BDNF and drug dependence: Meanwhile, our previous studies have investigated increased serum BDNF levels during the early withdrawal in heroin or methamphetamine dependence, and serum BDNF levels were positively related to psychotic symptoms such as depressive symptoms and impulsivity.[8,12] Also, some researchers have shown that promoter methylation of BDNF was associated with drug dependence such as heroin and alcohol.[13] Thus, it can be seen that BDNF played a significant role in heroin dependence.