Long-term effects of ketamine: evidence for a persisting impairment of source memory in recreational users

Rationale: Ketamine is an N-methyl-D-aspartate (NMDA)-receptor antagonist that is increasingly being used as a recreational drug. Previous research has shown gross generalised verbal memory impairments persisting 3 days after ketamine use in chronic users, however episodic memory has not specifically investigated in this population. Objective: To determine whether ketamine, on the night of drug use (day 0) and 3 days later, is associated with impaired episodic memory as assessed by a source memory task. Methods: Twenty ketamine users and 20 poly-drug controls were compared on a source memory task both on day 0 and 3. Participants also completed questionnaires on both days indexing schizophrenic-like and dissociative symptoms. Results: On day 0, ketamine abusers were impaired on both source memory and item recognition and scored more highly on schizophrenic and dissociative symptom scales compared to poly-drug controls. On day 3 ketamine abusers only displayed source memory impairments and these positively correlated with the level of schizophrenic-like symptoms on day 0. No differences on day 3 in schizophrenic-like or dissociative symptoms were observed. Conclusions: Ketamine abusers exhibit a persisting deficit in source memory on day 3 but not in item recognition. These findings suggest that repeated use of ketamine produces chronic impairments to episodic memory.