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
Is It Ok To Take Mounjaro A Day Early?
Taking Mounjaro a day early is not recommended without speaking to your prescriber first. Mounjaro is a once-weekly injection of tirzepatide, and the dosing schedule is carefully designed to maintain consistent medication levels in your body. While missing a dose by one day occasionally is unlikely to cause serious problems, altering your regular injection schedule without guidance could affect how well the medication works or increase the risk of side effects. Your GP or the prescriber managing your Mounjaro treatment can advise whether early dosing is safe in your specific situation.
10,000+ calculations today · NHS-aligned thresholds · 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/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 — Why Consistency Matters
Mounjaro is designed to be injected once every 7 days — ideally on the same day each week. This consistent schedule helps your body maintain stable levels of tirzepatide, the active ingredient in the KwikPen device. The dose titration schedule is also planned around weekly intervals: you start at 2.5 mg for 4 weeks, then increase to 5 mg, then 7.5 mg, and so on, with a minimum of 4 weeks at each dose level before escalating.
Taking Mounjaro early breaks this pattern and could mean:
- Overlapping doses if you inject a day early and then inject again at your normal time the following week
- Uneven medication levels in your bloodstream
- Potential impact on appetite suppression or weight loss effectiveness
- Increased risk of side effects like nausea or digestive upset
The NHS and MHRA guidance emphasises that prescription medications should be taken exactly as directed. If you're considering changing your injection day, always contact your prescriber first rather than adjusting the schedule yourself.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — 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 — 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.
When It's Safe To Adjust Your Injection Day
If you want to change your regular injection day for practical reasons — such as moving it from Wednesday to Thursday — this is something your prescriber may be able to help with. The key is that you need medical guidance to do this safely.
If you've missed a dose: If you're more than 4 days late for your injection, skip that dose and take your next dose on your normal scheduled day. Never take a double dose to make up for a missed one. If you're only 1–3 days late, you can take the injection as soon as you remember and then resume your normal weekly schedule.
Use our free GLP-1 weight loss calculator above to track your projected outcomes on Mounjaro, and our progress tracker to log your actual results week by week. Your prescriber will monitor your response and adjust your dose or schedule if needed based on your individual progress and tolerance.
- 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 take Mounjaro one day early by accident?
Taking Mounjaro one day early by accident is unlikely to cause serious harm, but it's not ideal. Contact your prescriber for guidance on whether to resume your normal schedule the following week or adjust slightly. Do not take a double dose.
Can I change my Mounjaro injection day permanently?
Yes, but only with prescriber approval. If you need to shift your injection day from, say, Monday to Friday for practical reasons, contact your GP or the online prescriber who manages your treatment. They can advise the safest way to do this.
What's the minimum time between Mounjaro injections?
The minimum safe interval between Mounjaro doses is 72 hours (3 days). Never inject more frequently than this. Most people inject every 7 days on the same day each week to maintain consistency.
If I miss my Mounjaro injection, when should I take it?
If you're fewer than 4 days late, take it as soon as you remember. If you're more than 4 days late, skip that dose and take your next dose on your normal scheduled day. Never double up to catch up.
Why does Mounjaro need to be taken on a strict schedule?
The weekly schedule maintains steady tirzepatide levels in your bloodstream, ensuring consistent appetite suppression and weight loss support. The dose titration (stepping up gradually) also requires weekly intervals to safely assess your tolerance before increasing.
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.