Experimental evidence from laboratory studies clearly demonstrates a robust temporal relationship between exposure to cannabinoids and psychotic symptoms. The onset of cannabis use may precede, follow, or co-occur with the onset of schizophrenia. Allebeck et al. [9] reported that in 69% of a schizophrenic patient sample from a Swedish case registry (n = 112), cannabis abuse preceded the onset of psychotic symptoms by at least 1 year. Further, in only 11% did the onset of psychotic symptoms precede the onset of cannabis abuse. Similarly, Linszen et al. [133] found that cannabis abuse preceded the onset of psychotic symptoms by at least 1 year in 23 of 24 cannabis-abusing recent-onset schizophrenic patients. Hambrecht and Hafner, [83, 84] in their study of first-episode schizophrenic patients, found that 14.2% of the sample had a lifetime history of drug abuse, with cannabis being the most frequently abused drug (88%). Furthermore, drug abuse preceded the first sign of schizophrenia by more than 1 year but typically by more than 5 years in 27.5% of patients. In 37.9% of individuals, drug abuse followed the first sign
