Tamoxifen is an effective treatment

The drug has a good record in helping to treat oestrogen receptor positive breast cancer, and to prevent the recurrence of the disease.

It appears to have no positive impact in a minority of cases, however.

The latest research suggests that high levels of two proteins produced by the cancerous tumour may reduce the drug's effectiveness.

The researchers, from Baylor College of Medicine in Houston, found that when two proteins, called AIB1 and HER-2, are present in high levels in the cells of women who have had breast cancer, their tumours are resistant to the cancer-fighting effects of tamoxifen.

Lead researcher Dr Kent Osborne said: "The finding could be important in predicting which women will respond to tamoxifen therapy."

It could be that the research enables scientists to develop a test to identify women who would be better off receiving a different breast cancer drug, such as anastrozole.

However, he stressed that the findings would need to replicated before they could be used in patient treatment.

Tests

Dr Osborne's team examined levels of the two proteins in 316 breast cancer patients.

They found that women with the highest levels were least likely to respond to tamoxifen treatment.

However, women with high levels of AIB1 seemed to respond better than others to other forms of cancer therapy.

Dr Richard Sullivan, of Cancer Research UK, told BBC News Online that the study was interesting but the theory needed to be tested on a larger group of patients.

He said: "We have made tremendous strides in reducing mortality from breast cancer over the last 20 years, but to make further progress we need to become increasingly sophisticated in the way we use new and currently available drugs."

The research is published in the Journal of the National Cancer Institute.