Disclosed is a pharmaceutical composition for treating oxidative stress comprising a therapeutically effective amount of a substantially pure compound of pterostilbene and a physiologically acceptable carrier. Pterostilbene is administered in an amount between about 2.5 mg to about 10 mg per kilogram of subject body weight. Also disclosed is a method for increasing a working memory of a subject, the method comprising administrating an effective amount of a substantially pure compound of pterostilbene, wherein the working memory for a subject increase and the therapeutic effectiveness is about 10 mg of pterostilbene per kilogram of subject body weight.