Anticholinergics such as atropine, hyoscyamine, and scopolamine antagonize the muscarinic acetylcholine receptors in the eye. By blocking these receptors, the pupils are no longer capable of constriction and dilation results. Such alkaloids present in many plants of the family Solanaceae may also induce mydriasis when used recreationally.[5]
Most hallucinogens such as LSD, psilocybin, and mescaline, in addition to some other phenethylamines and tryptamines, all produce mydriasis by agonizing the serotonergic 5-HT2A receptors in the brain. Dissociatives such as ketamine, DXM, and PCP do so as well via antagonism of the NMDA glutamate receptors. There have also been reports that the atypical psychedelic herb Salvia divinorum and its active constituent salvinorin A causes mydriasis. It works via agonism of the κ-Opioid receptors in the brain. How the neurological changes induced by these drugs ultimately causes pupil dilation is unknown.
