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
Why Am I So Tired on Mounjaro?
Feeling unusually tired whilst taking Mounjaro (tirzepatide) is a recognised side effect that affects some people. Tiredness can occur at any stage of your treatment, particularly during dose escalation. Whilst it's not listed as a very common side effect, fatigue is reported by a proportion of users and can impact daily life. Understanding why this happens and what options are available can help you manage it effectively with support from your GP or prescriber.
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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 Cause Tiredness
Mounjaro works by activating GLP-1 and GIP receptors, which regulate appetite, blood sugar, and how your body processes food. This dual action can affect energy metabolism in several ways. Firstly, your body is adapting to a significant reduction in calorie intake — if you're eating substantially less, your metabolic rate may temporarily slow, leaving you feeling fatigued. Secondly, the medication slows gastric emptying (how quickly food leaves your stomach), which can affect nutrient absorption and blood sugar stability. Thirdly, some users experience nausea or reduced appetite so severe that inadequate nutrition contributes to tiredness. Additionally, gradual weight management itself can trigger fatigue as your body adjusts. Individual factors like sleep quality, hydration, exercise intensity, and underlying conditions (such as anaemia or thyroid issues) also influence energy levels during treatment.
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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 Tiredness — What You Can Do
If you're experiencing tiredness on your Mounjaro UK prescription, start by reviewing basic wellness habits: ensure you're eating enough calories (your prescriber can advise on safe intake), staying well-hydrated, and getting adequate sleep. Gentle exercise like walking or swimming often helps more than intense activity when fatigued. Consider whether nausea is reducing your food intake — if so, speak to your GP about timing doses or dietary adjustments. Always inform your prescriber if tiredness is severe or affecting your ability to work or drive. Your GP may check for nutritional deficiencies (iron, B12, vitamin D) or thyroid problems, which are common causes of fatigue. In some cases, reducing your dose or adjusting your injection schedule may help whilst still providing weight loss benefits.
- 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 tiredness a common side effect of Mounjaro?
Fatigue is reported by some people taking Mounjaro but is not listed as a "very common" side effect. It's more likely during dose escalation or if your calorie intake drops significantly. If tiredness is severe, speak to your GP — it may be manageable with dietary or lifestyle adjustments.
Should I stop taking Mounjaro if I'm too tired?
Do not stop your Mounjaro without guidance from your prescriber. Instead, contact your GP or the clinic managing your treatment. They can assess whether tiredness relates to Mounjaro, check for other causes (anaemia, thyroid issues, poor nutrition), and discuss options like dose adjustment.
How long does Mounjaro tiredness last?
Fatigue often improves within weeks as your body adjusts to the medication and your appetite stabilises. However, if you're eating too little, tiredness may persist. Working with your prescriber to ensure adequate nutrition can help. Some people find energy returns once they reach a stable dose.
Can I compare Mounjaro side effects to Wegovy if I'm considering switching?
Both Mounjaro and Wegovy can cause tiredness, though the mechanisms differ slightly — Mounjaro is a dual GIP/GLP-1 agonist whilst Wegovy is GLP-1 only. Clinical trial data shows similar side effect profiles overall. Your prescriber can discuss which medication might suit you better based on your specific experience.
Should I use the GLP-1 weight loss calculator to track my progress if I'm feeling tired?
Yes — our free GLP-1 weight loss calculator can help you see your projected weight loss journey and understand realistic timelines. Sometimes recognising good progress despite fatigue can be motivating. However, prioritise managing tiredness first — weight loss is a marathon, not a sprint.
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.