New research suggested the benefits of the “breakthrough” new drug palbociclib, which is given to women with oestrogen receptor-positive breast cancer, was effectively reversed by xenoestrogens.
The compounds are present in products made from wheat, maize, barley and other staples.
Scientists said women currently taking the drug should consider altering their diet.
They also suggested that, as well as hindering the benefit of the palbociclib, xenoestrogens may accelerate the growth of oestrogen-fuelled cancers themselves.
Manufactured by Pfizer, the medication was approved for use on the NHS in November for women with oestrogen receptor-positive breast cancer - which accounts for about 70 per cent of cases - in whom the disease was advanced and unlikely to be cured.
The National Institute for Health and Care Excellence gave the green light after trials showed palbociclib could double the period progression-free survival in some patients.
Currently available to up to 8,000 women in the UK, it has been described as the “closest thing to a cure”, although scientists have hinted the drug may one day be improved to form a replacement to chemotherapy for women with early-stage breast cancer.
However, the new laboratory research by The Scripps Research Institute in California showed that even limited exposure to xenoestrogens largely reversed the impact of the medication when used in combination with letrozole.
"It's intriguing that even a low, background-level exposure to these xenoestrogens was enough to impact the effect of the therapy to this degree," said Dr Benedikt Warth, lead author of the study, which is published in the journal Cell Chemical Biology.
His colleague, Dr Gary Siuzdak, said: “Breast cancer patients taking palbociclib/letrozole should consider limiting their exposure to foods that contain xenoestrogens."
The Scripps team warned they had only tested the effect of two types of xenoestrogens and that many more were to be found in the human diet.
“There are probably a lot of clinically relevant discoveries yet to be made,” said Dr Warth.