 A large-scale, longitudinal study found that current frequent ketamine users have modest cognitive deficits, while infrequent or former heavy users do not.Many drugs have been found that lessen the risk of neurotoxicity from NMDA receptor antagonists.Centrally acting alpha 2 agonists such as clonidine and guanfacine are thought to most directly target the etiology of NMDA neurotoxicity.