There was a study with mice that showed "permanent" damage starting after about one month of daily dosing and getting consistently worse after 6 months of daily dosing. Of course by "permanent" damage, they mean that immediately after 6 months of daily injections of ketamine they gave them a few hours to metabolize the last dose given, and then the mice performed worse than control mice. They did not reevaluate the mice after some time had passed after the cessation of injections, but rather killed them and counted the neurons present in their brains, which revealed that no physical brain damage had taken place (ketamine group and control group had no statistically significant difference in the amount of neurons). They also noted significant weakening of the bladder walls in the ketamine group.