1.1 Definition A substance having a profound or significant effect on mental processes is known as psychoactive (Appleton 1967). A psychoactive drug is also known as psychotropic drug (Appleton 1971). Some people refer psychoactive medicinal plants as mind-altering herbal drugs (Crocq 2007). 1.2 Historical Aspects The psychoactive plants have been used for several purposes. The possible role of Amazonian psychoactive plants in the chemotherapy of parasitic worms has been discussed (Rodriguez et al. 1982). Hallucinogenic and stimulant plants of the Incas have been elaborated (Elferink 1974). In Ruiz Relación psychoactive plants used in Peru and Chile have been mentioned (Schultes 1983). Anadenanthera peregrina, Erythroxylon coca, Datura spp., and Trichocereus pachanoi were used by the ceramics of the ancient Nazca in rituals connected with shamanism (Dobkin and Cardenas 1980). Nymphaea ampla DC. (white water lily) has been used in Maya drug rituals (Emboden 1981). The plant is reported to contain isoflavones (Marquina et al. 2005). There has been mention of use of Nymphaea caerulea Sav. (narcotic water lily) and mandrake for shamanistic healing in Egypt (Emboden 1981, 1989). Datura stramonium L. (jimson weed), Nicotiana rustica L. (strong tobacco), Nymphaea ampla DC. (white water lily), and Salvia divinorum Epling & Játiva (sage of the diviners) have been used as hallucinogenic drugs in pre-Columbian Mesoamerican cultures (Carod-Artal 2015). Mexican phantastica is the early study of ethnobotanical sources of hallucinogenic medicinal plants (Guerra 1967). Similarly, hallucinogenic medicinal plants have been described in Aztec pharmacopeia (Elferink 1988). Several sacred psychoactive plants have been used by the Indians of Mexico (Díaz 1977). Ethnopharmacology and taxonomy of the psychoactive plants used in Mexico has been intensively studied (Díaz 1979). Fifty plants have been classified in the modern way of antipsychotic, antidepressant, minor tranquilizer, hallucinogens, sedatives, hypnotics, brain tonics, stimulants, and anticonvulsants (Jiménez-Olivares 1978). As many as twenty-two plant species are reutilized by the natives of middle and South America as ingredients of ritual fumigatories (De Smet 1985). Psychoactive properties of Kung Bushmen medicinal plants have been widely identified (Winkelman and Dobkin de Rios 1989). Ethnopsychiatric medicines of the Matsigenka are derived from medicinal plants of Rubiaceae, Solanaceae, and Cyperaceae (Shepard 1998). Krahô Indians in Brazil use 138 plant species directly or indirectly related to central nervous system. Majority of the medicinal plants appear to have psychoactive potential (Rodrigues and Carlini 2005). Forty-five plant species have been identified for the treatment of psychological/psychiatric disorders used by the Krahô Indians in Brazil (Rodrigues and Carlini 2006). Quilombolas, yet another Brazilian culture, use 169 plant species for the same purpose used by Krahô Indians (Rodrigues and Carlini 2006a). Psychoactive medicinal plants used in Brazil have been subjected to phytochemical investigations and monoterpenes, sesquiterpenes, carotenoids, and steroids have been reported as major chemical compounds (Otsuka et al. 2010). Psilocybe samuiensis Guzmán, Bandala, and Allen has been used for psychoactive purpose by Koh Samui and Koh Pha-Ngan in Thailand (Allen and Merlin 1992). The mushroom is known to psilocybin and psilocin (Jochen et al. 1994). In addition, a sesquiterpenoid metabolite psilosamuiensin A has been reported (Pornpakakul et al. 2009). Psilocybe atlantis Guzmán, Hanlin, and C. White found in Georgia is known to contain psilocybin and psilocin (Guzmán et al. 2003). Claviceps paspali Stev reported in the grains of Paspalum scrobiculatum L. (Kodo millet) is a fungus known to produce lysergic acid derivative and ergot alkaloids (Agurell 1966; Aaronson 1996). Psilocybe semilanceata (Fr.) P.Kumm. (liberty cap), Panaeolus cyanescens (Berk. & Broome) Sacc., and Psilocybe tampanensis Guzmán & S.H. Pollock are the hallucinogenic mushrooms of the German market. All three are sources of psilocybin and psilocin with P. cyanescens having highest concentration (Musshoff et al. 2000). Cases of poisoning with liberty cap have been reported characterized by visual and auditory hallucinations followed by the exogenous psychosis (Satora et al. 2005) and myocardial infaction (Borowiak et al. 1998). Phenylethylamine has been also identified in liberty cap (Beck et al. 1998). In addition, baeocystin has been reported in extracts based on liberty cap (Repke and Leslie 1977). Psilocybin is also reported in fungi genus Stropharia and Panaeolus (Carod Artal 2003). The psychoactive potential of Nutmeg (Myristica fragrans Houtt.) was initially explained in the year of 1968 (Beattie 1968). Lagochilus inebrians Bunge, Claviceps purpurea L., and Ipomoea pes-caprae (Linn.) R.Br. have been explained as possible substitutes for the Vedic plant Soma (Riedlinger 1993). DLysergamide obtained from the seeds of Argyreia nervosa, Ipomoea tricolor, Ipomoea violacea, and Ipomoea purpurea is used for recreational purpose in Poland (Juszczak and Swiergiel 2013). 1.3 Classification (Webb 1971) The classification of psychoactive medicinal plants is based on the effect: Stimulant: These plants have stimulating effect on the mind or activity without altering perception. Hypnosedatives or narcotics: These plants induce sleep, alter perception, and cause euphoria and vivid dreams. Hallucinogenic: The plants have significant effect on emotions, space, and time.