Following oral administration of methylphenidate, 78-97% of the dose is excreted in the urine and 1-3% in the feces in the form of metabolites within 48-96 hours. The main urinary metabolite is ritalinic acid (a-phenyl-2-piperidine acetic acid, PPAA); unchanged methylphenidate is excreted in the urine in small quantities (<1%). Peak PPAA plasma concentrations occurred at approximately the same time as peak methylphenidate concentrations, however, levels were several-fold greater than those of the unchanged drug. The half-life of PPAA was approximately twice that of methylphenidate.