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 in the UK
There is no single "best day" to inject Mounjaro โ you can choose any day of the week that suits your routine, as long as you inject once every seven days. What matters most is consistency: picking a day you'll remember and sticking to it each week. Your prescriber will advise you on timing when you start treatment, and you can adjust your injection day if needed. Using our GLP-1 weight loss calculator can help you understand potential outcomes as you progress through your tirzepatide dose titration.
10,000+ calculations today ยท NHS-aligned thresholds ยท 100% free
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/mยฒ).
While BMI isn't a perfect measure โ it doesn't account for muscle mass, bone density, or fat distribution โ 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 โ 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 and Consistency
Mounjaro (tirzepatide) is a once-weekly injection, meaning you inject the same dose every seven days. The specific day you choose is flexible โ Monday, Wednesday, Friday, or any other day works equally well. What's important is that you maintain consistent timing between injections: if you inject on a Monday one week, aim to inject on a Monday the following week.
- Flexibility: You can change your injection day if needed โ for example, if Monday doesn't fit your schedule, switching to Thursday is fine as long as you maintain seven-day intervals
- Time of day: You can inject in the morning or evening โ there is no clinical advantage to either
- Timing window: If you're a day late or early, this is generally acceptable, but try to stay within a consistent seven-day schedule
- Missed doses: If you miss your injection, take it within four days; if more than four days have passed, skip that dose and continue your normal schedule
Your prescriber will help you select an injection day when you start tirzepatide and discuss how to manage the Mounjaro KwikPen device safely.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care โ from a UK-regulated online pharmacy.
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 โ 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 โ completely free.
Consistency and NHS Prescribing Guidance
The UK's NICE guidance emphasises that consistency in GLP-1 and dual GIP/GLP-1 receptor agonist use is key to effectiveness. This means maintaining regular weekly injections rather than worrying about a specific day of the week. Your GP or specialist prescriber at an NHS tier 3 weight management service will give you detailed instructions on injection technique, storage, and scheduling when you receive your first prescription.
Many people find it helpful to link their injection day to a routine habit โ such as Sunday evening or Friday morning โ making it easier to remember. If you're receiving Mounjaro privately through a UK-registered online pharmacy, your prescriber will also provide written guidance on timing and what to do if you miss a dose.
- 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 โ 5mg โ 7.5mg โ 10mg โ 12.5mg โ 15mg
- GLP-1 receptor agonist
- Up to 15โ17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg โ 0.5mg โ 1mg โ 1.7mg โ 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 | 15โ22.5% | 12โ17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20โ32 weeks | 16โ20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | ยฃ150โยฃ250/month | ยฃ150โยฃ300/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 โ 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 change my Mounjaro injection day?
Yes. If your current injection day doesn't suit your schedule, you can switch to a different day as long as you maintain seven-day intervals between injections. Inform your prescriber of any changes, but flexibility is one of the benefits of once-weekly tirzepatide injections.
Does it matter if I inject in the morning or evening?
No. There is no clinical difference between morning and evening injections. Choose a time that fits your routine and helps you remember to inject consistently each week.
What if I inject a day late?
Injecting one day late is generally acceptable and won't significantly affect your treatment. However, if you miss your dose by more than four days, skip that dose and resume your normal schedule the following week. Never inject double doses to catch up.
Should I set a reminder for my Mounjaro injection?
Yes, setting a phone reminder or linking your injection to a weekly routine (like Sunday evening) can help ensure consistency. Consistency is more important than the specific day you choose.
Does Mounjaro work better if injected at a certain time of week?
No. Clinical trials showing Mounjaro's effectiveness used once-weekly injections regardless of the specific day chosen. Consistency and adherence matter far more than timing on a particular day of the week.
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.