Psychedelic drugs currently have a circular definition. Classed as hallucinogenic psychoactive drugs which have the primary effect of triggering non-ordinary states of consciousness - termed 'psychedelic' experiences.

Many of the known psychedelic substances have been found to bind-to and activate the brain's serotonin 5-HT_2A receptors. ( see: Wikipedia )

Although it has been experimentally found that activating the 5-HT_2A receptors leads to a marked increase in spontaneous (glutamate-mediated) synaptic activity in the prefrontal cortex, exactly how and why this leads to 'psychedelic' experiences remains unknown.

Further reading : Proceedings of the National Academy of Sciences

Also see : Empathogenic drugsplugin-autotooltip__plain plugin-autotooltip_bigEmpathogenic drugs

Empathogenic - a.k.a. Entactogenic - drugs are a wide-ranging group of compounds (e.g. MDMA) which produce experiences described as emotional communion, empathy or sympathy for others, oneness, relatedness, and emotional openness.

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