This document discusses various factors that can modify drug action in the human body. It identifies physiological factors like age, sex, pregnancy and food; pathological factors like liver and kidney disease; genetic factors; and environmental factors like route of administration and disease conditions as influencing drug response. It also covers drug interactions, noting that some drug combinations can produce synergistic or additive effects while others result in antagonism where the drugs oppose each other's actions. Understanding these modifying factors is important for choosing the appropriate drug and dose for each individual patient.