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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Why Do You Feel Low Energy On Mounjaro? Comparison With Wegovy
Experiencing low energy or fatigue whilst taking Mounjaro is a reported side effect that some UK patients encounter. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can affect energy levels during treatment, though experiences vary significantly between individuals. This guide compares how each medication affects energy, explains why fatigue occurs, and helps you understand when to speak to your prescriber. Both are prescription-only weight management injections available in the UK following clinical assessment, and both require careful monitoring during dose titration.
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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.
How Do Mounjaro And Wegovy Compare For Energy Levels?
Fatigue is a reported side effect with both Mounjaro (tirzepatide) and Wegovy (semaglutide), though clinical trial data shows different patterns. In SURMOUNT trials, tiredness was reported by approximately 3–5% of Mounjaro users at the 15mg maintenance dose. In STEP trials, fatigue was less commonly reported with Wegovy, though both medications slow gastric emptying and reduce calorie intake — which can contribute to low energy if nutrition is insufficient.
- Mounjaro: Dual GIP and GLP-1 agonist; fatigue may relate to rapid metabolic changes or nutritional gaps as appetite decreases significantly
- Wegovy: GLP-1 agonist only; fatigue typically less severe but can still occur, especially during dose escalation (0.25mg to 2.4mg)
- Individual variation: Energy levels depend on diet quality, hydration, underlying health conditions, and how quickly your body adjusts to the injection
Both medications are available through NHS specialist weight management services or private prescribing in the UK. Your prescriber will assess your individual suitability during clinical assessment, considering your medical history and current medications.
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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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Managing Fatigue: When To Speak To Your Prescriber
If you're experiencing persistent low energy on either medication, contact your GP or prescriber immediately — do not stop the injection without guidance. Fatigue can be managed by ensuring adequate calorie intake, staying hydrated, eating nutrient-dense foods (protein, iron-rich foods, complex carbohydrates), and getting sufficient sleep. Your prescriber may recommend pausing dose escalation, staying at your current dose longer, or switching medications if fatigue is severe and affecting your daily function. Track your energy levels alongside weight loss progress using our free progress tracker to identify patterns. Some patients experience improved energy after the first 4–8 weeks as their body adjusts; others may require prescriber intervention. This is a discussion point at every follow-up appointment — energy and wellbeing matter as much as weight loss outcomes.
- 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 worse on Mounjaro than Wegovy?
Clinical trial data suggests fatigue is reported slightly more frequently with Mounjaro than Wegovy, but individual experiences vary considerably. Some people feel energised on Mounjaro; others experience significant tiredness. Your prescriber can help determine which medication is more suitable based on your specific health profile and tolerance.
Why does Mounjaro make me feel tired?
Mounjaro's dual GIP and GLP-1 action causes rapid appetite suppression and changes to how your body processes energy. If you're eating significantly less without ensuring adequate nutrition, low energy can result. Additionally, metabolic adjustment during dose titration can cause temporary fatigue. Ensure you're eating enough protein, iron, and complex carbohydrates.
When should I contact my GP about fatigue on these medications?
Contact your GP if fatigue persists beyond 2–3 weeks, worsens despite adequate nutrition and sleep, or is affecting your ability to work or exercise. Severe or sudden fatigue can indicate an underlying issue requiring medical assessment. Never stop the injection without prescriber guidance.
Can I switch from Mounjaro to Wegovy if I'm too tired?
Potentially yes, but only under prescriber guidance. A switch requires a clinical review to ensure Wegovy is appropriate for you, and involves an approximate dose conversion (Mounjaro 15mg roughly equates to Wegovy 2.4mg, though conversions are individualised). Your prescriber will manage the transition carefully.
Will my energy improve over time on Mounjaro or Wegovy?
For many people, yes. Initial fatigue often improves after 4–8 weeks as your body adjusts and you establish a stable eating pattern that supports energy. However, if fatigue persists or worsens, prescriber review is essential. Energy levels are a valid health outcome — discuss them openly at follow-up appointments.
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