A controlled, randomized study on the treatment of schizophrenic psychosis with cannabidiol, a phytocannabinoid is performed. This approach is based upon recent findings indicating that the human endogenous cannabinoid system is significantly involved in the pathogenesis of schizophrenia. Our group has shown, for example, that Δ9-tetrahydrocannabinol (Δ9-THC) is able to provoke schizophrenia-like psychotic symptoms in healthy volunteers. This, as well as the capability of Δ9-THC to exacerbate productive psychotic symptoms in schizophrenic patients, has recently been confirmed by others. Furthermore, we found that the en-dogenous brain constituent anandamide, an endogenous Δ9-THC agonist, is significantly elevated in the CSF of schizophrenic patients. Cannabinergic substances such as anandamide may enhance dopaminergic neurotrans-mission by increasing dopamine turnover. They may also influence the onset or course of schizophrenia by as yet unidentified mechanisms We seek to investigate the efficacy of cannabidiol in the treatment of schizophrenic and schizophreniform psy-choses, because there is evidence that CB1 antagonists such as SR141716 and cannabidiol have antipsychotic effects comparable to those of classic neuroleptic drugs. Furthermore, cannabidiol is well tolerated showing few side effects in humans. Cannabidiol may serve as an antipsychotic medication that is not primarily based upon an antidopaminergic but upon different mechanisms, especially anticannabinergic ones. It may therefore be an effec-tive medication in at least a subgroup of schizophrenic and schizophreniform patients and may be expected to show additional anxiolytic effects and only minor side effects.

The control condition in this parallel design will be an established neuroleptic treatment with amisulpride that is primarely an antidopaminergic drug. Thus, we will study not only the antipsychotic efficacy of cannabidiol, but we will also compare the effects of both treatment strategies on side effects and neuropsychological functioning.

Inclusion Criteria:

• Clinical diagnosis of schizophrenia or schizophreniform psychosis according to DSM-IV.
• BPRS score >36 and BPRS psychosis cluster > 12.
• Ability to provide written informed consent.
• Participants are required an adequate contraception.

Exclusion Criteria:

• Any severe neurological or somatic disorder.
• Other psychiatric disorders including addictive disorders.
• Positive urine drug screening for any compound except benzodiazepines.
• No pregnancy or breast feeding.