The association between the 3′-UTR VNTR and response to sub-chronic methylphenidate has also been examined in a larger cohort of children and adolescents with ADHD (Purper-Ouakil et al. 2008). Individuals received placebo and varying doses of methylphenidate until no further clinical improvement or limiting side effects occurred (mean dose 31.19 mg/day). They were phenotyped with the ARS, the Stroop test (Stroop 1935), the Trail Making Test (Reitan 1958), and the Continuous Performance Test (Rosvold et al. 1956). Subjects that were homozygotes for the 10-repeat allele had significantly lower treatment responses. However, the final methylphenidate doses reached did not differ across genotype group. These results suggest that the maximal response to methylphenidate may be lower in 10/10 homozygotes and cannot be overcome with larger doses. Similarly, another study of adults with ADHD found that the 10/10 genotype was associated with non-statistically significant lower therapeutic response to 3 weeks of treatment with methylphenidate as assessed by the CGI-S and ARS (Kooij et al. 2008).
