GLP-1 Half-Life Calculator & How to Use It

This GLP-1 half-life calculator estimates how long a GLP-1 receptor agonist (like semaglutide or liraglutide) remains active in your body after your last dose. You enter the drug’s half-life in hours and the time since your last injection, and the calculator returns the estimated percentage of the drug still present, plus the time until it drops to a negligible level. It helps you understand dosing intervals, missed-dose timing, and when side effects may subside.

Formula

The calculator uses the standard exponential decay formula:

Remaining (%) = 100 × 0.5^{(hours_since_dose / half_life)}

Where:

• half_life (t) – the time it takes for the drug concentration in your blood to reduce by half, measured in hours.
• hours_since_dose (h) – the elapsed time since your last injection, in hours.
• Remaining (%) – the estimated percentage of the original dose still present in your body.

The second output, time to 1% remaining, is calculated as half_life × (log(100) / log(2)), which equals about 6.64 times the half-life. This is the time it takes for the drug to fall below 1% of its initial concentration, a common threshold for considering it effectively eliminated.

Worked Example

Using the defaults in the calculator above: a drug with a half-life of 168 hours (7 days, typical for semaglutide) and 72 hours since the last dose.

1. Divide the time since dose by the half-life: 72 / 168 = 0.4286
2. Raise 0.5 to that power: 0.5^0.4286 ≈ 0.743
3. Multiply by 100: 0.743 × 100 = 74.3%

So after 3 days, about 74% of the drug remains. The time to 1% remaining is 168 × (log(100)/log(2)) = 168 × 6.644 ≈ 1116 hours, or about 46.5 days. That matches the calculator’s output.

Why Half-Life Matters for GLP-1 Drugs

GLP-1 receptor agonists are used for type 2 diabetes and weight management. Their half-lives vary widely:

• Short-acting – exenatide (Byetta) has a half-life of about 2-3 hours, requiring twice-daily injections.
• Long-acting – liraglutide (Victoza, Saxenda) has a half-life of about 13 hours, so it’s given once daily.
• Ultra-long – semaglutide (Ozempic, Wegovy) has a half-life of about 168 hours (7 days), enabling once-weekly dosing.

Knowing the half-life helps you predict when the drug’s concentration will drop to a level where its effects (and side effects) diminish. This is especially useful if you miss a dose, switch medications, or experience prolonged nausea.

How to Interpret the Results

The “remainingâ€� percentage tells you how much of the drug is still in your system relative to your last dose. For example, if you take a weekly injection, at day 7 you have about 50% remaining (after one half-life). By day 14, you’d have about 25%. This doesn’t mean the drug stops working at 50%—it means the concentration is half of what it was right after the injection.

Most clinicians consider a drug effectively cleared after 4-5 half-lives (about 94-97% eliminated). For semaglutide, that’s roughly 28-35 days. If you’re planning surgery or switching to a different medication, this timeline is often used to avoid overlapping effects.

Common Mistakes and Limitations

• Using the wrong half-life – Always check the specific drug’s prescribing information. Half-lives can vary slightly between individuals and formulations.
• Assuming linear elimination – The exponential model is accurate for most GLP-1 drugs, but it’s an approximation. Real-world metabolism can be affected by kidney function, age, and other medications.
• Forgetting that half-life is not duration of action – A drug can have clinical effects even at low concentrations. The half-life tells you about concentration, not necessarily when the effect stops.
• Unit confusion – Ensure both the half-life and time since dose are in the same unit (hours). If you have days, multiply by 24.

Practical Uses and Safety Considerations

This calculator is an educational tool, not a substitute for medical advice. If you miss a dose, the general guidance for weekly GLP-1 drugs is to take it within 5 days of the scheduled dose; if more than 5 days have passed, skip it and resume the normal schedule. This calculator can help you see how much drug is still in your system at that point.

If you experience severe side effects like persistent vomiting or signs of pancreatitis, seek medical help immediately—don’t rely on half-life estimates. Always consult your healthcare provider before changing your dosing schedule.

Frequently Asked Questions

What is the half-life of semaglutide?

Semaglutide (Ozempic, Wegovy) has a half-life of approximately 168 hours, or 7 days. This allows for once-weekly dosing. After 4 weeks, about 6% of the original dose remains.

How long does it take for GLP-1 to leave your system completely?

It takes about 5 half-lives for a drug to be considered eliminated (less than 3% remaining). For semaglutide, that’s about 35 days. For liraglutide, it’s about 65 hours (2.7 days).

Can I use this calculator for any GLP-1 drug?

Yes, as long as you know the half-life. Common values: exenatide immediate-release ~2.4 hours, liraglutide ~13 hours, semaglutide ~168 hours, and dulaglutide ~120 hours (5 days). Check your medication’s package insert for the exact value.

Why is my drug still working after the half-life has passed?

Because the half-life is the time for concentration to halve, not to reach zero. After one half-life, 50% remains; after two, 25%. The drug continues to have an effect as long as the concentration is above the therapeutic threshold, which can be much lower than the peak.

Does kidney function affect GLP-1 half-life?

Yes, for some GLP-1 drugs. For example, exenatide is cleared renally, so its half-life can be prolonged in patients with kidney impairment. Semaglutide is less affected. Always consult your doctor if you have kidney issues.

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in t | Half-life (hours) | h | 168 | 0
in h | Hours since last dose | h | 72 | 0
out remaining | Drug remaining | % | 100 * 0.5^(h/t) | 1
out time_to_1pct | Time to 1% remaining | h | t * log(100) / log(2) | 0