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 Mounjaro Last In Your System?
Mounjaro (tirzepatide) remains active in your body for approximately 4–5 days after each weekly injection. However, the full effects of the medication build over several weeks as you increase your dose. Understanding how long Mounjaro lasts helps you manage expectations about weight loss timing and what happens if you miss a dose. This guide explains the pharmacology behind tirzepatide, how it works in your system, and practical information for UK users taking this prescription-only weight management injection.
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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 Long Mounjaro Stays Active In Your Body
Mounjaro's half-life is approximately 4–5 days, meaning this is the time it takes for your body to eliminate half of the medication from your system. After about 10–14 days, most of the previous dose has been cleared before your next weekly injection. However, tirzepatide has a long-acting formulation, so effects typically persist throughout the week and into the next injection cycle. The medication works by activating GIP and GLP-1 receptors, which continue to influence appetite signalling, blood sugar regulation, and gastric emptying even as drug concentration gradually decreases. This is why Mounjaro is dosed once weekly rather than daily — the sustained effect covers the full 7-day period. When you start Mounjaro or increase your dose, the cumulative effect builds over your first few weeks on each dose level, which is why the prescriber recommends waiting at least 4 weeks before escalating to the next strength during dose titration.
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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.
What Happens If You Miss A Dose
If you miss your Mounjaro injection, UK prescribing guidance suggests that within 4 days of the missed dose, you can take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed since you were due to inject, skip that dose and take your next dose on your normal scheduled day — do not double up. The minimum safe interval between Mounjaro injections is 72 hours (3 days). Missing one dose is unlikely to cause sudden weight regain, since the medication remains partly active in your system, but consistent weekly dosing is important for maintaining steady appetite control and optimal effects. Always contact your GP or prescriber if you have missed multiple doses or are unsure about your injection schedule.
- 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 quickly does Mounjaro work after injection?
Mounjaro begins working within hours of injection as tirzepatide enters the bloodstream and activates GIP and GLP-1 receptors. However, the full appetite-suppressing effects are usually most noticeable after several days and build over weeks as you escalate through dose titration. Most users report clearer effects by weeks 3–4 on each dose level.
Does Mounjaro stay in your system permanently?
No. Mounjaro is eliminated gradually, with a half-life of 4–5 days. After approximately 2–3 weeks, virtually all of the medication has left your body. If you stop taking Mounjaro, its effects typically diminish within days to weeks, though this varies by individual and depends on how long you have been taking it.
What happens to weight loss when you stop Mounjaro?
When you stop Mounjaro, the appetite-suppressing effects wear off as tirzepatide is metabolised. Some people experience weight regain, particularly if diet and exercise habits change back to pre-treatment levels. Clinical trials show that maintaining lifestyle changes (reduced calories, physical activity) helps prevent rapid regain.
Can you switch from Mounjaro to Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications — Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1 agonist only. Any switch must be managed by your prescriber. There is no automatic conversion dose; your prescriber will assess the best starting point for Wegovy based on your individual response to Mounjaro.
How long does it take to see weight loss results with Mounjaro?
Most people notice changes in appetite within the first week or two, but significant weight loss typically becomes measurable after 4–8 weeks on the same dose. The clinical SURMOUNT trials showed average weight loss of 15–21% over 72 weeks when combined with reduced-calorie diet and increased physical activity. Individual results vary considerably.
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.