Following the administration of a drug, the body immediately begins the process of absorption, distribution, metabolism, and elimination. The study of these processes is known as pharmacokinetics. Absorption is the transfer of drugs to the bloodstream. Various factors influence the rate and extent of absorption. Many orally administered drugs are generally metabolized by the liver before they enter the systemic circulation. Such drugs are only partially absorbed. On the contrary, drugs delivered through IV directly reach the bloodstream and are almost completely absorbed. The absorbed drug is then distributed to the tissues. Elimination is the removal of the drug from the body in the unaltered or metabolized form. Renal elimination is limited to hydrophilic substances since lipophilic substances tend to get reabsorbed. So, metabolizing lipid -soluble drugs into water-soluble substances becomes necessary before excretion.