The Absorption, Distribution, Metabolism and Excretion of Drugs in the Elderly

As age increases, the functions of tissues and organs in the body gradually decline. Due to this decline in organ function, drug absorption, distribution, metabolism and excretion (ADME processes) in elderly people are worse than those of young people. Furthermore, drug sensitivity is different in the elderly, who are prone to have adverse reactions to drugs. Thus, it is very important to design drugs according to the characteristics of the elderly.

Drug dissolution is effected by the aging of the gastrointestinal mucosa in the elderly and the decrease of gastric acid secretion (25% - 20% reduction). However, due to the weakening of gastrointestinal movement in the elderly and slow gastric emptying rate, drugs stay longer in the gastrointestinal tract, which is conducive to greater drug absorption. The combination of these negative and positive factors usually results in normal drug absorption rate. 

Due to a decrease in the amount of plasma proteins, an increase of fat percentage and decrease of lean tissues (skeletal muscle, liver, brain, kidney, etc.), when the same dose of drug is used in elderly and young people, it has a high level of free state and greater functionality in the elderly. The elderly are therefore more prone to toxic reactions.

There were no abnormal changes in liver function indexes in the elderly, but the activity of drug metabolism enzyme in the liver is decreased so that the half-life of the drug is prolonged. Also, the age associated reduction of parenchymal cells in the liver and a reduction of liver blood flow affects the ability of the liver to metabolise drugs.  These factors further compound the drug scavenging capacity of the elderly, causing drug effect enhancement and more adverse reactions.

The kidney is the main organ involved in drug excretion; therefore the pharmacokinetics of aging induced change mainly results from reduced kidney functionality. It is the most important factor in producing toxic drug reactions in the elderly.