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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Can You Change Your Mounjaro Injection Day?
If you're taking Mounjaro (tirzepatide), a weekly weight loss injection licensed in the UK, you may need to change your injection day for practical reasons. The good news is that adjusting your scheduled injection day is usually possible — but it must be done correctly to maintain consistent dosing. This guide explains how to safely change your Mounjaro injection day, the rules you need to follow, and when to contact your prescriber or pharmacy for support.
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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.
How to Safely Change Your Mounjaro Injection Day
Changing your Mounjaro injection day is safe provided you follow one key rule: maintain at least 3 days (72 hours) between injections. This spacing is critical to ensure your body receives the correct dose on schedule. When you change your day, think of it as a one-off adjustment rather than permanently skipping a dose.
The safest approach: If you normally inject on a Monday and want to move to Wednesday, simply take your next dose on Wednesday instead. From then on, Wednesday becomes your new injection day. The gap between your last Monday injection and your first Wednesday injection will be longer than usual (likely 8–10 days), but this single longer gap is safe.
- Never inject more than once every 72 hours
- Plan your change in advance with your prescriber or pharmacy
- Note the date of your last injection before switching
- Confirm your new injection day with your pharmacy to avoid confusion
If you're using our GLP-1 weight loss calculator, you can also track your injection schedule alongside your projected weight loss results to stay organised.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS and Private Prescribing Considerations
If you receive Mounjaro through an NHS specialist weight management service (tier 3), your prescriber should support schedule changes — discuss timing with your clinic before switching days. For private prescriptions, contact your online prescriber or pharmacy directly before making any changes; they'll confirm your new injection day and update your prescription records accordingly.
Keep a clear record of your injection dates, especially when changing your day. This helps your prescriber monitor your progress accurately and ensures any side effects or results are properly tracked. Your pharmacy can also issue a new prescription schedule aligned with your preferred injection day, preventing mix-ups when ordering future KwikPen devices.
- 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.
What happens if I accidentally skip a Mounjaro injection?
If you miss your injection, take it within 4 days of the missed date and continue your normal schedule from there. If more than 4 days have passed, skip that dose and take your next injection on your regular scheduled day. Never double-dose to catch up.
Can I change my Mounjaro injection day permanently?
Yes. You can switch to a new injection day permanently by adjusting once and then maintaining that new day going forward. Ensure your prescriber knows your new schedule so they can update your records and prescription timing.
What's the minimum time between Mounjaro injections?
The minimum safe interval is 72 hours (3 days) between injections. Do not inject more frequently than this, as it could lead to overdosing and increased side effects.
Do I need my GP's permission to change my Mounjaro injection day?
If you're on an NHS prescription, inform your specialist weight management service before changing your day. For private prescriptions, contact your prescriber or online pharmacy to notify them of the change and update your records.
How do I track my injection day if I change it?
Mark your new injection day on a calendar or set a phone reminder for your preferred day each week. Many patients use their pharmacy's reminder service or prescription app. You can also use our free progress tracker to log your injection dates alongside your weight loss journey.
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.