The researchers concluded: "The provisional results from our Phase II study suggest high-dose oral GLA to be a valuable new agent in the treatment of endocrine-sensitive breast cancer."
Borage, also known as the starflower, has been used in medicine for more than 700 years.
It contains a substance known as gamma linolenic acid (GLA).
Work being carried out in the Netherlands shows that a diet rich in GLA reduces the risk of prostate tumours.
Lab tests in America and South Africa show GLA can kill brain and prostate cancer cells.
And in Wales, researchers have discovered that the chemical can inhibit the spread of maligant tumours by restricting growth of the blood vessels that supply them with vital nutrients.
Now research published in the Journal of Cancer indicates that GLA can also boost the impact of the breast cancer drug Tamoxifen.
The research focused on 38 breast cancer patients who were given eight oral capsules of GLA, a total of 2.8 grams a day, plus Tamoxifen, and compared them with a control group of women who took just Tamoxifen.
The women taking the cocktail responded to the drug significantly faster than the controls.
The researchers concluded: "The provisional results from our Phase II study suggest high-dose oral GLA to be a valuable new agent in the treatment of endocrine-sensitive breast cancer."
Dr Alan McGown, the head of drug development at the Cancer Research Campaign (CRC), said borage contained the most potent concentration of GLA in nature. Oil from the plant is 24% GLA.
The compound is also found in evening primrose oil which is used to treat premenstrual problems.
Professor Gordon McVie, the director general of the CRC, said women should be aware of the potential benefits of GLA but he advised patients to consult their doctors.