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 and Tiredness: How It Compares to Wegovy
Tiredness (fatigue) is a reported side effect of both Mounjaro and Wegovy, though it affects people differently. This guide compares these two prescription weight loss injections side by side, explaining why fatigue occurs, how common it is, and what the clinical evidence shows. Both are GLP-1 based medications licensed in the UK, but they work slightly differently. Understanding the distinction can help you and your GP decide which might suit you better following a clinical assessment.
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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: Mechanism, Trials, and Tiredness Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. This dual mechanism may affect energy levels differently compared to single-pathway medications. In SURMOUNT clinical trials, fatigue was reported as a common side effect in up to 1 in 10 participants across dose levels.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one pathway. In STEP clinical trials, tiredness was also reported as a common side effect, though exact prevalence data varies by trial phase. Both medications slow gastric emptying and may affect nutrient absorption initially, which can contribute to energy dips. The key difference: Mounjaro's dual mechanism may theoretically produce different fatigue patterns, but individual variation is high and clinical data does not show one is significantly more or less fatiguing than the other. Dose escalation (the titration process over several weeks) is managed carefully in both cases to minimise side effects.
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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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Which Medication Might Be Right for You?
There is no evidence that Mounjaro causes substantially more or less tiredness than Wegovy. Fatigue severity depends on individual factors: your metabolism, dose level, existing health conditions, diet, and hydration. Some people experience mild tiredness that resolves within weeks; others report persistent low energy that improves with dose adjustment or dietary support. Both medications require a UK prescription following clinical assessment from a licensed prescriber. Your GP or specialist will consider your full health picture—including any existing fatigue, thyroid function, anaemia risk, and other medications—before recommending one over the other. Using our GLP-1 weight loss calculator can help you understand potential outcomes, whilst the BMI calculator provides context for your starting point.
- 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 Mounjaro more tiring than Wegovy?
No robust clinical evidence shows Mounjaro causes more tiredness than Wegovy. Both have fatigue reported as a common side effect in trials. Individual experience varies widely based on dose, metabolism, hydration, and overall health. Your prescriber will monitor you and adjust doses if fatigue becomes problematic.
Why do these weight loss injections cause tiredness?
Both slow stomach emptying and affect appetite hormones, which can impact energy temporarily. gradual weight management itself can reduce energy levels. Some tiredness is also psychological (adjusting to new eating patterns). Most people adapt within 4–6 weeks, though individual timelines vary.
Can I reduce tiredness while on Mounjaro or Wegovy?
Yes. Stay hydrated, eat balanced meals with adequate protein, maintain gentle exercise, and ensure good sleep. If fatigue is severe, contact your prescriber—a dose reduction or temporary pause may help. Never adjust doses yourself without medical guidance.
How common is tiredness with these weight loss injections?
Fatigue is listed as "common" (affecting up to 1 in 10 people) in both SURMOUNT and STEP trials. Many people experience no tiredness at all. Side effects vary significantly between individuals and often improve as your body adjusts over the first month.
Will my GP prescribe Mounjaro or Wegovy based on tiredness risk?
Your prescriber will assess your full health history, including baseline energy levels, anaemia, thyroid function, and other medications. Both medications require a UK prescription and clinical assessment. They'll recommend the option most suitable for you, considering all risks and benefits together.
Ready to take control of your weight loss journey?
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