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
How Long Does Mounjaro Stay In Your System?
Mounjaro (tirzepatide) has a half-life of approximately 4–5 days, meaning it takes this long for your body to eliminate half the injected dose. Most of the medication is cleared from your system within 2–3 weeks after your final injection. However, the full effects — including appetite reduction and metabolic changes — may take several weeks longer to wear off completely. Your GP or prescriber can advise on what to expect during this transition period and whether stopping is right for you.
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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.
Understanding Mounjaro's Half-Life and Clearance
Half-life is the time it takes your body to eliminate 50% of a medication. For Mounjaro (tirzepatide), the half-life is approximately 4–5 days. This is longer than some other GLP-1 medications, allowing for once-weekly dosing.
- After 1 week: Approximately 90% of the previous dose remains in your system
- After 2 weeks: Most of the medication is still present, which is why you take your next dose weekly
- After 3 weeks: The majority has been cleared; however, some metabolic effects persist
- After 4–5 weeks: Most of the drug has been eliminated from your bloodstream
Complete clearance varies between individuals depending on kidney and liver function, body weight, and metabolism. Your prescriber can discuss individual factors affecting how long Mounjaro stays active in your system.
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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 When You Stop Mounjaro?
When you stop Mounjaro, appetite and food cravings typically return gradually over 2–4 weeks as the medication leaves your system and hormonal signals normalise. Weight regain is common if dietary and lifestyle changes are not maintained. Stopping should always be discussed with your GP or prescriber — do not stop abruptly without guidance.
If you are considering stopping Mounjaro or switching to another weight loss medication like Wegovy (semaglutide), your prescriber can advise on the safest transition plan. Some individuals continue with maintenance doses; others transition to lifestyle management alone. Our GLP-1 weight loss calculator can help you understand dose-related outcomes and support informed conversations with your healthcare provider.
- 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 Mounjaro stay in your system after you stop injecting?
Yes. Mounjaro takes approximately 2–3 weeks to clear from your bloodstream after your final injection, though appetite-suppressing effects may gradually wear off over 4 weeks. Individual clearance varies based on metabolism, kidney function, and body composition. Your GP can advise on your personal timeline.
Why does Mounjaro have a 4–5 day half-life?
The 4–5 day half-life is deliberate — it allows for once-weekly dosing while maintaining steady medication levels in your body. This is longer than semaglutide (Wegovy's ingredient), which has a shorter half-life. The extended half-life means steady appetite control throughout the week.
Can Mounjaro cause long-term changes after stopping?
Mounjaro does not cause permanent metabolic changes. However, if you have developed healthy eating habits during treatment, these may persist. Without continued effort, appetite and eating patterns typically return to pre-treatment baseline within 4–6 weeks. Weight maintenance requires sustained lifestyle changes.
How does Mounjaro clearance compare to Wegovy?
Mounjaro (tirzepatide) has a half-life of 4–5 days, while Wegovy (semaglutide) has a half-life of approximately 7 days. Wegovy clears slightly slower. Both are once-weekly injections, but the different pharmacokinetics may affect individual experience when stopping or switching between medications.
What should I do if I'm stopping Mounjaro?
Always discuss stopping with your GP or prescriber — do not stop abruptly. Plan continued dietary and activity changes to support weight maintenance. If you are switching to Wegovy or another medication, your prescriber will advise on the transition. Our progress tracker can help you maintain momentum during the transition period.
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.