The aim of the study was to investigate a possible contribution of the cannabinoid receptor gene (CNR1) to the development of i.v. drug addiction. Allele and genotype frequencies of a previously associated flanking triplet repeat polymorphism were compared between patients and controls, and the whole coding region of the CNR1 gene of all patients were screened for presence of mutations. The study took place at the Addiction Treatment Unit of the Medical School Hannover, and two outpatients' departments in Hannover, Germany. Forty German unrelated opioid addicts (27 males and 13 females; mean age 37.9 years; range 16-53 years), took part, all of them satisfying ICD-10 and DSM-IV diagnostic criteria for opioid dependence and 81 age- and sex-matched controls (German blood donors). Measurements used were lengths of alleles, genotyping and single strand conformation polymorphism (SSCP) analysis. Neither the >/= 5 alleles of the extragenic triplet repeat (AAT) marker nor the alleles of an intragenic biallelic CNR1 polymorphism (1359G/A) were associated with i.v. drug use in our study group. In addition, we did not detect any sequence variation within the CNR1 gene which could confer susceptibility to i.v. drug abuse. In contrast to previous investigations, we found no evidence for an involvement of the CNR1 gene in i.v. drug addiction.