Patients, and some doctors, assumed it was due to the effects of the harsh chemicals in the chemotherapy.
A few months ago, a study proved for the first time that "chemo fog" is real. Now, a separate study suggests it may not be caused by chemotherapy, but by the disease itself.
That's according to a new study from Florida's Moffitt Cancer Center.
Researchers there found breast cancer survivors scored lower on memory tests than those who never had cancer, regardless of whether or not they had chemotherapy.
They also scored lower on tests measuring speed and concentration.
And they found the effect sometimes lasted several years.
But they found no difference in cognitive problems for patients who were taking hormonal therapy, such as tamoxifen.
The research was published in the online version of the journal "Cancer," by the American Cancer Society.