Ketamine er et dissociative anaesthetic closely related with phencyclidine (PCP). Phencyclidine is a non-competitive NMDA-antagonist, and it is assumed that the pharmacodynamic mechanism of action for ketamine is the same. Receptor binding studies shows that ketamine has affinity to many receptor types, including opioid mu and kappa receptors. Ketamine has only about 25 times lower affinity for kappa receptors than for the NMDA-receptor complex. Naloxone is a specific antagonist for opioid receptors and block both mu og kappa receptors. A dose of naloxone 10 times larger than required to block mu receptors is required to block kappa receptors. Experiments with naloxone suggest that ketamine is not a mu agonist, but experiments with sufficient large naloxone doses to block kappa receptors have not been carried out in humans.