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
Free tracker included with every download this week
Can I Inject Mounjaro A Day Early?
No, you should not inject Mounjaro a day early. Mounjaro (tirzepatide) is designed as a once-weekly injection with a minimum of 72 hours (3 days) between doses. Injecting too soon could affect how the medication works and increase the risk of side effects. If you're considering adjusting your injection timing, it's important to understand the correct scheduling and speak with your prescriber about your specific situation.
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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.
Mounjaro Injection Timing and Dose Schedule
Mounjaro is a once-weekly injection delivered via the KwikPen device. The standard schedule means you inject on the same day each week — for example, every Monday at the same time. The minimum interval between injections is 72 hours (3 days), meaning you cannot safely inject closer together than this without medical guidance.
- Injecting a day early would be only 6 days after your last dose instead of the intended 7 days
- While one day early is unlikely to cause immediate harm, it disrupts your dose titration schedule and blood levels of tirzepatide
- Your prescriber has calculated your current dose based on a 7-day interval — changing this without guidance can affect efficacy and side effects
- If you consistently inject early, you'll exhaust your monthly supply faster and run short before your next prescription
If you want to change your injection day permanently (for example, moving from Monday to Wednesday), your prescriber can advise on the safest way to do this with the minimum gap of 72 hours.
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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.
What If You Miss Your Regular Injection Day?
Missing a Mounjaro injection is different from injecting early. If you miss your scheduled day, NHS guidance and the Mounjaro patient information leaflet recommend: Within 4 days of your missed dose, inject as soon as you remember, then continue your normal weekly schedule from that point. If more than 4 days have passed since your scheduled day, skip that dose and take your next injection on your normal scheduled day.
- Never take a double dose to make up for a missed injection
- Always maintain at least 72 hours between injections
- Consult your prescriber if you're unsure about your timing, especially if you're in the dose escalation phase
Use our free GLP-1 weight loss calculator to track your progress while maintaining proper injection timing, and speak with your prescriber or GP if you have concerns about your schedule.
- 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 inject Mounjaro one day early?
Injecting one day early means only 6 days between doses instead of 7, which can disrupt your blood levels and may affect how well the medication works. It also means your monthly supply will run out early. While unlikely to cause immediate serious harm, it's not recommended. Maintain the 7-day schedule your prescriber set.
Can I change my Mounjaro injection day?
Yes, you can change your regular injection day, but this must be discussed with your prescriber. They can advise on the safest way to adjust, ensuring at least 72 hours (3 days) between your last injection and your new scheduled day. Do not make changes without medical guidance.
What's the minimum time between Mounjaro injections?
The minimum interval between Mounjaro injections is 72 hours (3 days). You should not inject sooner than this without specific guidance from your prescriber. The standard once-weekly schedule means 7 days between doses.
Will injecting Mounjaro early affect weight loss results?
Yes, changing your injection timing can affect how well Mounjaro works. Your dose titration and blood levels are calculated based on a 7-day interval. Injecting early disrupts this schedule, potentially affecting appetite suppression and weight loss effectiveness. Stick to your prescribed schedule.
What should I do if I'm about to miss my Mounjaro injection?
If you're within 4 days of your missed dose, inject as soon as you remember, then continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your normally scheduled day. Never inject earlier than planned or take a double dose.
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.