BDNF and major depressive disorder: Angelucci et al., (2007) found a reduction in BDNF serum levels in patients addicted to heroin. However, Heberlein et al., showed an increase in BDNF serum level in patients addicted to heroin, substituted by opioids (Heberlein et al., 2011). The reasonably small sample size (N=15) in the case of Angelucci et al., can be blamed for their results. In addition, nicotine and alcohol (Joe et al., 2007), depression (Molendijik et al., 2011), and stress (Miltoma et al., 2008) can play a role in obtaining the different results.