Drugs used to destroy cancer cells may actually be more harmful to healthy cells in the brain, research suggests.
Chemotherapy can cause severe side effects
A team from New York's University of Rochester found several types of key brain cell were highly vulnerable to the drugs.
They say it might help explain side effects such as seizures and memory loss associated with chemotherapy - collectively dubbed 'chemo brain'.
The research is published in the Journal of Biology.
Drug therapy for cancer can prompt a wide range of neurological side effects, even the onset of dementia.
But they were thought not to be directly linked to the drug treatment itself.
Instead, some doctors have put them down to the patient's vulnerable psychological state.
The latest study found that dose levels typically used when treating patients killed 40% to 80% of cancer cells - but 70% to 100% of human brain cells grown in the lab, and caused serious damage to brain cells when given to mice.
Several types of healthy brain cell continued to die for at least six weeks after exposure.
Common drugs tested
Lead researcher Dr Mark Noble said: "This is the first study that puts chemo brain on a sound scientific footing, in terms of neurobiology and cellular biology."
The Rochester team carried out tests with three drugs used to treat a wide range of cancers: carmustine, cisplatin and cytosine arabinoside.
All three drugs were toxic to several types of brain cell whose job is to repair other cells in the brain - even at very low concentrations.
They also killed off oligodenrocyte cells, which play a key role in the transmission of messages around the nervous system.
The researchers suggest damage to cells in the hippocampus, which is responsible for memory and learning, is most likely to explain chemo brain symptoms.
Professor John Toy, Cancer Research UK¿s medical director, said: "The doses of therapy needed to treat cancer while leaving the body's healthy cells as unharmed as possible is a fine balance judged by experienced specialists.
"They aim to maximise benefits and minimise damage. Unfortunately side-effects can include toxicity to the brain.
"This research in mice may hopefully suggest new ways of researching how this toxicity might be overcome.
"It is important to remember, however, that all presently available cancer treatments have gone through extensive clinical trials to ensure that their benefits outweigh unwanted effects.
"No patient should stop their treatment because of this research."
The researchers said it might be possible to add protective agents to chemotherapy drugs.
They also suggest further work to pinpoint which cells are most at risk.