Chemobrain refers to a syndrome (a bunch of symptoms) that some people get when they take chemotherapy drugs. A more formal name is Chemotherapy-Related Cognitive Dysfunction. Another informal name is "chemo fog". Chemotherapy is a systematic treatment, meaning it goes all over the body. An unfortunate side effect of many chemotherapy agents is a diminshment in mental energy and acuity. As a stimulant, modafinil has found some interest from doctors for treatment of this condition.

Management of Cognitive Changes in Oncology Patients

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A phase III clinical trial of modafinil on people taking the chemotherapy drug doclataxel found "modest" benefit for the fatigue those patients experience.

Mayo Clinic on Chemo Brain.