Sublingual
Sublingual, literally 'under the tongue', from Latin, refers to a pharmacological route of administration in which certain drugs are entered directly into the bloodstream via absorption under the tongue.  Many pharmaceuticals are prepared for sublingual administration.  These commonly include cardiovascular drugs, steroids, barbiturates, some enzymes and increasingly frequently, certain vitamins and minerals.

# Principle
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 diffusion, the substance quickly enters the venous circulation, which returns to the heart and then travels to the systemic arterial circulation.  In contrast, substances absorbed by the bowel are subject to "first pass metabolism" in the liver before they are distributed to the rest of the body.
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. Because of its size and relative fragility, salvinorin A cannot pass the GI tract intact and must instead be absorbed across a mucous membrane.

# Substance
Almost any form of substance is appropriate for sublingual administration, so long as in that form the substance can readily enter into solution with the saliva in the mouth.  Chemicals prepared as powders, solutions, or even aerosol sprays may all make use of this method.  However, a number of factors, such as pH, molecular weight, and lipid solubility (to name just a few) of a substance may determine whether the route is practical or not.  Based on these properties, it is entirely possible that a drug, which will readily become a solution with saliva, simply diffuses too slowly (or not at all) in the buccal mucosa to be effective.

# Psychoactives
In addition to Salvinorin A, other psychoactives may too be applied sublingually.  LSD, MDMA, morphine, alprazolam and many other drugs including the psychedelic tryptamines and phenethylamines are all viable candidates for administration via this route.  Most often, the drug in question is powdered and placed in the mouth (often directly under the tongue).  If held there long enough, the drug will diffuse into the blood stream, bypassing the GI tract.  This is definitely a preferred method to simple oral administration, because MAO is known to oxidize many drugs (especially the tryptamines such as DMT) and because this route translates the chemical directly to the brain, where most psychoactives act.  However, this method is severely limited because only a small amount of the substance can be entered in this way, and often a large amount remains to be swallowed. Also, many alkaloids have a repulsive taste which makes them difficult to hold in the mouth.

# Allergens
Allergens may also be applied under the tongue, and the FDA is reviewing this method of allergen immunotherapy but it is not yet approved in the US. Roder published recent work showing sublingual immunotherapy with grass pollen is not effective in symptomatic youngsters in primary care.

# Footnotes
-  Roder (2007). "". J Allergy Clin Immunol. 119 (4): 8928. PMID 17321581..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}