Due to a slower biotransformation most half-lives of drugs are increased. Also a lower renal excretion rate contributes to an extended half-life. On the other hand, much more free drug is available in the plasma of young children and thus more drug can be eliminated. All these factors together determine the half-life. A significant change in one of the factors can lead to shortening or prolongation of the half-life.
Half-life = (0.693 x volume of distribution) / clearance
The elimination half-life of theophylline varies during early life and childhood. In premature neonates the t1/2 is 30 hours. In neonates the half-life is 24 hours. During childhood the half-life is much lower: only 3.5 hours for children from 1 to 9. So, for this age group, increased frequency of administration of theophylline is necessary. Towards the adult life the t1/2 increases again to 8 hours.
Drug A has a longer half-life than drug B. This means that the concentration of drug A in the body is higher than that of drug B when both drugs are given intravenously at the same time.
Extra info: The concentration of each drug depends on the starting dose of that drug. If B was given at a very high dose, its concentration would remain higher than drug A’s concentration until A has been fully eliminated from the system.
Generally speaking, the half-lives of corticosteroids compared with those in other drugs are:
Extra info: Corticosteroids are lipophilic molecules and thus are hard to eliminate from the body. Corticosteroids can stay in the body for weeks to months after the last administration of the drugs.