Modafinil acid is a metabolite of modafinil, a wake-promoting agent for oral administration. Orally administered modafinil is extensively biotransformed in the liver to the inactive metabolites modafinil acid and modafinil sulphone, before being eliminated primarily in the urine (elimination half-life 9 to 14 hours). Modafinil acid was the major urinary metabolite, which accounted for 35% to 60% of the dose. Two major metabolites of modafinil, modafinil acid, and modafinil sulfone, do not appear to contribute to the CNS-activating properties of modafinil.