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
Best Day to Inject Mounjaro — Scheduling Your Weekly Dose
There is no single "best day" to inject Mounjaro — what matters most is choosing a day that works for you and sticking to it consistently every week. Mounjaro (tirzepatide) is a once-weekly injection, and the key to safe, effective use is maintaining a regular schedule with at least 72 hours between doses. Your prescriber will guide you on timing based on your lifestyle, but the day you choose matters far less than the consistency with which you use it.
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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.
How to Choose Your Injection Day
Mounjaro is designed for once-weekly dosing, meaning you inject the same day every seven days. Rather than a specific "best day," focus on selecting a day that fits naturally into your routine. Many people choose a day when they're at home and relaxed — perhaps a weekend day or a regular weekday morning. The consistency matters far more than the calendar date.
- Pick a day you're unlikely to forget
- Choose a time when you can rest afterward if needed (some people experience nausea initially)
- Ensure at least 72 hours (3 days) between each injection
- Set a phone reminder to help you remember
- If you miss a dose within 4 days, inject as soon as you remember; then continue on your normal schedule
Your prescriber will confirm your chosen schedule during your clinical assessment. Using our GLP-1 weight loss calculator can help you understand how consistent dosing may support your weight management goals over time.
Clinically BackedWeight Loss
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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 Happens If You Inject at Different Times
While Mounjaro works best on a consistent weekly schedule, occasionally injecting a day or two earlier or later won't harm you — as long as there's a minimum of 72 hours between doses. The tirzepatide in the Mounjaro pen is designed to work over a seven-day cycle, so minor timing variations are acceptable. However, regularly changing your injection day could make it harder to remember when to take your next dose, which is why consistency is emphasised. If you're travelling or your routine changes temporarily, discuss adjusted timing with your GP or prescriber.
---COL_J_TOOLS--- Use our free BMI calculator to monitor how your weight management progresses with consistent Mounjaro use, and our progress tracker to log your journey week by week.- 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.
Can I inject Mounjaro on a different day each week?
Not ideally. Mounjaro works best when injected the same day every week. While occasional timing variations (within 72 hours) won't cause harm, keeping a consistent day makes it easier to remember and maintain your schedule. Ask your prescriber to help you choose a day that fits your routine.
What happens if I inject too early or too late?
If you inject less than 72 hours after your previous dose, you should wait. If it's been more than 4 days since your last dose, inject as soon as you remember, then resume your normal weekly schedule. Your prescriber will clarify this during your assessment.
Is it better to inject Mounjaro in the morning or evening?
There's no clinical evidence that morning or evening is "better." Choose whichever time suits your routine best — morning if you're more alert then, evening if you prefer to rest afterward. Consistency with your chosen time is more important than the specific hour.
Can I change my injection day if needed?
Yes, but discuss this with your prescriber or GP first. If your routine changes, you can adjust your injection day — just maintain at least 72 hours between doses and pick a new consistent day moving forward.
Does the day I inject affect how well Mounjaro works?
No. The effectiveness of tirzepatide (Mounjaro's active ingredient) depends on consistent weekly dosing and maintaining the right dose for your needs — not on which specific day of the week you inject. Consistency matters far more than the calendar date.
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.