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 I Take Mounjaro 2 Days Late?
Yes, you can take Mounjaro up to 4 days late without affecting your treatment schedule. If you miss your weekly injection and remember within 96 hours (4 days), take it as soon as possible and continue your normal weekly schedule from there. However, if more than 4 days have passed, skip that dose and take your next injection on your scheduled day. Always speak to your prescriber if you have concerns about timing. Our free weight loss calculator can help you track progress alongside your Mounjaro treatment plan.
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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/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 Timing: What the Data Shows
Mounjaro KwikPen dosing is designed for once-weekly administration — each injection contains a 7-day dose of tirzepatide. Clinical guidance allows flexibility within a specific window to account for real-life scheduling challenges.
- Within 4 days (96 hours) late: Take your injection as soon as you remember, then continue your normal weekly schedule
- More than 4 days late: Skip the missed dose entirely and take your next dose on your scheduled day
- Never double-dose: If you accidentally take two doses within a short timeframe, contact your prescriber immediately
- Minimum spacing: Always leave at least 72 hours (3 days) between injections
The flexibility built into Mounjaro dosing recognises that busy schedules happen — the medication is formulated to tolerate this minor variation without compromising efficacy. Your prescriber can provide personalised guidance if you're concerned about consistency.
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.
Mounjaro vs Wegovy: Injection Timing Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are UK-licensed GLP-1 weight loss injections given once weekly, so timing flexibility is similar. However, there are key mechanism differences: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. In clinical trials, Mounjaro showed average weight loss of approximately 20.9% at the highest dose (15 mg) in the SURMOUNT trial, compared to around 14.9% with Wegovy in the STEP-1 trial. Neither medication is superior — which is right for you depends on individual health factors, which your prescriber will assess during your NHS NICE–aligned clinical assessment.
- 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 happens if I'm 3 days late taking my Mounjaro injection?
If you're within 4 days (96 hours) of your scheduled dose time, you can take your Mounjaro KwikPen as soon as you remember. Once you've taken it, continue your normal weekly injection schedule from that date. There's no need to adjust your future doses — this built-in flexibility is intentional in the medication design.
Is there a penalty if I miss my Mounjaro dose?
Occasional missed doses within the 4-day window don't significantly affect treatment outcomes. However, consistency matters for sustained weight loss results. If you're struggling to remember your weekly injection, set a phone reminder, use a calendar alert, or ask your prescriber about strategies to help you stay on track.
Can I take Mounjaro early instead of late?
Mounjaro is designed for once-weekly dosing. Taking injections closer together than 72 hours (3 days) apart is not recommended and may increase side effects. If you're considering changing your injection day for convenience, speak to your prescriber — they can help you adjust to a new consistent schedule.
What's the difference between Mounjaro and Wegovy timing?
Both are once-weekly injections with similar flexibility — you can take either within 4 days of your scheduled time. The main differences are the mechanism (Mounjaro has dual GIP+GLP-1 action vs Wegovy's GLP-1 only) and dose ranges. Your prescriber will advise which is appropriate based on your UK clinical assessment.
What should I do if I'm more than 4 days late?
If you've missed your Mounjaro dose by more than 4 days, skip that dose and take your next injection on your normal scheduled day. Don't try to catch up with a double dose. Contact your prescriber if you're unsure, especially if this happens regularly.
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.