The most important CNS effects are psychotropic. Psychedelic effects are dose dependent but are not uncommon even at the low doses used in the treatment of chronic pain (20–30 mg/h). In a study in healthy volunteers, ketamine caused distortion of reality, auditory hallucinations, paranoid ideas, anxious feelings (panic attacks) an inability to control thoughts, derealization in time and space, visual hallucinations, and increased awareness of sound and color. An intense feeling of a high was felt, which some described as extremely unpleasant; others expressed an intense feeling of euphoria. Other CNS adverse effects are dizziness, blurred vision, vertigo, nausea/vomiting, dysphasia, nystagmus, nightmares or vivid dreams, impaired motor function, and memory deficits [11, 12]. These effects decrease rapidly after termination of ketamine administration, although unpleasant dreams may persist up to three nights afterward. In clinical studies the incidence of psychological or psychedelic effects, although common, is still low. Sedation is most common during the infusion; the incidence of other effects is approximately less than 5% [10, 13].

Adverse effects were overall considered mild by patients in a retrospective study where subanesthetic doses of ketamine was administered over 30–60 min (Table 6.2) [10].