Aims: Recreational use of the N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine, is increasing. The present study aimed to examine both the acute and residual effects of this drug on cognitive function, dissociation and schizotypal symptomatology in recreational users.
Design and participants: A parallel group design was used to compare 20 volunteers who reported having taken ketamine with 19 volunteers who reported no consumption of ketamine on the relevant night (day 0). All 39 participants were tested on day 0 and again 3 days later. On each test occasion a battery of tests was administered which tapped a wide range of memory functions, attention, dissociation, schizotypal symptomatology and mood.
Findings: Groups were broadly matched for polydrug use apart from ketamine. Acute effects on day 0 replicated previous laboratory studies showing a broad spectrum of cognitive impairments following ketamine administration as well as marked dissociative effects and schizotypal symptomatology. Three days later, ketamine users had significantly higher scores than controls on both dissociation and schizotypal symptomatology. On some cognitive measures there were no group differences on day 3; however, on tests tapping semantic memory, the ketamine users showed persisting impairments compared with controls.
Conclusions: Ketamine appears to induce acute and severe impairments of working, episodic and semantic memory as well as psychotogenic and dissociative effects. Three days after drug ingestion, recreational users display semantic memory impairment and dissociative and schizotypal symptomatology which could reflect chronic or residual effects of taking the drug or pre-existing differences in ketamine users.