Rare but striking individual differences in responsiveness to cannabinoids have been observed that might involve mutations in the gene encoding the brain-expressed cannabinoid receptor. In a preliminary study, the human CB1 cannabinoid receptor coding region was comparatively sequenced in different groups of individuals: one group showed acute psychotic symptoms after cannabis intake, while another group did not develop any psychopathology after long-term heavy cannabis abuse. No evidence for structural mutations was obtained, which might provide some insight into the molecular basis of individually different responsiveness to cannabinoids. Comparison of CB1 cannabinoid receptor amino acid sequences between species substantiated evidence that the protein sequence is relatively well conserved.