A drug widely used to treat cancer may cause brain damage, with the effects lasting for years after the end of treatment, research suggests.
The drug, 5-fluorouracil (5-FU), is used, alongside others, to treat cancers of the breast, ovaries, colon, stomach, pancreas and bladder.
Tests on mice showed it destroys vital cells in the brain that help to keep nerves functioning properly.
The University of Rochester study features in the Journal of Biology.
The researchers say their findings could explain some of the neurological side effects associated with chemotherapy - a phenomenon often known as "chemo brain".
These include memory loss, poor concentration, and in more extreme cases, seizures, impaired vision and even dementia.
Until recently they were often dismissed as the by-products of fatigue, depression and anxiety related both to the diagnosis and treatment of cancer.
But many patients show symptoms: a previous study by the Rochester team found more than 80% of breast cancer patients reported some form of mental impairment after chemotherapy.
The latest study found 5-FU attacks oligodendrocyte cells in the brain and the precursor stem cells from which they originate.
These cells play a crucial role in the central nervous system, producing myelin, the protective sheath that keeps nerve fibres in working order.
If myelin is not constantly renewed, communication between nerve cells is damaged.
The researchers showed that oligodendrocytes virtually disappeared from the brains of mice six months after the animals were treated with 5-FU.
Lead researcher Dr Mark Noble said: "It is clear that, in some patients, chemotherapy appears to trigger a degenerative condition in the central nervous system.
"Because these treatments will clearly remain the standard of care for many years to come, it is critical that we understand their precise impact on the central nervous system, and then use this knowledge as the basis for discovering means of preventing such side effects."
The latest study builds on previous work by the Rochester team, which found that three widely used chemotherapy drugs were more toxic to healthy brains than the cancers they were supposed to treat.
Martin Ledwick, of the charity Cancer Research UK, stressed the study had been carried out on animals, and that more work would be needed before any firm conclusions could be drawn on the effect on cancer patients.
He added: "It must be remembered that this drug (5-fluorouracil) can offer significant benefits for people who need it which far outweigh the changes which some patients report."