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 Does Mounjaro Make You So Tired?
Fatigue and tiredness are common side effects reported by people taking Mounjaro (tirzepatide) in the UK. This happens because Mounjaro works by slowing how quickly food moves through your stomach and signalling fullness to your brain — both of which require energy. Nausea and reduced food intake can also contribute to tiredness. The good news is that fatigue typically improves as your body adjusts to the medication over the first few weeks. Understanding why this happens and knowing what to expect can help you manage the side effect while you're taking this prescription-only weight management injection.
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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.
Why Mounjaro Causes Fatigue and How It Relates to Cost
Tiredness on Mounjaro happens for several interconnected reasons. First, dose titration matters — when you start at 2.5 mg and increase gradually to 5 mg, 10 mg, or 15 mg, your body is adjusting to increasingly powerful GIP and GLP-1 receptor activation. This dual mechanism (unlike Wegovy's single GLP-1 action) places higher metabolic demands on your system. Second, reduced food intake means fewer calories and nutrients reaching your bloodstream, which naturally reduces available energy. Third, nausea — listed as a very common side effect in clinical data — drains mental and physical energy. Many people also report improved appetite suppression means they're eating smaller portions and may not be consuming enough iron, B vitamins, or protein, all essential for energy production. Most users find fatigue peaks in weeks 1–4 after dose increases, then improves as the body acclimates. If tiredness persists beyond 4–6 weeks, discuss this with your prescriber — you may need a dose adjustment or additional support.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
UK Prescription Costs and NHS vs Private Access Routes
In the UK, accessing Mounjaro depends on your route. NHS access is through tier 3 specialist weight management services following a GP referral — it's free but waiting lists can extend months or years depending on your ICB (integrated care board) region. Private online prescribing typically costs £129–£320 per month depending on your dose, with the KwikPen containing four weeks of injections. A private online consultation with a UK-licensed prescriber usually costs £20–£50 and is conducted remotely. Once approved, your prescription is dispensed by a GPhC-registered pharmacy and delivered to your home. Many people choose private access to avoid NHS waiting times, though eligibility and pricing vary between providers. Always check that any prescriber is GMC-registered and the pharmacy is GPhC-approved before proceeding.
- 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.
How long does Mounjaro fatigue typically last?
Most users experience peak tiredness in the first 4 weeks after starting or after each dose increase. For many people, fatigue improves significantly by weeks 5–8 as the body adjusts. If tiredness persists beyond 6 weeks, contact your prescriber — it may indicate you need a slower dose escalation or nutritional support.
Can I reduce Mounjaro fatigue by eating more?
Partially. While eating smaller meals is part of how Mounjaro works, you still need adequate nutrition. Focus on nutrient-dense foods — lean proteins, iron-rich vegetables, whole grains, and B vitamins — rather than simply eating larger volumes, which can trigger nausea.
Is Mounjaro fatigue worse than Wegovy tiredness?
Some users report more pronounced tiredness on Mounjaro compared to Wegovy, likely because tirzepatide's dual GIP/GLP-1 mechanism is more potent. However, individual responses vary greatly — some find Mounjaro more manageable. Use our free GLP-1 weight loss calculator to explore both options.
What's the cheapest way to get Mounjaro in the UK?
NHS access is free if you qualify and can access a tier 3 service, though waiting lists are long. Private online prescribing costs £129–£320/month depending on dose. Compare providers carefully — prices and consultation fees vary, but always verify the prescriber is GMC-registered.
Should I stop Mounjaro if I'm too tired?
Do not stop without guidance. Contact your prescriber first — tiredness often improves with time, and stopping abruptly can cause rebound hunger. Your prescriber may adjust your dose, suggest nutritional changes, or recommend a slower titration schedule instead.
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.