Drug Half-Life Calculator
Estimate a drug's elimination half-life using initial/final plasma concentrations and elapsed time.
Input values
Provide two concentration readings and the time between them to solve for the half-life assuming first-order elimination.
How to Use This Calculator
Enter the drug's initial and final plasma concentrations alongside the time elapsed between those samples. Hit "Calculate" to see the elimination half-life expressed in hours; the calculator assumes a single-compartment, first-order elimination model.
Methodology
We apply the standard pharmacokinetic formula that relates the change in concentration to the natural logarithm of the ratio, scaled by the time interval. The calculator also derives the elimination rate constant for quick reference.
Full original guide (expanded)
Data Source and Methodology
All calculations are based on standard pharmacokinetic equations and data from authoritative medical sources. For more details, consult your pharmacology resources.
The Formula Explained
Half-life (t₁/₂) is calculated as:
\( t_{1/2} = \frac{t \cdot \ln(2)}{\ln(\frac{C_0}{C_t})} \)
Glossary of Terms
- Initial Concentration (C₀): The starting drug concentration.
- Final Concentration (Cₜ): The concentration after a given period.
- Time Elapsed (t): Duration between the two measurements.
- Half-Life (t₁/₂): Time required for concentration to halve.
How It Works: A Step-by-Step Example
If the initial concentration is 100 mg/L, the final is 25 mg/L after 6 hours, the formula yields approximately 3 hours for the half-life.
Frequently Asked Questions (FAQ)
What is a drug half-life?
A drug half-life is the time required for the concentration in the bloodstream to reduce by half.
Why is knowing the half-life of a drug important?
It helps determine dosing schedules and how the drug is cleared from the body.
Can the half-life of a drug change?
Yes, factors like age, liver function, and interactions with other drugs can alter the half-life.
Is the half-life the same for all drugs?
No, every drug has a different half-life based on its chemical characteristics.
How accurate is this calculator?
It provides estimates based on first-order models; please consult a healthcare professional for individualized advice.
About the author
Ugo Candido builds tools and educational content to make complex calculations approachable, all while keeping the assumptions transparent.
Contact: info@calcdomain.com
Editorial policy
CalcDomain content is educational and reviewed for clarity and transparency. Inputs and assumptions appear directly in the interface so you can verify how the results are produced.