Taken from Wikipedia -
http://en.wikipedia.org/wiki/Mydriasis
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 plants of the genus Brugmansia may also induce mydriasis.[2]
Many hallucinogens induce mydriasis. The serotonergic hallucinogens; such as LSD; psilocybin, the active substance contained in some psychotropic mushrooms; mescaline, the active substance contained in some psychotropic cacti such as San Pedro; hallucinogenic phenethylamines such as 2C-B; and DMT, naturally produced by the human body and several hallucinogenic plants, all produce mydriasis by agonizing the serotonergic 5-HT2A receptors in the brain. The 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 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.
Drugs that increase overall serotonin levels in general are capable of causing mydriasis in the same way as the 5-HT2A-mediated psychedelics. This is because serotonin (5-HT) itself is naturally responsible for normal 5-HT2A stimulation. Hence, in sufficient quantities serotonin is mydriatic and can even be mildly psychedelic, though the potentially fatal serotonin syndrome usually ensues before the psychedelia becomes overly-pronounced. Examples of such drugs include MDMA (as well as other MDxx compounds), fenfluramine, chlorphentermine, cocaine, Amphetamines, Stimulants such as Adderall and many widely-prescribed antidepressants such as the SSRIs, SNRIs, and MAOIs. Natural serotonin-boosting supplements such as L-Tryptophan and 5-HTP are also potentially capable of this in excessive doses.
The neurotransmitter norepinephrine (noradrenaline) regulates many physiological processes in the body and brain. One of them is the autonomic constriction and contraction of certain muscles. The psychoactive drug cocaine potently inhibits the normal reuptake of norepinephrine into presynaptic nerve terminals resulting in an increased level of extracellular norepinephrine. The released norepinephrine then proceeds to bind to adrenergic receptors, and the biological effects of norepinephrine finally occur. When a solution of cocaine is dropped into the eye, this process takes place, and the end result is dilation of the pupil. Cocaine itself is not typically used for this task however. Any potent norepinephrine reuptake inhibitor or release agent should be capable of such an effect.
The opiates and opioids such as morphine and heroin cause miosis. Subsequently, mydriasis occasionally occurs during opioid rebound and withdrawal. [3]
