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 Feel Tired?
Yes, fatigue is a common side effect of Mounjaro (tirzepatide), affecting many people during the early weeks of treatment. Tiredness typically occurs as your body adjusts to the medication, often improving as you continue. This UK health guide explains what causes fatigue on Mounjaro, how long it usually lasts, and whether you should report it to your prescriber. We'll also cover UK costs and how to access this prescription-only weight management injection through both NHS and private routes.
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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.
Mounjaro Fatigue: What the Evidence Shows
Fatigue is listed as a common side effect of Mounjaro, meaning it may affect up to 1 in 10 people during clinical trials. The tiredness often appears during the first few weeks as your body adjusts to the tirzepatide dose, and typically improves over time as you progress through dose titration. Some users report that fatigue is worst immediately after injections, then gradually reduces throughout the week. Unlike nausea or stomach issues, which some people experience consistently, tiredness often becomes less noticeable after 4–6 weeks on each dose level.
- Fatigue may be linked to rapid blood sugar stabilisation
- Reduced appetite and calorie intake can contribute to tiredness
- Dose increases (during titration between 2.5 mg and 15 mg) may temporarily worsen fatigue
- Staying hydrated and maintaining nutrition supports energy levels
- Most people find fatigue improves as their body adapts
If tiredness persists or worsens, it's important to discuss this with your prescriber, as it could also indicate that you're not eating enough or have other health factors affecting energy. Your GP or prescribing clinic can assess whether the fatigue is medication-related or needs further investigation.
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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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Accessing Mounjaro in the UK: NHS vs Private Cost
In the UK, you can access Mounjaro through two routes: NHS specialist services or private prescription. NHS access requires referral to a tier 3 weight management service, which varies significantly by region and often has waiting lists of several months. Private prescription is available through UK-licensed prescribers following an online clinical assessment, with costs typically ranging from £130–£320 per month depending on your dose level and which private provider you use. The KwikPen device itself is the same regardless of route — the difference is the clinical assessment process and cost. Always ensure any prescriber is registered with the GMC or MHRA to ensure safety and legitimacy.
Before starting Mounjaro, you'll need a clinical assessment to confirm eligibility. Your prescriber will review your BMI, medical history, and any weight-related health conditions. You can use our free GLP-1 weight loss calculator to estimate potential results at different doses, and our BMI calculator to check whether you're likely to qualify. Once prescribed, you'll receive guidance on injection technique, storage (refrigerate unopened pens at 2–8°C), and what to expect during dose titration. Common questions about Mounjaro vs Wegovy, side effect management, and medication interactions should be discussed with your prescriber before treatment begins.
- 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 tiredness a serious side effect of Mounjaro?
Fatigue is common but usually not serious. It's listed as a side effect affecting up to 1 in 10 people and typically improves within 4–6 weeks as your body adjusts. However, persistent severe tiredness should be reported to your prescriber, as it could indicate you need nutritional support or have an underlying health issue.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs typically range from £130–£320 per month depending on your dose level (lower doses cost less). NHS access is free but requires specialist referral and varies by region. Always check current pricing with your chosen prescriber, as costs can vary.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is available through NHS tier 3 specialist weight management services. You'll need a referral from your GP and must meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Availability and waiting times vary significantly by region.
How long does Mounjaro fatigue last?
Tiredness usually peaks in the first 1–2 weeks after starting or increasing your dose, then gradually improves over 4–6 weeks as your body adapts. Most people find energy levels normalise, though staying hydrated and eating enough helps. If fatigue persists beyond 8 weeks, discuss with your prescriber.
What's the difference between Mounjaro and Wegovy for tiredness?
Both are GLP-1 medications and both can cause fatigue. Mounjaro is a dual GIP/GLP-1 agonist, Wegovy is GLP-1 only. Clinical trial data shows similar rates of fatigue with both. Individual responses vary — your prescriber can advise which is more suitable based on your health profile and tolerance.
Ready to take control of your weight loss journey?
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