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
What To Do If You Take Mounjaro A Day Late
If you've taken your Mounjaro injection a day late, don't worry — this is a common concern and there's clear guidance. Mounjaro (tirzepatide) is a prescription-only once-weekly GLP-1 and GIP receptor agonist used for weight management in the UK. As long as you take your dose within 4 days of your usual scheduled time, you can proceed as normal and continue your regular injection schedule the following week. This flexibility is built into the medication protocol to account for real-life scheduling challenges. Always check the patient information leaflet that comes with your KwikPen, and contact your prescriber if you have specific concerns about your individual 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 — The 4-Day Window
Mounjaro is designed to be injected once every 7 days. The key guideline is the 4-day window — if you take your dose within 4 days of your scheduled injection day (either before or after), you can safely continue as normal without needing to adjust anything.
- If you're up to 4 days late: inject as soon as you remember, then return to your normal weekly schedule
- If you're more than 4 days late: skip that dose entirely and take your next dose on your regular scheduled day
- Never take a double dose to make up for a missed injection
- The minimum time between two Mounjaro doses is 72 hours (3 days)
This 4-day window gives you flexibility whilst maintaining consistent dosing. If you're unsure whether you fall within the window, calculate the time from your last injection and contact your prescriber or pharmacy for reassurance.
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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.
Staying On Track With Your Mounjaro Schedule
Being a day late occasionally won't significantly impact your weight management progress with Mounjaro. Consistency matters over time, but the medication is robust enough to handle minor timing variations. To avoid future delays, try setting a phone reminder for your usual injection day each week — many people find this simple step eliminates the stress of remembering.
If you frequently struggle with timing, talk to your prescriber about adjusting your scheduled injection day to a time that fits your routine better. For example, if you always forget on Wednesdays, switching to Monday might work better. Your KwikPen should be stored in the fridge between injections, so plan around a time when you're typically at home.
- 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 if I'm only 1 day late with my Mounjaro injection?
One day late is well within the safe 4-day window. Simply inject as soon as you remember and continue with your normal weekly schedule the following week. There's no need to skip doses or adjust anything — this minor timing variation is expected and accounts for normal life happening.
Is it dangerous to take Mounjaro late?
Taking Mounjaro within the 4-day window is safe and does not increase your risk of side effects or complications. The medication is designed with this flexibility built in. If you're concerned about any aspect of your treatment, contact your GP or prescriber for personalised reassurance.
What's the difference between being 1 day late and 4 days late?
Both timings fall within the safe 4-day window, so the guidance is the same for either scenario — inject and continue your regular schedule. The 4-day window exists because the body can tolerate variation in once-weekly dosing without losing efficacy or safety.
Should I skip my dose if I'm a day late?
No, you should not skip your dose if you're a day late. Only skip a dose if you're more than 4 days late. Being 1 day late is minor — inject as soon as you remember and return to your normal weekly schedule the following week.
How do I remember to take Mounjaro on time each week?
Set a phone alarm or calendar reminder for your scheduled injection day and time. Many people choose the same day each week (e.g. "every Monday morning") to build it into their routine. If timing is genuinely difficult, ask your prescriber about changing your injection day to a time that suits your lifestyle better.
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.