According to the report in the Journal of Clinical Oncology, this affects about one-third of cancer patients.
In the study, about 33 percent of those on high-strength opioids, such as morphine, scored lower on mental tests measuring attention and work recall.
Researchers say there's no proof the painkillers are causing these problems and more studies are needed.
But they insist there needs to be better approaches to the management of pain in cancer because people have to choose being pain free or not being able to think clearly.
