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 More Tired Than Wegovy?
Fatigue is a reported side effect of both Mounjaro and Wegovy, though it affects people differently. If you're experiencing tiredness on Mounjaro, you may wonder whether switching to Wegovy would help—or whether the fatigue is an inevitable part of weight loss injections. The truth is more nuanced. Both medications work via similar pathways but use different mechanisms, and clinical trial data shows varying fatigue rates. Understanding the differences can help you and your GP or prescriber decide whether to continue, adjust your dose, or explore alternatives.
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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.
Fatigue in Mounjaro vs Wegovy: What the Clinical Trials Show
In clinical trials, fatigue (tiredness) was reported as a common side effect of both medications, but at different rates. In the SURMOUNT-1 trial for Mounjaro (tirzepatide), fatigue affected approximately 6–8% of participants across all doses. In the STEP-1 trial for Wegovy (semaglutide), fatigue affected roughly 4–5% of participants. These are modest differences, and both are significantly lower than nausea or diarrhoea rates. The key mechanism difference is important: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. This dual action in Mounjaro may affect energy regulation differently in some people, though individual responses vary widely. Fatigue can also be linked to gradual weight management, calorie restriction, nutrient deficiencies, dehydration, or the body adjusting to hormonal changes—not necessarily the medication itself. Switching from Mounjaro to Wegovy will not automatically resolve tiredness if the underlying cause is diet or lifestyle-related.
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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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Is Tiredness Permanent, and Should You Switch Medications?
Fatigue during the first few weeks of Mounjaro often improves as your body adapts—many people report energy levels returning to normal by weeks 4–8. Before considering a switch to Wegovy, discuss with your GP or prescriber whether your tiredness is dose-related, lifestyle-related, or medication-related. If you're on a higher Mounjaro dose (10 mg or 15 mg), a temporary dose reduction or pause may help identify the cause. Ensure you're eating adequate protein, staying hydrated, and maintaining consistent sleep patterns. Use our free GLP-1 weight loss calculator to track your expected weight loss rate—rapid loss combined with low calorie intake is a common cause of fatigue. Your prescriber can also check for underlying issues like anaemia, thyroid dysfunction, or nutrient deficiencies. If fatigue persists despite these measures, switching to Wegovy is an option, though results vary by individual.
- 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 trials show Mounjaro fatigue rates of 6–8% vs Wegovy at 4–5%. The difference is modest, and both medications affect energy differently in individuals. Fatigue may also relate to diet, hydration, or gradual weight management rather than the medication itself. Your GP can help identify the cause.
Why might Mounjaro make me tired but not Wegovy?
Mounjaro's dual GIP and GLP-1 mechanism may regulate energy differently than Wegovy's single GLP-1 pathway. Some people are more sensitive to this dual action. Dose titration, individual metabolism, and lifestyle factors (sleep, nutrition, stress) also play major roles.
Will switching from Mounjaro to Wegovy help my fatigue?
Not necessarily. If your tiredness stems from calorie deficit, dehydration, or nutrient deficiency, switching medications won't help. Before switching, confirm with your GP that the fatigue is medication-related. A dose reduction or pause may clarify this.
How long does Mounjaro tiredness usually last?
Many people experience fatigue in the first 2–4 weeks as their body adjusts. Energy often returns by weeks 4–8. If fatigue persists beyond 8 weeks, discuss with your prescriber whether a dose adjustment or investigation for other causes (anaemia, thyroid issues) is needed.
What should I do if I'm tired on Mounjaro?
Check your calorie intake, protein, hydration, and sleep quality. Use your GP's advice or our free progress tracker to monitor patterns. Ensure you're eating enough and not over-exercising. Contact your prescriber if fatigue is severe or persistent—dose adjustment or a medication switch may be considered.
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.