Now the medical science has well recognized the role played by biological rhythm on the effects of drugs. It is an established fact that rythmicity is an integral part of all life ,hence in drug administration, importance has to be given not only to the right substance of the right amount at the right place but also to the right time.
Not to speak of the optimum utilization of drugs given at the right time, even the onset of symptoms of certain diseases like asthma attacks, angina pectoris etc do not occur randomly with in 24 hours of the day. Drugs of different classes used as painkillers show periodic variations in their pharmacokinetics and can greatly depend on the time of the day.