any of various forms of stimulation thought to arouse sexual excitement. Aphrodisiacs may be classified in two principal groups: (1) psychophysiological (visual, tactile, olfactory, aural) and (2) internal (stemming from food, alcoholic drinks, drugs, love potions, medical preparations).

Despite long-standing literary and popular interest in internal aphrodisiacs, almost no scientific studies of them have been made. Scientific research is limited to occasional tests of drugs or hormones for the cure of male impotence. Most writings on the subject are little more than unscientific compilations of traditional or folkloric material. Of the various foods to which aphrodisiac powers are traditionally attributed, fish, vegetables, and spices have been the most popular throughout history. In none of these foods, however, have any chemical agents been identified that could effect a direct physiological reaction upon the genitourinary tract, and it must be concluded that the reputation of various supposedly erotic foods is based not upon fact but upon folklore.

It has been suggested that man's universal attribution of libidinous effects to certain foods originated in the ancient belief in the therapeutic efficacy of signatures: if an object resembled the genitalia, it possessed, so it was reasoned, sexual powers. Thus the legendary aphrodisiac powers of ginseng root and powdered rhinoceros horn.

With the exception of certain drugs such as alcohol (alcohol consumption) or marijuana, which may lead to sexual excitation through disinhibition, modern medical science recognizes a very limited number of aphrodisiacs. These are, principally, cantharides and yohimbine, both of which stimulate sexual arousal by irritating the urinary tract when excreted. Cantharides, or cantharidin, consists of the broken dried remains of the blister beetle (q.v.) Lytta vesicatoria. It has been a traditional sexual stimulant fed to male livestock to facilitate breeding. In humans the substance produces skin blisters on contact, and attempts to ingest it as an aphrodisiac are considered extremely hazardous. Yohimbine is a crystalline alkaloid substance derived from the bark of the yohimbé tree (Corynanthe yohimbe) found in central Africa, where it has been used for centuries to increase sexual powers. Although it has been promoted as an aphrodisiac, most investigators feel that any clinical change in sexual powers after its use is probably due to suggestion, because stimulatory effects are elicited only with toxic doses.