Factors Modifying Drug Response

Key points

• • People differ in how they respond to the same drug, so the effect of a given amount can vary widely from one person to another.
• • Body size and composition matter: a smaller person or a child generally needs a smaller amount, which is why amounts for children are worked out by weight.
• • The very young and the very old often handle drugs differently, because the liver and kidneys that clear them are immature in babies and may decline in old age.
• • Disease of the liver or kidneys slows the breakdown and removal of many drugs, so they can build up to harmful levels.
• • Inherited differences (pharmacogenetics) mean some people break a drug down fast and others slowly, changing its effect.
• • Other factors include pregnancy, other drugs being taken at the same time, and whether the body has become tolerant with repeated use.