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 — UK Dosing Timeline
Mounjaro (tirzepatide) has a half life of approximately 3–4 days, meaning it takes about 3–4 days for half the medication to leave your body. This relatively short half life is why Mounjaro is injected once weekly — the dose builds up in your system over time with regular weekly injections. Understanding how long tirzepatide stays in your system helps explain the dose titration schedule, why missed doses matter, and what to expect from NHS prescribing guidance in the UK.
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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.
What Is Half Life and Why Does It Matter for Mounjaro?
Half life is the time it takes for your body to eliminate half of a medication's concentration from your bloodstream. For Mounjaro (tirzepatide), the half life is 3–4 days. This means if you inject 5 mg on Monday, approximately 2.5 mg remains in your system by Thursday or Friday.
Because Mounjaro has a relatively short half life, the medication is designed as a once-weekly injection — this allows the drug to accumulate to effective therapeutic levels week by week. Unlike daily medications that reach steady state in days, Mounjaro reaches steady state (maximum accumulation) after approximately 4–5 weeks of consistent weekly dosing.
This is why the NHS and all UK prescribing guidelines recommend waiting at least 4 weeks at each dose level before increasing — your body needs time to build up the medication to its full effect. If you switch doses too quickly, you won't see the true effect of your current dose, and you risk dose escalation that may not be necessary.
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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.
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Mounjaro Dose Titration Schedule and Half Life Implications
The standard UK Mounjaro titration schedule respects the medication's half life and accumulation pattern:
- Weeks 1–4: 2.5 mg once weekly (starting dose)
- Weeks 5–8: 5 mg once weekly (minimum 4 weeks before increase)
- Weeks 9–12: 7.5 mg once weekly
- Weeks 13–16: 10 mg once weekly
- Weeks 17–20: 12.5 mg once weekly
- Week 21+: 15 mg once weekly (maximum dose)
Each dose increase requires a 4-week interval because that's roughly how long it takes tirzepatide to reach steady state. If you miss a dose, the half life becomes relevant — if you're more than 4 days late, you may need to skip that injection and resume your normal schedule to avoid dosing errors.
If you miss a Mounjaro injection, the short half life means your levels will drop relatively quickly. Within 4 days of a missed dose, take it as soon as remembered and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and resume your regular injection day — this prevents accidentally overlapping doses. The 3–4 day half life also means that if you accidentally take two doses very close together, the medication leaves your system more gradually than if you'd taken a single larger dose, which is why timing between injections should be at least 72 hours (3 days) apart.
For NHS prescribing, understanding the half life helps explain why specialist tier 3 weight management services emphasise consistent weekly injection timing — staying on schedule allows tirzepatide to accumulate properly and lets your prescriber accurately assess whether a dose increase is needed. Erratic injection timing can mask the medication's true effect.
- 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 system completely?
Mounjaro (tirzepatide) has a half life of 3–4 days. After approximately 10–14 days (roughly 3–4 half lives), the vast majority of the medication is eliminated from your body. However, trace amounts may remain longer. Full washout typically takes around 2–3 weeks, which is why if you stop Mounjaro, appetite suppression effects usually fade within 1–2 weeks.
Why does Mounjaro have a 4-week titration schedule if the half life is only 3–4 days?
Although individual half life is short, tirzepatide accumulates in your body with repeated weekly doses, reaching steady state (maximum effective concentration) after approximately 4–5 weeks. This is why UK guidelines recommend waiting at least 4 weeks at each dose before increasing — your body needs that time to fully build up the medication to its true therapeutic effect.
What happens if I miss a Mounjaro injection?
Within 4 days of a missed dose, inject as soon as you remember and continue your regular weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on your normal day. The short half life means your tirzepatide levels drop relatively quickly after a missed dose, so timing the next injection correctly is important.
How does Mounjaro's half life compare to Wegovy (semaglutide)?
Mounjaro (tirzepatide) has a half life of 3–4 days. Wegovy (semaglutide) has a slightly longer half life of approximately 7 days. This means semaglutide stays in your system roughly twice as long, but both are once-weekly injections designed to accumulate over time with regular dosing.
Can I take Mounjaro every 10 days instead of weekly?
No — Mounjaro should always be taken once weekly on your chosen day. Although tirzepatide takes 10–14 days to mostly leave your system, the medication is engineered for weekly dosing. The 4-week accumulation pattern that guides dose escalation depends on consistent weekly injections. Changing injection frequency should only be done under guidance from your NHS prescriber.
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