Ketamine, a dissociative anaesthetic in use since 1970, produces prominent psychoactive effects in humans. Its non-medical use has raised concerns in many countries, including Singapore. This paper narrates the psychedelic and psychotic effects of ketamine in two ketamine dependent patients who have presented to the psychiatric service. These effects were dose-related and comprised multimodal hallucinatory experiences, a sense of slowing, paranoid ideation and enhancement of sexual, musical and sensory enjoyment. In both ketamine users the psychotic symptoms resolved quickly with symptom-targeted treatment. However, breaking the ongoing addiction cycle seemed more difficult. The neuro-pharmacological mechanisms of these phenomena are largely due to its complex multi-receptors actions, notably through the excitatory amino acids through mainly the N-methy-D-aspartate NMDA receptors. The detection of ketamine abuse requires a high index of suspicion and needs to be considered when there is an acute presentation with multi-modal hallucinations and psychosis.