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
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Mounjaro Half-Life Chart — How Long It Stays in Your System
Mounjaro (tirzepatide) has a half-life of approximately 5 days, meaning it takes about 5 days for your body to eliminate half of the dose you've injected. This extended half-life is why Mounjaro is given once weekly — the medication builds up gradually in your system over several weeks. Understanding how long tirzepatide remains active helps explain why dosing adjustments take time and why missing doses requires specific guidance. Our GLP-1 weight loss calculator can help you understand how different Mounjaro doses may affect your treatment timeline.
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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.
How Mounjaro's Half-Life Affects Your Dosing Schedule
Mounjaro's 5-day half-life means the medication accumulates in your body over time. After your first injection, only half the dose remains after 5 days. By day 10, only 25% remains. However, your next weekly injection adds a fresh dose, so levels build steadily. It typically takes 4–5 weeks of consistent weekly injections to reach steady-state levels — where the amount of tirzepatide in your system stabilises.
This is why your prescriber recommends staying on each dose for at least 4 weeks before increasing. This allows you to experience the full effect of that dose and assess how well you're tolerating it. The accumulation pattern also explains why if you miss a dose by a few days, you can still take it without major impact — some tirzepatide from your previous injection remains active.
- Week 1–4: 2.5 mg dose — initial adjustment phase
- Week 5–8: 5 mg dose — first maintenance level
- Week 9+: 7.5 mg, 10 mg, 12.5 mg, or 15 mg — escalation as tolerated
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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.
Missed Doses and Half-Life Guidance
Because Mounjaro remains in your system for several days, missing a single injection isn't an emergency. If you miss a dose within 4 days of when you should have taken it, inject as soon as you remember and resume your normal schedule the following week. If more than 4 days have passed, skip that dose and take your next injection on the regular scheduled day.
Never take a double dose to make up for a missed injection. The minimum time between injections should be 72 hours (3 days). If you regularly miss doses, discuss this with your prescriber — they may recommend switching to a different medication or adjusting your reminder system. Understanding the half-life helps clarify why the NHS and your prescriber give these specific guidance, as leaving significant gaps in your treatment affects the medication's effectiveness. Track your doses and progress using our free progress tracker.
- 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.
How long does Mounjaro stay in your body after one injection?
Mounjaro's half-life is approximately 5 days. After 5 days, half the dose remains in your system; after 10 days, roughly 25% remains. Because of this extended half-life, it's given once weekly rather than daily. The medication accumulates over 4–5 weeks to reach steady-state levels where consistent therapeutic effects are experienced.
Why does my prescriber want me to stay on each Mounjaro dose for 4 weeks?
The 4-week period allows tirzepatide to accumulate to steady-state levels in your body. This gives you and your prescriber time to assess tolerability, side effects, and effectiveness before increasing the dose. Changing doses too quickly prevents you from experiencing the full benefit of each dose level.
What happens if I miss a Mounjaro injection?
Because Mounjaro has a 5-day half-life, missing one dose isn't immediately critical. If you remember within 4 days, inject and resume your normal schedule. If more than 4 days have passed, skip and take your next dose on schedule. Never double-dose. For missed doses, the half-life means some medication remains active from your previous injection.
How does Mounjaro's half-life compare to Wegovy?
Mounjaro (tirzepatide) has a half-life of approximately 5 days. Wegovy (semaglutide), which is also a once-weekly injection, has a similar extended half-life of about 7 days. Both medications' longer half-lives allow for weekly dosing rather than daily injections, making them convenient for weight management.
Does tirzepatide's half-life affect how quickly side effects go away?
Yes. Because Mounjaro remains in your system for several days after each injection, side effects like nausea may persist for a few days. This is why prescribers recommend staying on each dose for several weeks — side effects often improve as your body adjusts. If severe side effects occur, contact your prescriber rather than suddenly stopping.
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.