Serotonergic psychedelics (also known as serotonergic hallucinogens) are a class of hallucinogenic drugs (specifically psychedelic drugs) with a method of action strongly tied to the serotonin neurotransmitter. Serotonin (often referred to as 5-HT, short for its full chemical name 5-Hydroxy Tryptamine) is a naturally occurring neurotransmitter which is tied to positive mood, certain involuntary muscle control, and countless other functions, many of which are not yet fully understood.
While the method of action of serotonergic psychedelics is not fully understood, different serotonergic psychedelics are known to show affinities for different 5-HT receptors in different ways and at different levels, and may be classified by their activity at different 5-HT sub-sites, such as 5-HT1A, 5-HT1B, 5-HT2A, etc. Many serotonergic psychedelics, such as the family of tryptamines, have very strong structural similarities to serotonin itself, which partially explains the affinity for certain 5-HT sites. It is almost unanimously agreed that serotonergic psychedelics produce their effect by acting as strong partial agonists at the 5-HT2A receptors. How this produces the psychedelic experience is unclear, but it is likely that it acts by increasing excitation in the cortex, possibly by specifically facilitating input from the thalamus, the major relay for sensory information input to the cortex [1]. Worth noting is that selective serotonin reuptake inhibitors (a class of antidepressants including Paxil, Prozac, and Zoloft) strongly decrease the hallucinogenic effects of serotonergic psychedelics.[citation needed] MDMA's effect is also strongly negated by the use of SSRIs, however MDMA is generally not considered a serotonergic psychedelic, as the majority of MDMA's action appears to be through stimulating a release of serotonin, which does not appear to be the method of action of the serotonergic psychedelics.
Examples of Serotonergic psychedelics include LSD, psilocybin, DMT and mescaline. The tryptamines, such as DMT, psilocybin, etc structurally resemble serotonin itself, see Erowid.org's Chem-Compare tool, here shown comparing psilocybin and serotonin. The family of phenethylamines, which includes Mescaline and many of the research chemicals such as 2C-I more strongly resembles dopamine, as shown here in a comparison of dopamine and mescaline.
The most widely known use of serotonergic psychedelics is as recreational drugs, providing the user with a psychedelic experience known in the common slang as a trip.