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 I Take My Mounjaro Shot A Day Early?
Taking your Mounjaro injection a day early isn't recommended without speaking to your GP or prescriber first. Mounjaro is designed as a once-weekly injection with a specific dosing schedule, and altering the timing could affect how the medication works. This guide explains the recommended injection schedule, how Mounjaro timing compares to Wegovy, and what you should do if you've missed or need to reschedule a dose. Always consult your prescriber for personalised guidance on your specific situation.
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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.
Mounjaro Injection Schedule vs Wegovy — Timing and Safety
Mounjaro (tirzepatide) is designed to be taken as a once-weekly injection, ideally on the same day each week. The minimum time between doses is 72 hours (3 days), but this is the lower safety threshold, not the ideal schedule.
Why timing matters: Mounjaro works by maintaining steady levels of the GIP and GLP-1 receptor activation in your body. Taking it too close to your last dose could affect blood levels and tolerability, whilst taking it too far apart may reduce effectiveness.
Wegovy comparison: Wegovy (semaglutide) also requires once-weekly dosing with the same 72-hour minimum gap between injections. Both medications follow the same strict weekly schedule because their effectiveness depends on consistent, spaced dosing.
If you're considering taking your Mounjaro shot early, the safest approach is to contact your GP, prescriber, or the pharmacy that dispensed your KwikPen. They can advise based on your individual health and medication history.
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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've Missed or Need to Reschedule Your Dose?
If you've missed your Mounjaro dose, the guidance is straightforward:
- If it's within 4 days of your scheduled injection day, take it as soon as you remember, then resume your normal weekly schedule
- If it's more than 4 days late, skip that dose and take your next dose on your usual day
- Never take a double dose to make up for a missed one
If you need to reschedule regularly (for example, changing your injection day from Monday to Friday), speak to your prescriber — they may adjust your schedule formally to prevent confusion. Use our free GLP-1 weight loss calculator to track your progress and plan your injection calendar alongside your weight loss journey.
- 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 between Mounjaro injections?
The minimum time between Mounjaro doses is 72 hours (3 days). However, Mounjaro is designed for once-weekly dosing, and the 72-hour gap is a safety threshold, not a target. Taking it exactly 3 days apart repeatedly isn't recommended — stick to your weekly schedule as prescribed by your GP or prescriber.
Will taking Mounjaro a day early affect its effectiveness?
Taking Mounjaro too close to your previous dose could affect how your body processes the medication and may increase the risk of side effects. Mounjaro's effectiveness depends on consistent weekly spacing. If you're concerned about effectiveness, discuss this with your prescriber rather than adjusting your schedule yourself.
How does Mounjaro dosing compare to Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections with a 72-hour minimum gap between doses. The key difference is their mechanism: Mounjaro is a dual GIP + GLP-1 agonist, whilst Wegovy is a GLP-1-only agonist. Both require the same consistent weekly schedule for optimal results.
What should I do if I need to change my Mounjaro injection day?
If you need to change your regular injection day (for example, from Monday to Wednesday), contact your prescriber or GP before making the change. They can formally adjust your schedule and clarify when to take your next dose. Don't attempt to reschedule without professional guidance.
Can I take Mounjaro early if I'm going on holiday?
If you're travelling and need to adjust your Mounjaro schedule for holiday dates, contact your GP or prescriber in advance. They may be able to adjust your injection day slightly, but this should always be done with professional approval. Plan ahead to avoid taking doses too close together or too far apart.
Ready to take control of your weight loss journey?
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