Calculating a half life requires measuring the level of the drug at two different points in time. It needs to be far enough after the dose of the medicine that there is no further absorption of the drug to confuse the assessment. The calculation requires calculation of logarithmic values to the base of 2 (log2).
The formula for calculation of half-life (T1/2) requires the knowledge of the initial concentration (C1), and the subsequent concentration (C2) obtained an amount of time later (t). The formula is:
T1/2 = t / [log2(C1/C2)]
Today, there are computer programs that will allow the numbers to be plugged in and the half-life result returned. It is also possible to build the formula in Excel, since Excel includes a logarithmic function key to the desired base number.
The process and formula for calculating half-life is the same, regardless of whether it is a psychotropic or other medicine. For example, Prozac has a very long half-life, ranging from 4-16 days, so relatively little is metabolized over the course of a day. The baseline level has to be taken well after the daily dose, such as 8 hours later. A representative level is 200 ng/ml. A repeat level is checked before the next days dose, 16 hours later, and may be 180 ng/ml. Using the above formula, the half-life for this person would be slightly over 105 hours, or 4.4 days.