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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How Long Does 2.5Mg Mounjaro Stay In Your System?
The length of time Mounjaro (tirzepatide) remains in your system depends on several factors, including your individual metabolism, body composition, and kidney and liver function. Generally, tirzepatide has a half-life of approximately 4–5 days, meaning half the dose is eliminated from your body within this timeframe. However, the medication can be detected in your system for longer periods. Since Mounjaro is a prescription-only injection, your prescriber will monitor how your body responds and adjust your dose accordingly during the titration phase. Understanding how long the medication stays active helps explain why it's given as a once-weekly injection.
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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.
Understanding Mounjaro Half-Life And Elimination
Half-life refers to how long it takes for half of a drug dose to be removed from your body. Mounjaro (tirzepatide) has a half-life of approximately 4–5 days, meaning that after 4–5 days, roughly 50% of your dose has been eliminated through your kidneys and liver.
- After 1 week, approximately 75% of the dose is eliminated
- After 2 weeks, around 93% is eliminated
- Small amounts may be detectable for 3–4 weeks after your injection
This relatively long half-life is why Mounjaro works as a once-weekly injection—the medication builds up slightly in your system week to week during the dose titration phase, which is why your prescriber increases the dose every 4 weeks. The Mounjaro KwikPen device delivers a consistent dose each week, allowing your body to reach a stable level of the medication.
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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.
Factors That Affect How Long Mounjaro Stays Active
Your individual metabolism, age, weight, kidney function, and liver health all influence how quickly tirzepatide is eliminated from your system. People with reduced kidney or liver function may have tirzepatide remain active for longer periods. This is why your GP or prescriber will assess your full health history before starting Mounjaro and may monitor your kidney function during treatment.
Additionally, as you progress through dose titration—moving from 2.5mg to 5mg, 7.5mg, and potentially higher doses—the cumulative effect builds gradually. This is why each dose is maintained for at least 4 weeks before escalating. If you miss a dose, it can be taken within 4 days of the missed injection without significantly disrupting the medication's 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.
Does stopping Mounjaro mean it leaves your system immediately?
No. Even after your final injection, tirzepatide remains active for several days. You may continue to experience reduced appetite and other effects for 1–2 weeks. If you stop Mounjaro, your prescriber will discuss what to expect and whether continued weight management support is appropriate.
How does the 4–5 day half-life relate to weekly dosing?
The once-weekly schedule is timed so that when you receive your next injection, approximately 25% of the previous dose remains in your system. This allows for a gradual, steady accumulation that reaches equilibrium by week 4–5 of treatment, which is why dose adjustments happen every 4 weeks.
Will Mounjaro stay longer in my system if I'm overweight?
Body weight can affect how medications distribute and are metabolised, but tirzepatide's half-life of 4–5 days is relatively consistent across individuals. Your prescriber will monitor your individual response and adjust your dose based on your personal progress and tolerance.
What happens if I accidentally take two doses close together?
If you inject Mounjaro within 72 hours (3 days) of your previous dose, contact your prescriber or the pharmacy immediately. Do not take a double dose. Your prescriber can advise you on whether to skip the next scheduled dose or adjust your schedule.
How do I use our GLP-1 weight loss calculator alongside Mounjaro treatment?
Our free weight loss calculator can help you estimate potential outcomes at different Mounjaro doses based on clinical trial data. Use it alongside your BMI calculator to track your starting point and progress over time with your prescriber's support.
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.