both medications can cause fatigue as they both work on appetite regulation and weight loss, though the exact mechanisms differ slightly.
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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.
When to Speak to Your GP or Prescriber
Contact your prescriber if:
- Fatigue is severe and prevents you from daily activities
- Tiredness worsens significantly or persists beyond 4 weeks
- You feel dizzy, faint, or experience chest discomfort alongside fatigue
- You suspect another health condition may be developing (e.g., thyroid problems, anaemia)
Your GP or the prescriber managing your Mounjaro UK prescription can assess whether the tiredness is a normal adjustment response or a sign that your dose needs adjusting. They may recommend staying on your current dose longer before titration, or they may suggest lifestyle changes to support your energy levels. Never stop taking Mounjaro without professional guidance.
Managing tiredness whilst on Mounjaro involves practical lifestyle adjustments. Ensure you're eating nutritious, balanced meals—even though appetite is reduced, consuming adequate protein and micronutrient-rich foods supports energy production. Stay hydrated, as dehydration can worsen fatigue. Incorporate gentle physical activity like walking, which can paradoxically boost energy rather than drain it. Prioritise sleep quality and aim for 7–9 hours nightly. If nausea is contributing to tiredness, eating smaller, more frequent meals may help. Track your symptoms and energy levels using our free progress tracker to identify patterns and share with your GP. Avoid sudden changes to other medications without discussing with your prescriber, as some medications interact with tirzepatide and may affect how you feel.
- 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 normal side effect of Mounjaro?
Yes. Fatigue is listed as a common side effect, particularly during the first few weeks or when increasing doses. Many people report that tiredness improves as their body adjusts to the medication, typically within 2–4 weeks. However, individual experiences vary significantly.
Can Mounjaro fatigue be related to gradual weight management?
Yes. gradual weight management requires significant metabolic energy, which may leave less energy available for daily activities. Additionally, the reduced calorie intake that supports weight loss can contribute to tiredness in some people.
What's the difference between Mounjaro side effects and Wegovy tiredness?
Both Mounjaro and Wegovy (semaglutide) can cause fatigue because both regulate appetite and support weight loss. Mounjaro works as a dual GIP and GLP-1 agonist, whilst Wegovy is a GLP-1 agonist only, but tiredness may occur with either medication during dose titration.
How long does Mounjaro fatigue typically last?
Many people experience the most noticeable tiredness in the first 2–4 weeks of starting Mounjaro or when increasing doses. For others, fatigue may persist longer or be minimal. Your GP can advise based on your individual response and may adjust your dose titration schedule if needed.
What should I do if I feel very tired on Mounjaro?
Contact your prescriber or GP. Ensure you're eating balanced meals, staying hydrated, sleeping well, and engaging in gentle activity. They may suggest staying on your current dose longer, adjusting nutrition, or in some cases, pausing dose escalation until energy improves.
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.