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Cornell Pediatrics

Half Life, Drug

Calculate the half-life of a drug from two plasma levels separated by a time interval.

Input Peak concentration mcg/mL
Input Trough concentration  mcg/mL
Input Time interval hrs


Note: The "peak" should be obtained after the drug has been fully distributed, and the "trough" obtained at some interval later. These levels need not be the "true" peak or trough, as long as they are separated in time. The longer the time interval between the levels, the better the estimate.

If the levels are drawn around a dose (trough, dose, peak) rather than after the same dose (dose, peak, trough), the calculations here would be invalid if steady-state conditions are not satisfied. The conditions of steady state would be met if 4-5 half-lives have elapsed since the last change of the dosing regimen to the measurement of the first level. Alternatively, if two measured trough levels are the same, the patient would be in steady state.

Also note that the time interval entered is not the same as the dosing interval.

The formulas used are:

kelim = ln(Cpeak) - ln(Ctrough)

t = 0.693 / kelim


Created: January 7, 2000
Revised: October 19, 2000.

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