In and out of the K-hole: a comparison of the acute and residual effects of ketamine in frequent and infrequent ketamine users

Aims. Ketamine, a non-competitive NMDA receptor antagonist, produces acute impairments of working, episodic and semantic memory along with psychotogenic and dissociative effects when a single dose is given to healthy volunteers. In recreational users, Curran & Morgan (2000) showed that ketamine produced the same acute effects but that 3 days after ingestion, ketamine users showed persisting memory impairment and elevated psychotogenic symptoms compared with controls. To explore whether such persisting effects re? ect chronic effects of ketamine use, the present study compared frequent with infrequent users of ketamine on the night of drug use and again 3 days later. Design and participants. Eighteen frequent and 19 infrequent ketamine users were assessed on each test day on a range of cognitive tasks tapping memory and attentional function and on subjective scales (schizotypal symptomatology, dissociation, mood). Findings. Groups were broadly matched for polydrug use apart from ketamine which frequent users took significantly more often and in larger quantities than infrequent users. Acute effects on day 0 replicated previous findings. On day 3 frequent users showed significant impairments on tasks tapping episodic and semantic memory but there was no evidence of persisting dissociative or schizotypal symptoms. Conclusion. These findings indicate that frequent use of ketamine produces long-lasting impairments in episodic memory and aspects of retrieval from semantic memory. Such effects accord with animal evidence of the effects of NMDA receptor blockade on memory. Those using, or contemplating using ketamine should be informed of these persisting, detrimental effects of the drug upon human memory.