Approximately 1,400 young adult twin pairs from the Norwegian
Institute of Public Health Twin Panel were interviewed and assessed
for their lifetime use of illicit drugs, including marijuana, stimulants, opiates, cocaine and psychedelics. Researchers defined the significant lifetime use of illicit substances as use 10 or more times.
Previous theories have suggested that genetic factors might be of less importance in influencing drug use in societies where drugs were not widely available.