Pharmacokinetics is the field of pharmacology where we learn how the body affects a certain substance after their administration. As the word 'kinetic' means movement, Pharmacokinetics is the process of how the body carries the certain substance. This certain substance can be anything varying from drugs to fertilisers.

Administration - Intake of a certain substance.

IUPAC Definition of Pharmacokinetics - Process of the uptake of drugs by the body, the biotransformation they undergo, the distribution of the drugs and their metabolites in the tissues, and the elimination of the drugs and their metabolites from the body over a period of time.

IUPAC - International Union of Pure and Applied Chemistry

A number of phases occur once the drug enters into contact with the organism, these are described using the acronym ADME (or LADME if liberation is included as a separate step from absorption):

• Absorption

• Distribution

• Metabolism

• Excretion

Absorption is the movement of a drug from its site of administration to the bloodstream.

The process of drug distribution is important because it can affect how much drug ends up in the active sites, and thus drug efficacy and toxicity. A drug will move from the absorption site to tissues around the body, such as brain tissue, fat, and muscle. Many factors could influence this, such as blood flow, lipophilicity, molecular size, and how the drug interacts with the components of blood, like plasma proteins.

Cytochrome P450 (CYP450) enzymes are responsible for the biotransformation or metabolism of about 70-80% of all drugs in clinical use.

Elimination involves both the metabolism and the excretion of the drug through the kidneys, and to a much smaller degree, into the bile. Excretion into the urine through the kidneys is one of the most important mechanisms of drug removal.