The main processes involved in pharmacokinetics are absorption, distribution, and the two routes of drug elimination, metabolism and excretion. Together they are sometimes known by the acronym ‘ADME’. Distribution, metabolism and excretion are sometimes referred to collectively as drug disposition. Absorption is the process by which drugs enter the body. Given by any route other than intravenously, drug molecules must cross tissue membranes (e.g. skin epithelium, subcutaneous tissue, gut endothelium, capillary wall) to enter the blood. Distribution is the process by which drugs move around the body. After entering the blood, drug molecules must cross capillary walls to enter the tissues, reach cell membranes and enter cells. Metabolism is the process by which drugs are chemically altered to make them sufficiently water-soluble for excretion in urine or faeces (via the biliary tract). Metabolism occurs in a variety of body organs and tissues, but chiefly in the liver, gut wall, kidney and skin. Excretion is the process by which drugs leave the body. Drugs that are sufficiently water-soluble will be excreted unchanged in the urine. Lipid-soluble drugs must be modified to water-soluble metabolites before excretion via the kidney or into the intestine via the bile. Medicines aim to prevent, cure or control various disease states. To achieve this goal, adequate concentrations of the medicine must be delivered to the target tissues so that therapeutic, yet nontoxic levels, are obtained. Pharmacological and toxicological actions of medicines are primarily related to their plasma concentrations. Consequently, healthcare professionals involved with medicines must recognise the onset speed of medicine action as well as the intensity and duration of its effect. In turn, these are controlled by the following four fundamental pathways of drug movement and modification in the body. Pharmacokinetics influences the decided route of administration for a specific medication, the amount and frequency of each dose and its dosing intervals. On the other hand, pharmacodynamics is the study of how a medicine acts on a living organism. This includes the pharmacological response and its duration and magnitude observed, relative to the medicines concentration at an active site in the organism; i.e. the study of a medicine’s effect and the mechanisms of action. Absorption from the site of administration permits entry of the therapeutic agent (either directly or indirectly) into plasma. Medicine-related factors include ionisation state, molecular weight, solubility, and formulation. Small, nonionised, lipid-soluble medicines permeate plasma membranes most readily. Absorption is the process by which drug molecules gain access to the bloodstream from the site of drug administration. The speed of this process (the rate of drug absorption) and its completeness (the extent of drug absorption) depend on the route of administration. Routes of administration can be considered in two categories: Enteral. Drugs given by mouth are normally swallowed before being absorbed in the stomach or small bowel, after which they enter the portal venous system and pass through the liver before gaining access to the systemic circulation. Some drugs introduced into the alimentary tract are absorbed directly into the systemic circulation without passing through the liver (e.g. via the buccal, sublingual or rectal routes), thereby avoiding the potential hazards of gastric acid, binding to food, and metabolism by gut wall or liver enzymes (first-pass metabolism).Parenteral. This includes any route that avoids absorption via the gastrointestinal tract such as administration by injection, inhalation or by application to the skin. Absorption after an oral dose is a lengthy process, during which drug molecules may be damaged (e.g. denatured by gastric acid), sequestered (e.g. bound to food preventing absorption) or modified by first-pass metabolism. As a consequence of all these hazards, it is not surprising that absorption is frequently incomplete following oral administration. The proportion of a dose that reaches the systemic circulation unscathed is known as the bioavailability of the drug.