Common cancer drugs may be more harmful to the brain than the tumour cells they are meant to destroy.

Laboratory tests have shown that dose levels typically used when treating patients killed 70-100 per cent of neural cells but just 40-80 per cent of cancer cells. Several types of healthy brain cell continued to die for at least six weeks after exposure.

The findings, published in the Journal of Biology, may help explain the little understood cancer therapy side effect of "chemo brain".

Patients can suffer symptoms ranging from memory loss to seizures, loss of vision and even dementia. Until recently, these problems were often blamed on a patient's mental state.

However, a growing body of evidence is now leading doctors to accept the reality of "chemo brain".

A study this year suggested that more than 82 per cent of cancer patients may suffer some form of mental impairment. While scientists have suspected that chemotherapy could have an impact on the central nervous system, it was not clear how this might occur.

Mark Noble, from the University of Rochester Medical Centre in New York, who led the research, said: "This is the first study that puts 'chemo brain' on a sound scientific footing, in terms of neurobiology and cellular biology."

The brain is populated with several types of cells that produce or repair normally functioning neurons. These are classified as dividing stem cells, dividing intermediate cells, precursors and progenitors, and non-dividing mature cells. Dr Noble's team exposed healthy brain cells as well as cancer cells to three chemotherapy drugs, carmustine, cisplatin and cytosine arabinoside, used to treat a wide range of diseases, including breast cancer, leukaemia and brain tumours.Tests showed that the drugs were toxic to all the different cell types even at very low concentrations.

The research points to several strategies for making cancer treatments safer, such as applying protective agents and screening to see what cell populations are most at risk.