The medical doctors, who use drugs to address disease, usually just cover up the symptoms of that disease and make the patient feel better. With cancer, they use the drug to attack these cells and tumors which attack a health body. The theory behind chemotherapy is that the drugs attack fast growing cells. The cancer cell is a fast growing cell as is hair and red blood cells which is why hair loss and anemia are big side effects of this procedure.
For as long as chemotherapy has been around, there has been theories as to why it creates something called chemo-brain or chemo fog; that loss of mental faculties that slows the person down in their thinking and ability to multi-task.
There are lots of theories about what creates chemo brain.
We found a study that was done which has a very good validity as to why this phenomena happens. You can read about it here Chemo Fog Research