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A definitive enantioselective pharmacokinetic evaluation of dl-threo-methylphenidate (MPH) was carried out in 11 healthy volunteers, all of whom received, in a randomized crossover design, three oral administrations of MPH: immediate release (IR), slow release (SR), and SR chewed before swallowing (CH). In addition, all subjects received MPH intravenously (IV) on a separate occasion. Both plasma and urine samples were collected for up to 16 hr after each drug administration. Significant enantioselective differences were found in pharmacokinetic parameters such as CL, MRT, Vd_ss, AUC₀ ^∞, and t_1/2. A profound distortion of the enantiomeric ratio for MPH (d ≫ 1) was evident in all plasma samples harvested after oral administration. After IV MPH, however, there was no significant distortion in the plasma d/1 ratio until 1.5 hr after dosing, whereafter there was a divergence of the plasma levels of the …