to one another can also contribute to the lower probability of remission among individuals with dependence on other drugs [55]. Genetic studies also suggest a significant overlap across substances in the genetic liability to dependence. For example, nicotine and alcohol dependence may share over 60% of their genetic vulnerabilities and alleles of several genes have been associated with alcohol, nicotine and polysubstance dependence [56]. The existence of comorbid SUDs poses a therapeutic challenge for clinicians, given the mixed evidence supporting sequential treatment over simultaneous treatment [57]. The findings of this study also emphasize the need to understand common mechanisms underlying the rewarding effects of drugs and the sequential neuropharmacological neuroadaptations once an addiction has developed.[54].
