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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Mounjaro Tiredness vs Wegovy: What's the Difference?
Tiredness and fatigue are common side effects reported with both Mounjaro (tirzepatide) and Wegovy (semaglutide) during weight management treatment in the UK. While both are prescription-only GLP-1 receptor agonists (Mounjaro is technically a dual GIP+GLP-1 agonist), they have different mechanisms and clinical trial data that may influence fatigue profiles. This guide compares how tiredness presents with each medication, what clinical evidence shows, and how your prescriber decides which is suitable for you based on individual health factors. Calculate your personal weight loss potential and BMI context using our free tools to support conversations with your GP.
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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.
Mounjaro vs Wegovy: Tiredness and Fatigue in Clinical Trials
Both Mounjaro and Wegovy list fatigue and tiredness as common side effects affecting up to 1 in 10 people during treatment. However, the clinical trial data shows subtle differences in how frequently tiredness was reported.
In the SURMOUNT-1 trial for Mounjaro (tirzepatide), fatigue was reported in approximately 8–10% of participants at the maximum 15 mg dose. In the STEP-1 trial for Wegovy (semaglutide), tiredness affected roughly 7–9% at the 2.4 mg maintenance dose.
These differences are modest and not statistically significant in most comparisons. Tiredness with both medications typically occurs in the first 1–4 weeks as your body adjusts, and often improves as you continue treatment. The Mounjaro KwikPen's dose titration schedule (starting at 2.5 mg and increasing every 4 weeks) allows your body to adapt gradually, which may help reduce early fatigue.
- Both medications slow gastric emptying, which can contribute to early tiredness
- Weight loss itself can sometimes cause temporary fatigue (calorie deficit effect)
- Individual responses vary significantly — some people experience no tiredness at all
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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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Why Fatigue Occurs and How Your Prescriber Chooses
Tiredness with GLP-1 and GIP+GLP-1 medications typically stems from reduced appetite leading to lower calorie intake and your body's adjustment to new hormone signalling. Nausea and poor appetite can reduce nutrient absorption, contributing to fatigue. Dehydration from diarrhoea (another common side effect) also plays a role.
Your UK prescriber will assess which medication suits you by considering: your medical history, other medications, previous side effect tolerance, and personal health goals. Neither Mounjaro nor Wegovy is "better" — they work differently and affect individuals differently. A prescriber may choose Mounjaro's dual mechanism if you have type 2 diabetes or prediabetes (additional glucose control benefit), or recommend Wegovy if you prefer a simpler, single-pathway approach. Both require clinical assessment before prescription and are available through NHS specialist weight management services or private online prescribing in the UK.
- 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 worse with Mounjaro than Wegovy?
Clinical trial data shows no significant difference — both cause fatigue in roughly 7–10% of people. Individual responses vary widely. Some people experience no tiredness at all, while others find it more pronounced. Your prescriber will monitor your response and can adjust your dose if fatigue becomes problematic.
How long does tiredness last with Mounjaro?
Tiredness typically peaks in weeks 1–4 and often improves by week 6–8 as your body adjusts. If fatigue persists beyond 8 weeks, contact your prescriber — it may indicate dehydration, nutrient deficiency, or that this medication isn't the right fit for you.
Can I reduce Mounjaro tiredness by eating more?
Eating slightly more nutritious food can help, but the purpose of Mounjaro is to reduce overall intake. Focus on nutrient-dense foods (protein, leafy greens, healthy fats) rather than increasing calories. Ensure you're drinking enough water and getting adequate sleep — these help combat fatigue.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways (dual action), while Wegovy (semaglutide) activates GLP-1 only. The dual mechanism may provide extra appetite suppression and better glucose control, but both can cause similar side effects. Your prescriber chooses based on your individual health profile and NHS NICE eligibility.
How do I know if my tiredness is from Mounjaro or something else?
Keep a symptoms diary noting tiredness onset, severity, and patterns. Contact your GP if tiredness is severe, worsening, or accompanied by new symptoms. Your prescriber can assess whether it's a normal adjustment effect, medication-related dehydration, or an unrelated health issue requiring investigation.
Ready to take control of your weight loss journey?
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