Ketamine and rapidly acting antidepressants: Breaking the speed of sound or light?

There is an urgent need for rapidly acting antidepressants. Current therapies share a delayed onset of action, contrasting with drugs of abuse that have rapid psychotropic effects but cause tolerance and dependence. A key uncertainty is whether there is a finite speed limit imposed by the critical role of homeostatic adaptive mechanisms that underpin the efficacy and onset of available psychotropic agents and whether this is mutable with emerging agents with potential rapid onset, in particular ketamine.