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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What Happens If You Take Mounjaro A Day Early?
Taking your Mounjaro injection a day early is generally safe, but it's important to understand how dose timing works and how to avoid confusion with future injections. Mounjaro (tirzepatide) is a once-weekly injection, and consistent timing helps maintain steady medication levels in your body. This guide explains what to do if you've taken your dose early, how to reset your schedule, and when to contact your GP or prescriber for personalised advice.
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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.
Understanding Mounjaro Dose Timing
Mounjaro is designed to be taken exactly once per week. Each dose should be roughly 7 days apart. The standard guidance is to inject on the same day each week — for example, every Monday or every Thursday. If you've taken your injection one day early, the gap between your last dose and this one will be shorter than usual (6 days instead of 7), and your next scheduled dose will be 8 days away.
The minimum time between doses is 72 hours (3 days). Taking a dose one day early does not violate this safety rule. However, to keep your dosing schedule consistent going forward, you'll need to reset your injection day. The manufacturers and NHS guidance recommend that if you take Mounjaro early, you should then continue with your new injection day — don't try to get back to your original day as this can lead to doses being too close together.
Our GLP-1 weight loss calculator can help you track your expected progress, and it's useful to note your actual injection dates so your prescriber can give you accurate advice if anything changes.
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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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What To Do If You've Taken Mounjaro Early
If you've accidentally taken your Mounjaro injection one day early, there's no need to panic. One day is a minor variation and is unlikely to cause harm. The key is to establish a new consistent weekly schedule going forward. Choose your new injection day and stick to it — this might mean your next dose is 8 days away instead of the usual 7, but this is acceptable.
Contact your prescriber if:
- You've taken two doses within 72 hours (3 days) — this is outside safe guidance
- You're unsure about resetting your schedule
- You experience unusual symptoms or side effects
- You're taking other medications that might be affected by timing changes
Your GP or online prescriber can confirm the best new injection day and update your records so there's no confusion at your next refill.
- 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.
Is it dangerous to take Mounjaro a day early?
No, taking Mounjaro one day early is not dangerous. The minimum safe interval between doses is 72 hours (3 days), so one day early poses no safety risk. The important thing is to establish a new consistent weekly schedule afterwards to maintain steady medication levels.
What's the difference between Mounjaro and Wegovy for dose timing?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections with the same 72-hour minimum interval rule. Dose timing guidance is identical for both medications — consistency week-to-week is what matters most for maintaining therapeutic effect.
Should I adjust my next dose if I took Mounjaro early?
No, you should not take an extra dose or skip a dose. Simply pick your new weekly injection day and continue as normal. If you took today's dose one day early, your next dose should be seven days from today. Your prescriber can confirm the best new schedule.
What if I took Mounjaro early and now I'm confused about my schedule?
Contact your prescriber or GP immediately with the date you took your early dose. They can review your records, confirm the correct next injection date, and reset your prescription schedule. This avoids future confusion and ensures you receive the right doses at the right times.
Does taking Mounjaro early affect weight loss results?
One dose taken one day early will not significantly affect your weight loss results. Mounjaro's effectiveness depends on consistent weekly dosing over weeks and months, combined with dietary changes and physical activity. What matters is maintaining your new weekly schedule going forward.
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.