Nicotine use and nicotine dependence (or tobacco use disorder) are influenced by both genetic liability and environmental factors1,2. Several different smoking-related phenotypes have been studied, including smoking initiation3, cigarettes per day3, the Fagerström Test for Nicotine Dependence (FTND)4, problematic tobacco use5, tobacco use disorder defined by codes from the International Classification of Diseases (ICD-TUD)6, and nicotine dependence7 defined by the Diagnostic and Statistical Manual of Mental Disorders (DSM-NicDep) (Table 1).
