The gene responsible for encoding SERT has a functional polymorphism at the 5′-regulatory promoter region, which results in two forms, long (L) and short (S). The LL-genotype is hypothesized to play a key role in the early onset of alcohol use. The present invention discloses the differences in treatment and diagnosis based on the L or short genotypes as well as on a single nucleotide polymorphism of the SERT gene, the 3′ UTR SNP rs1042173. The present invention demonstrates the efficacy of using the drug ondansetron and similar drugs for treatment based on variations in the polymorphisms of the SERT gene as well as methods for diagnosing susceptibility to abuse of alcohol and other addiction-related diseases and disorders.