Why can children and older adults respond differently to the same medicine?

Children and older adults respond differently to medications due to significant variations in pharmacokinetics—how the body absorbs, distributes, metabolizes, and excretes drugs.1

Children
Children are not simply small adults. Their physiological differences include:
* Absorption: Irregular bowel motility and decreased gastric acidity can alter how drugs are absorbed.1
* Distribution: Infants, particularly those born prematurely, have different body compositions, such as higher proportions of body water and less fatty tissue.1
* Metabolism: Metabolic rates are generally much higher during childhood, necessitating dosages calculated by weight or body surface area rather than age.1

Older Adults
The elderly often require lower doses due to:
* Organ Function: Reduced liver and kidney function slows the metabolism and excretion of drugs.1
* Physiological Changes: Decreased gastric acid secretion and reduced blood flow to organs, caused by conditions like arteriosclerosis, impact drug processing.1
* Complexity: Older adults frequently manage multiple illnesses, increasing the risk of drug incompatibility and adverse effects.1

Because of these factors, the fundamental principle of drug therapy is to use the lowest effective dose to ensure safety.1