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 Lethargy vs Wegovy: What's the Difference?
Lethargy and tiredness are reported side effects of both Mounjaro and Wegovy, though frequency and severity vary between individuals. Both medications are prescription-only weight loss injections licensed in the UK, but they work through different mechanisms. Understanding how each one affects energy levels can help you and your GP make an informed decision about which treatment might suit you better. This guide compares the two factually, based on clinical trial data and UK prescribing information.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways in the body. This dual mechanism may affect energy metabolism and appetite regulation differently than a single-pathway medication. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one hormone pathway. In the SURMOUNT clinical trials, fatigue was reported in common frequency with Mounjaro across various doses. In the STEP trials for Wegovy, tiredness appeared at similar rates. The difference isn't dramatic between the two—both can cause lethargy, particularly during dose escalation when your body is adjusting to the medication. Your prescriber will consider your individual health profile, other medications, and tolerance when deciding between them. Individual responses vary considerably, and what causes fatigue in one person may not affect another.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Fatigue, Dose Titration, and Individual Suitability
Lethargy often appears during the first 4–8 weeks of treatment, particularly when increasing doses. With Mounjaro, dose titration happens weekly (starting 2.5 mg, escalating to 15 mg maximum), whilst Wegovy escalates more gradually over 16+ weeks to reach 2.4 mg. A longer titration schedule may mean less intense side effects for some people, though this isn't assured. Both medications work best alongside a reduced-calorie diet and increased physical activity—and fatigue can sometimes be linked to undereating or insufficient energy intake. Your GP or prescriber will assess whether lethargy is manageable or whether a different dose, slower escalation, or alternative medication would be more suitable for you. Only a licensed prescriber can decide which medication is right for your circumstances.
- 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 lethargy than Wegovy?
Clinical trials show similar fatigue rates for both, though individual experiences vary. Mounjaro's faster dose escalation may cause more noticeable early tiredness, whilst Wegovy's slower titration might feel gentler. Your prescriber will assess your personal risk factors and tolerance. Results are highly individual.
How long does lethargy last on these medications?
Fatigue often improves within 4–8 weeks as your body adjusts. If lethargy persists beyond this or significantly affects daily life, tell your prescriber immediately. Dose adjustments or a different medication may be recommended.
Can I take Mounjaro or Wegovy if I'm already tired?
Yes, but inform your prescriber about any existing fatigue, anaemia, thyroid problems, or other conditions. They'll assess whether the medication is safe for you and monitor closely. Prescription requires a full clinical assessment.
Is lethargy a reason to stop Mounjaro or Wegovy?
Not necessarily, but persistent lethargy warrants discussion with your prescriber. They may reduce your dose, slow escalation, adjust your diet, or recommend an alternative. Never stop a prescription medication without medical guidance.
Which medication should I choose to avoid lethargy?
Neither medication assures freedom from fatigue—both can cause it. Your prescriber will consider your medical history, other medications, and personal circumstances to recommend the most suitable option. Individual factors matter more than the medication choice alone.
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