When two or more drugs are taken at the same time complex interactions may occur. Drugs can interact to change biological functions within the body through PHARMACOKINETIC or PHARMACODYNAMIC mechanisms or through their combined toxic effects. Changes in the pharmacokinetic properties of a drug can include changes in absorption, distribution, metabolism, and excretion of the drug, and each of these can affect blood and plasma concentrations and, ultimately, brain levels of the drug. Although a change in the speed at which a drug reaches the bloodstream is rarely clinically relevant, a change in the amount of drug absorbed can be important, because this can lead to changes in the plasma levels of the drug, which can influence the amount of drug that reaches the brain.

The distribution of a drug throughout the body can be affected by changes in the...