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?
Yes, you can change your Mounjaro injection day if needed, but it requires careful planning to maintain the correct dosing schedule. Mounjaro is a once-weekly weight loss injection, and changing your regular injection day is possible as long as you follow specific timing guidelines. This guide explains how to adjust your injection schedule safely and what to discuss with your prescriber.
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Calculate Your BMI Instantly
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 Change Your Mounjaro Injection Day Safely
Changing your Mounjaro injection day is straightforward but must follow strict timing rules to ensure you receive the correct dose. The key rule is maintaining at least 72 hours (3 days) between injections. This means you cannot give yourself two injections closer together than 3 days apart.
If you want to move your injection day earlier, you can inject as soon as 72 hours have passed since your last injection. If you want to move it later, simply wait until your normal schedule allows. For example, if you normally inject on Monday, you could change to Thursday (4 days later) or Friday (5 days later), but not Wednesday (only 2 days).
- Wait at least 72 hours from your last injection before the next one
- Plan your new day in advance with your prescriber if possible
- Mark your new injection day clearly on a calendar to avoid confusion
- Check the KwikPen storage requirements after any schedule change (pens can be stored at room temperature for up to 30 days after opening)
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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.
Why You Might Want to Change Your Injection Day
Many people on Mounjaro find it helpful to change their injection day for practical reasons. Common reasons include:
- Work or travel schedules that make your regular day inconvenient
- Clinic or GP appointment availability
- Personal preference for a different day of the week
- Life circumstances that make a new day more manageable
Whatever your reason, your prescriber can help you plan a safe transition. Always discuss any schedule change with your GP or the healthcare professional managing your Mounjaro treatment. They can confirm the new timing, update your records, and provide specific guidance based on your current dose and treatment stage.
- 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 is the minimum time between Mounjaro injections?
The minimum time between injections is 72 hours (3 days). You cannot inject Mounjaro closer together than this. If you miss a dose and remember within 4 days, inject as soon as you remember, then continue your normal weekly schedule.
Can I change my Mounjaro injection day multiple times?
Yes, you can adjust your injection day as needed, provided you always maintain at least 72 hours between injections. Each time you change your day, ensure the new schedule is sustainable long-term, and inform your prescriber of the adjustment.
What should I do if I accidentally inject too soon after my last dose?
If you inject closer than 72 hours from your previous dose by mistake, contact your prescriber or NHS 111 immediately for guidance. They can assess whether any action is needed and help you establish a safe new injection schedule going forward.
How do I remember my new Mounjaro injection day after changing it?
Set a phone reminder or alarm for your new day, mark it on a calendar, use a pill organiser, or sync it with another weekly habit (like grocery shopping or a gym day). Consistency is key, so pick a routine that works best for you.
Will changing my injection day affect my Mounjaro dose or weight loss?
As long as you maintain the correct 7-day spacing overall and follow the 72-hour minimum rule, changing your regular day will not affect your dose or weight loss results. Your prescriber can use the GLP-1 weight loss calculator to project outcomes once your new schedule is established.
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.