The principle behind sublingual administration is fairly simple. When a chemical comes in contact with the mucous membrane, or buccal mucosa, it diffuses into the epithelium beneath the tongue. This region contains a high density of blood vessels, and as a result, via osmosis, the substance quickly enters the blood stream at the sublingual artery. Quickly thereafter, the substance is carried by blood flow to the connected lingual artery, which takes up the chemical and translates it to its source: the carotid artery. This artery is connected directly to the brain.
In theory, sublingual routes of administration have certain advantages over simple oral administration. This route is often faster, and entering a drug into one's body sublingually ensures that the substance will only come in contact with the enzymes in saliva prior to entry into the bloodstream. Drugs otherwise orally administered must instead survive the incredibly hostile environment of the gastrointestinal tract. This may mean a much greater percentage of the original substance is degraded either by the myriad of enzymes in the GI tract, such as monoamine oxidase, or the strong acids it contains. Additionally, after GI absorption, the drug is sent to the liver where the drug may be extensively metabolized; this is known as the first pass effect of drug metabolism. Due to the degradative qualities of the stomach and intestine, or the solubility of the GI tract, certain substances, such as salvinorin A may only be administered orally via the sublingual route.