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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Tiredness on Mounjaro vs Wegovy — What's the Difference?
Fatigue and tiredness are common side effects reported with both Mounjaro (tirzepatide) and Wegovy (semaglutide), but the frequency and severity can vary between individuals. Understanding how these two prescription-only weight loss medications compare helps you make an informed decision with your prescriber. Both are once-weekly injections licensed in the UK for weight management, but they work through different mechanisms — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. This guide compares tiredness reports, trial data, and practical considerations to help you understand what to expect.
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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.
Tiredness Reports: SURMOUNT vs STEP Trial Data
Mounjaro (tirzepatide) — SURMOUNT trial data: Fatigue was reported as a common side effect, affecting up to 1 in 10 participants across all doses. Most cases were mild to moderate and typically improved after the first 4–8 weeks as the body adjusted to the medication. Severe tiredness requiring dose reduction or discontinuation was rare.
Wegovy (semaglutide) — STEP trials: Fatigue was also reported as common, affecting a similar proportion of participants. Most cases were mild and transient, though some participants reported persistent low energy during the first month of treatment.
Importantly, both medications cause nausea and appetite suppression, which can indirectly contribute to fatigue if calorie intake drops too rapidly or nutritional intake becomes unbalanced. Using our free GLP-1 weight loss calculator can help you estimate realistic weight loss projections and understand the importance of maintaining adequate nutrition alongside treatment.
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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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Why Might You Feel Tired, and Which Is Right for You?
Tiredness on either medication can result from several factors: gradual weight management itself (which requires energy), appetite suppression leading to lower calorie intake than your body needs, or the medication's direct effect on metabolism. Individual responses vary significantly — some people experience no fatigue at all, while others report noticeable tiredness for a few weeks.
Neither Mounjaro nor Wegovy is inherently "more tiring" than the other based on clinical trial data. Your prescriber will assess your individual health, medical history, and suitability for each medication. Factors like your current energy levels, any underlying thyroid or metabolic conditions, and how quickly you're losing weight may influence which is more appropriate for you. Always discuss tiredness or other side effects with your GP or prescriber — they may adjust your dose, suggest dietary changes, or recommend a different medication if needed.
- 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.
Does Mounjaro cause more tiredness than Wegovy?
Clinical trial data shows similar frequencies of fatigue reported on both medications (both affecting roughly 1 in 10 people). Individual experiences vary widely — some people feel no fatigue, others experience temporary tiredness during the first few weeks. Neither medication is specifically known for causing more severe tiredness than the other.
How long does tiredness usually last on these injections?
Most people who experience tiredness find it improves within 2–8 weeks as their body adjusts. If fatigue persists beyond 8 weeks, discuss it with your prescriber — it may relate to gradual weight management, inadequate nutrition, or another underlying factor that can be addressed.
Can I reduce tiredness by adjusting my Mounjaro or Wegovy dose?
Your prescriber may suggest staying at a lower effective dose rather than titrating to the maximum dose, which can help reduce side effects including fatigue. Never adjust your dose without medical guidance. Ensuring adequate protein, calories, and micronutrients also helps combat tiredness.
Is tiredness a sign that the medication isn't working?
No — tiredness is a common but temporary side effect unrelated to how well the medication is helping you lose weight. Weight loss and side effects are separate. If you're concerned about either, discuss both with your prescriber.
Which medication should I choose if I'm worried about tiredness?
Both medications have similar fatigue profiles in clinical trials. Your prescriber will recommend based on your overall health, BMI, any weight-related conditions, and other medications you take — not based on tiredness risk alone. Use our free BMI calculator to understand your starting point, then discuss all treatment options with your GP.
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.