**Standards message here**

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

*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:

k_{elim} = |
ln(C_{peak}) - ln(C_{trough}) |

t_{interval} |

t_{½} = 0.693 / k_{elim}

*Created:* January
7, 2000

*Revised:* October
19, 2000.