Variation in DRD4 has also been tested for association with therapeutic response following sub-chronic methylphenidate administration in children diagnosed with ADHD (Hamarman et al. 2004). Children with the 7-repeat allele of the exon III polymorphism required higher doses of methylphenidate (about two times more) to achieve the same therapeutic response over a 2-week period as measured by the CGI-S. Cheon et al. (2006) undertook a similar study and found that the 4-repeat allele was associated with the best treatment responses based on the ARS and the Kiddie-Schedule for Affective Disorders and Schizophrenia-Present and Lifetime Version (K-SADS-PL; Kaufman et al. 1997). Only one individual in that study had the 7-repeat allele, and therefore the previous finding could not be examined; conversely, the 4-repeat allele was not analyzed in the Hamarman et al. (2004) study. Therefore, these two sub-chronic studies found associations with different alleles of the exon III polymorphism and different measures of methylphenidate response.
