Seventy detoxified heroin addicts were randomly assigned to one of two groups receiving ketamine-assisted psychotherapy (KPT) involving two different doses of ketamine. There were 35 heroin addicts (27 male and 8 female) in the experimental group, and 35 heroin addicts (28 male and 7 female) in the control group. The patients of the experimental group received existentially oriented psychotherapy in combination with a hallucinogenic ("psychedelic") dose of ketamine (2.0 mg/kg i.m.). The patients of the control group received the same psychotherapy combined with a very low, non-hallucinogenic (non-psychedelic), dose of ketamine (0.2 mg/kg i.m.). This low-dose induces some pharmacological effects without inducing a peak psychedelic experience. Both the psychotherapist and patient were blind to the dose of ketamine. Otherwise, all patients were treated alike and were given the same preparation. The KPT sessions, regardless of dose, also were similar. All patients' psychological and clinical evaluations during the treatment and follow-up period were performed by a clinician evaluator other than the psychotherapist providing KPT. This rater was also blind to the dose of ketamine. KPT included preparation for the ketamine session, the ketamine session itself, and the post session psychotherapy aimed to help patients to integrate insights from their ketamine session into everyday life. During the ketamine session, the psychotherapist provided emotional support for the subject and carried out psychotherapy. Psychotherapy was existentially oriented, but also took into account the subject's individual and personality problems.

The results of this double-blind, randomized clinical trial of KPT for heroin addiction showed that high-dose (2.0 mg/kg) KPT elicits a full psychedelic experience in heroin addicts as assessed quantitatively by the Hallucinogen Rating Scale. On the other hand, low-dose KPT (0.2 mg/kg) elicits "sub-psychedelic" experiences similar to ketamine-facilitated guided imagery. High-dose KPT produced a significantly greater rate of abstinence in heroin addicts within the first two years of follow-up than did low-dose KPT. High-dose KPT elicited a greater and longer-lasting reduction in craving for heroin (assessed with the Visual Analog Scale of Craving), as well as greater positive change in nonverbal unconscious emotional attitudes (assessed with the Color Test of Attitudes). Thus, the higher rate of abstinence in the high-dose group may be related to KPT's effects on craving (similar to other NMDA receptor ligands) and modification of nonverbal unconscious emotional attitudes. KPT-induced effects on depression, anxiety, anhedonia, and psychological changes as assessed by the MMPI, Locus of Control Scale, Questionnaire of Terminal Life Values, Purposes-in-Life Test, and Spirituality Scale were similar in the experimental and control groups. These results support the conclusion that high-dose ketamine-assisted psychotherapy may improve abstinence in heroin addicts through reduction in craving. However, it also appears that the acute psychedelic effects induced by psychedelic psychotherapy on the verbal level do not always lead to high rates of abstinence from drugs and alcohol. Further research should explicate how high-dose KPT improves relapse rates, and how to apply more optimally acute drug-induced psychological effects towards therapeutic ends.