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 Change My Mounjaro Injection Day?
Yes, you can change your Mounjaro injection day, but there are specific rules you need to follow. Mounjaro is a prescription-only weight loss injection that works as a dual GIP and GLP-1 receptor agonist, and timing matters for safety and effectiveness. If you need to reschedule your weekly injection, the key rule is maintaining at least 72 hours (3 days) between doses. Your prescriber can help you pick a new regular day that fits your schedule better. Always discuss any changes with your GP or the prescriber who issued your prescription.
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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.
The 72-Hour Rule for Rescheduling
The most important guidance for changing your Mounjaro injection day is the minimum 72-hour gap between doses. This means you must wait at least 3 full days between one injection and the next. If your regular day is Monday and you want to switch to Wednesday, you can do this — but you cannot inject on Tuesday (only 1 day gap) or Wednesday morning if you injected Monday morning. You need to allow the full 72 hours to pass. If you've missed a dose or want to reschedule, ask your prescriber for guidance on the best new schedule. They may suggest a gradual shift over 1–2 weeks to transition smoothly to your new preferred day. Do not attempt to make up a missed dose by injecting sooner than 72 hours after your last injection — this could affect your dose titration schedule and potentially increase side effects.
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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.
How to Change Your Mounjaro Injection Day Safely
Contact your prescriber or GP before making any permanent changes to your injection schedule. They have your medical records and can advise on the safest way to transition. If you're within 4 days of a missed dose, you can take it as soon as remembered and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your usual day. For a planned reschedule (for example, switching from Monday to Friday), discuss with your prescriber whether a gradual shift works better than a sudden change. Keep a note of your injection dates — use our free progress tracker to log each dose and help you remember your new day. Some people find setting a phone reminder on their chosen day helpful for staying consistent with the weekly injection 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's the minimum time I must leave between Mounjaro doses?
You must leave at least 72 hours (3 full days) between doses. If your regular injection is Monday, the earliest you can inject next week is Thursday. Never inject before 72 hours have passed, even if you want to change your injection day.
Can I switch my Mounjaro day from Monday to Friday?
Yes, you can switch days, but you must maintain the 72-hour gap. If you normally inject Monday, you can gradually shift to Friday over 1–2 weeks by discussing with your prescriber. They'll guide you through the safest transition without disrupting your dose titration.
What if I accidentally miss my Mounjaro injection?
If you remember within 4 days of the missed dose, inject as soon as you can, then continue your normal weekly schedule from that point. If more than 4 days have passed, skip that dose and resume your regular injection day. Never double-dose to catch up.
Can my prescriber help me change my injection schedule?
Yes — always discuss timing changes with your GP or prescriber first. They know your dose level and health history. They can recommend the safest way to reschedule or shift your injection day without affecting your treatment effectiveness.
Will changing my injection day affect my weight loss with Mounjaro?
No, changing your injection day should not affect results as long as you maintain the once-weekly schedule and the 72-hour minimum gap between doses. Consistency is more important than the specific day — choose a day that works for your routine and stick to it.
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.