The adventures of a drug inside a person Before a drug can be clinically effective, it must be absorbed, or more simply put, it must be absorbed into the bloodstream. The chemical composition of the drug, as well as the environment in which it is placed, work together to determine the speed and degree of absorption of the drug. This determines the timing of the drug's onset of action and, in part, the severity of its effect. And absorption is only the first "step of the journey." The medication goes through three more phases inside the body - distribution, metabolism and excretion.