Biological rhythms in the absorption, distribution, metabolism and excretion of drugs.

Studies carried out in the last 20 years indicated that biological rhythms can be detected in the pharmacokinetics of most classes of drugs. These time-dependent variations could be due to parallel changes in the physiological functions and variables involved in the absorption, distribution, metabolism and excretion of drugs. A review of the data available suggests that the peak and trough values of these functions and variables do not occur at the same hour of the day in every factor involved in drug disposition. This information could be used to predict the time-dependent changes in the pharmacokinetics. The presence of circadian variations in the kinetics of drugs raise the rather old question: "When to administer drug?"