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 Does Mounjaro Make Me Tired?
Fatigue and tiredness are common side effects of Mounjaro (tirzepatide) in the UK, particularly when starting the medication or moving to a higher dose. Many people on Mounjaro report feeling unusually tired during the first few weeks, though this often improves as their body adjusts. Understanding what's happening and when to contact your GP is important for managing this side effect safely and effectively.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Causes Fatigue and When It Typically Occurs
Fatigue with Mounjaro can happen for several reasons. As a GIP and GLP-1 receptor agonist, tirzepatide slows how quickly food leaves your stomach and affects nutrient absorption—your body may need time to adjust to these changes. During dose titration, when you increase from one Mounjaro dose to the next (moving from 2.5mg to 5mg, then to 7.5mg, 10mg, 12.5mg, or the maximum 15mg), tiredness often peaks in the first few days before settling down.
Other contributing factors include:
- Reduced calorie intake—if you're eating significantly less due to decreased appetite, your body has less energy available
- Dehydration—ensure you're drinking enough water daily
- gradual weight management—significant changes in body composition can temporarily affect energy levels
- Individual metabolism—some people's bodies adjust faster than others
Most NHS specialist weight management services report that fatigue typically improves within 7–10 days of starting a new dose. If tiredness persists beyond two weeks, contact your GP or prescriber.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 on Mounjaro: Practical NHS-Aligned Steps
If you're experiencing tiredness on Mounjaro, several practical strategies can help. First, ensure adequate nutrition—despite reduced appetite, aim for protein-rich foods and nutritious meals to support energy levels. Stay well hydrated, as dehydration significantly worsens fatigue. Sleep and rest are crucial: allow your body extra recovery time during dose increases. Light physical activity like walking can paradoxically boost energy, whereas intense exercise may worsen tiredness initially.
Consider keeping a simple progress tracker to log when fatigue occurs and its severity—this helps your GP identify patterns. If tiredness severely impacts daily functioning, interferes with work or caring responsibilities, or doesn't improve after two weeks, contact your prescriber immediately. Your GP can assess whether a dose adjustment or medical review is needed. Never stop Mounjaro suddenly; always consult your healthcare team first.
- 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 on Mounjaro normal?
Yes, fatigue is very common, especially during dose increases. Most people experience it within the first week of starting or escalating their Mounjaro dose. It typically improves as your body adjusts over 7–10 days. However, if tiredness persists beyond two weeks or is severe, contact your GP for advice.
Will the tiredness go away on Mounjaro?
For most people, yes. As your body adjusts to tirzepatide and your dose stabilises, fatigue usually decreases. However, individual experiences vary. Some people report sustained low energy on higher doses. Your GP or prescriber can discuss whether your current dose is appropriate or if adjustments might help.
Should I stop taking Mounjaro if I'm too tired?
No—never stop Mounjaro suddenly without medical guidance. Instead, contact your GP or prescriber if fatigue is severe or persistent. They can assess whether you need a dose reduction, a longer adjustment period, or further investigation. Stopping abruptly can affect your treatment plan.
Can I use the GLP-1 weight loss calculator to understand my Mounjaro dose better?
Yes, our free GLP-1 weight loss calculator can help you understand how different doses of tirzepatide (Mounjaro) are typically used and what side effects like fatigue commonly occur at each stage. It's a useful tool for tracking your dose journey and expectations.
When should I contact my GP about Mounjaro fatigue?
Contact your GP if tiredness persists beyond two weeks, severely affects daily functioning, is accompanied by other symptoms (dizziness, shortness of breath, chest pain), or prevents you from working or caring for others. Your GP can assess whether investigation or dose adjustment is needed.
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.