Researchers concluded that omega-3 fatty acids found in the fish oil could prove to be a safe and beneficial booster for tamoxifen therapy.
Tamoxifen blocks the female hormone oestrogen's ability to fuel breast cancer.
Rats with breast cancer were fed either a 17% fish oil diet or a 20% corn oil diet for eight weeks. In each case the oil supplement was given with or without tamoxifen.
Analysis of the tumours showed that omega-3 fatty acids in the fish oil increased the activity of genes promoting cellular specialisation, or differentiation.
This indicates an anti-cancer effect, since cancer cells are highly undifferentiated.
Combining fish oil and tamoxifen also reduced the activity of genes linked to tumour growth and cancer spread.
Study leader Dr Jose Russo, director of the Breast Cancer Research Laboratory at Fox Chase Cancer Center Philadelphia, said: "If a tumour was being treated with tamoxifen, the addition of an omega-3 fatty acid diet seemed to make the tumour, at least at the molecular level, more benign and less aggressive and responsive to tamoxifen."
The findings were presented at the annual meeting of the American Association for Cancer Research in Orlando, Florida.