Until recently, Catechol-O-methyltransferase (COMT) was the only candidate for a possible interaction between a genetic predisposition for schizophrenia and heavy cannabis abuse [(Caspi et al., 2005; O'Tuathaigh et al., 2010) but see also (Zammit et al., 2011)]. This evidence concerns the gene COMT and schizophrenia.