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 and tiredness are recognised side effects of Mounjaro (tirzepatide), a prescription-only GLP-1 and GIP receptor agonist injection used for weight management in the UK. Fatigue is listed as a common side effect, meaning it may affect up to 1 in 10 people taking this medication. Understanding why this happens and how long it typically lasts can help you manage this side effect effectively whilst your body adjusts to the tirzepatide dose.
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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.
Why Does Mounjaro Cause Fatigue?
Mounjaro works by activating both GLP-1 and GIP receptors, which affects how your body regulates appetite, blood sugar, and energy metabolism. Fatigue can develop for several reasons:
- Rapid changes in appetite and calorie intake: If you're eating significantly less food, your body may not be getting enough energy to maintain normal activity levels, leading to tiredness.
- Blood sugar stabilisation: As tirzepatide improves blood sugar control, some people experience initial dips in energy as their metabolism adjusts.
- Slowed gastric emptying: The medication slows how quickly food leaves your stomach, which can affect nutrient absorption and energy availability in the short term.
- Hormonal adjustment: Your body is adapting to new hormone signals, which requires metabolic energy and can temporarily affect how you feel.
Most people find that fatigue improves after the first few weeks as their body adjusts to the tirzepatide dose.
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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.
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How Long Does Mounjaro Fatigue Last?
Fatigue from Mounjaro typically improves within 2–4 weeks as your body adjusts to the medication and you stabilise your eating patterns. However, every person responds differently. Some people notice energy levels improving after the first injection, whilst others may experience tiredness for several weeks into their treatment.
If fatigue persists beyond 4 weeks, or if it significantly impacts your daily life, contact your GP. This may indicate that you need to adjust your calorie intake, your tirzepatide dose, or explore other factors affecting your energy levels. Using our free progress tracker can help you log how you're feeling alongside your weight changes, making it easier to spot patterns and discuss them with your prescriber.
- 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 serious side effect of Mounjaro?
Fatigue is a common but not serious side effect of Mounjaro. It's typically mild to moderate and improves within weeks as your body adjusts. However, if tiredness is severe or persistent beyond 4 weeks, contact your GP to rule out other causes and discuss whether your dose or calorie intake needs adjustment.
Can I prevent fatigue when starting Mounjaro?
Whilst you can't prevent fatigue entirely, you can manage it by eating enough calories to fuel your body (not undereating), staying hydrated, getting adequate sleep, and maintaining light physical activity. Work with your prescriber to ensure your reduced-calorie diet is balanced and nutritionally complete, not extremely restrictive.
Should I stop taking Mounjaro if I feel very tired?
Do not stop Mounjaro without speaking to your prescriber first. If fatigue is severe, your GP may advise pausing treatment, adjusting your dose, or investigating other causes. Never self-stop a prescription medication. Contact your GP or the clinic that prescribed your tirzepatide if side effects are affecting your quality of life.
Is fatigue worse at higher Mounjaro doses?
Fatigue is more commonly reported during dose escalation phases, when your body is adjusting to higher tirzepatide concentrations. The 2.5 mg starting dose and each increase (5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg) may trigger temporary tiredness. Most people adjust within 1–2 weeks at each dose level.
How does Mounjaro fatigue compare to Wegovy fatigue?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) list fatigue as a common side effect. Individual experiences vary widely. Some people report more fatigue with Mounjaro due to its dual mechanism, whilst others experience similar levels with both medications. Your prescriber can discuss which medication may suit you better based on your health profile.
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