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
Can You Do Mounjaro A Day Early?
No, you should not do Mounjaro a day early. Mounjaro is designed as a once-weekly injection, and the guidance is clear: maintain at least 72 hours (3 days) between doses. If you've missed your scheduled injection, you can take it within 4 days of the missed date and then continue your normal weekly schedule. Taking it early risks disrupting your dose titration schedule and may increase side effects. Always check your patient information leaflet or contact your prescriber if you're unsure about timing.
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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 Dosing Schedule And Timing
Mounjaro (tirzepatide) comes in a KwikPen pre-filled device and is prescribed as a once-weekly injection. The standard guidance from MHRA-approved patient information is:
- Minimum gap between injections: 72 hours (3 days)
- If you miss a dose within 4 days, inject as soon as you remember, then continue your normal weekly schedule
- If you miss a dose by more than 4 days, skip that dose and take your next injection on the scheduled day
- Never take a double dose to make up for a missed injection
Taking your injection early (less than 72 hours after your last dose) is not recommended because it disrupts the dose titration protocol that your prescriber has planned for you. The NHS tirzepatide pathway assumes consistent weekly dosing, as clinical trials (SURMOUNT-1) that established safety and efficacy used this schedule.
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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.
Why Consistency Matters With Mounjaro Dosing
Your prescriber has timed your Mounjaro dose escalation carefully. Most patients spend 4 weeks on each dose level before increasing—this allows your body to adjust and helps your prescriber monitor for side effects. Taking an injection early could compress this timeline unexpectedly and increase nausea, vomiting, or other gastrointestinal effects. If you're considering taking Mounjaro early because you're travelling or have a schedule change, contact your GP or prescriber first. They may advise adjusting your injection day going forward to fit your routine, rather than taking it early once. Your UK prescription was issued with a specific schedule in mind, and consistency helps ensure the treatment works safely.
- 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 safe gap is 72 hours (3 days) between doses. Never inject before this time has passed. Once-weekly dosing means you should maintain approximately 7 days between injections unless your prescriber advises otherwise.
Can I move my Mounjaro injection day forward permanently?
Possibly, but only with your prescriber's approval. If your current day doesn't suit your schedule, contact your GP or the clinic where you were prescribed Mounjaro. They can advise whether it's safe to adjust your regular injection day going forward.
What if I accidentally do Mounjaro a day early?
If you've injected less than 72 hours after your last dose by accident, contact your GP or NHS 111 for guidance. Monitor for increased side effects like nausea or vomiting. Don't inject again until at least 72 hours have passed since the early dose.
Does the SURMOUNT trial data support early dosing?
No. SURMOUNT-1 and clinical trials that led to MHRA approval used strict once-weekly scheduling. The efficacy and safety data (average clinically studied weight loss at 15 mg) is based on consistent weekly timing, not early or variable dosing.
Can I use MyWeightCheck's GLP-1 calculator with adjusted Mounjaro timing?
Our weight loss calculator uses standard once-weekly dosing intervals. If you've altered your injection schedule with prescriber approval, the projections may shift. Always discuss any dosing changes with your prescriber and recalculate based on your actual new schedule.
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.