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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Fatigue on Mounjaro vs Wegovy — What's the Difference?
Fatigue is a reported side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but occurs at different rates between the two medications. Understanding how these GLP-1 weight loss injections compare can help you and your prescriber decide which option might be more suitable for your individual health needs. This guide breaks down the evidence from clinical trials and explains the difference in how these medications work.
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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 Rates in Clinical Trials — Side-by-Side Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT-1 clinical trial, fatigue was reported as a common side effect affecting approximately 5–7% of participants across all dose levels. Fatigue typically emerged during the dose titration phase and often improved as the body adjusted to the medication.
Wegovy (semaglutide) is a single GLP-1 receptor agonist. In the STEP-1 clinical trial, fatigue occurred less frequently, reported by approximately 3–4% of participants on the 2.4 mg maintenance dose. Both medications showed that fatigue was generally mild to moderate and transient.
- Mounjaro: ~5–7% of trial participants reported fatigue
- Wegovy: ~3–4% of trial participants reported fatigue
- Both: fatigue typically improved within weeks of dose adjustment
- Mechanism: Mounjaro's dual action (GIP + GLP-1) may contribute to slightly higher fatigue rates during initial use
Individual experiences vary significantly. Some people experience no fatigue at all, while others report it as a more persistent symptom. Your own medical history, medications, and overall health affect how you respond to either medication.
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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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What Causes Fatigue and When to Seek Advice
Fatigue on weight loss injections can stem from several sources: rapid metabolic changes as your body adjusts to reduced calorie intake, changes in blood sugar regulation, or the medication itself altering gut hormone signalling. During dose titration, your body is adapting to increasing amounts of the active ingredient, which can temporarily increase tiredness.
Fatigue typically improves within 2–4 weeks as your body acclimatises. However, persistent or worsening fatigue should always be discussed with your prescriber or GP. Other causes—such as anaemia, thyroid dysfunction, or medication interactions—should be ruled out. Your healthcare provider may adjust your dose, recommend dietary changes, or switch to an alternative medication if fatigue is significantly affecting your quality of life. Use our GLP-1 weight loss calculator to explore projected outcomes with either medication, and discuss any concerns with your prescriber before starting treatment.
- 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 slightly more common with Mounjaro (5–7%) than Wegovy (3–4%), but the difference is modest. Fatigue rates vary widely between individuals regardless of which medication you take. Your prescriber can help assess which option suits your health profile and fatigue tolerance.
How long does fatigue last on Mounjaro or Wegovy?
Most participants in clinical trials who experienced fatigue reported improvement within 2–4 weeks as their body adjusted. If fatigue persists beyond 4 weeks or worsens, contact your GP or prescriber to discuss whether a dose adjustment or medication change is appropriate.
Can I prevent fatigue when starting a weight loss injection?
There's no assured way to prevent fatigue, but staying well-hydrated, eating nutrient-dense foods, getting adequate sleep, and ensuring regular gentle physical activity can help. Discuss any fatigue concerns with your prescriber—they may recommend a slower dose titration if you're particularly sensitive to side effects.
Should I stop my medication if I feel fatigued?
No—do not stop taking your medication without speaking to your prescriber first. Fatigue often improves naturally within weeks. Your GP or prescriber can assess whether the fatigue is medication-related and discuss alternatives, dose adjustments, or support strategies tailored to you.
Which medication causes fewer side effects overall—Mounjaro or Wegovy?
Both have similar common side effects (nausea, diarrhoea, constipation, stomach pain), though reported rates vary slightly between trials. No medication is universally "better"—suitability depends on your individual health, preferences, and how your body responds. Your prescriber will assess your eligibility and recommend the most appropriate option for you.
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