Scientists have pinpointed how evening primrose oil fights breast tumours.
Nearly a third of cancers carry the Her-2/neu receptor
It is down to a substance in the oil called gamma-linolenic acid that acts on the same receptor in tumours as the powerful breast cancer drug Herceptin.
Unlike Herceptin, which blocks the Her-2/neu receptor, GLA interferes with the gene carrying the DNA code needed to make the receptor work.
The US work in the Journal of the National Cancer Institute applies to about 30% breast cancers.
Her-2/neu-positive tumours tend to be particularly aggressive. Activation of this receptor makes the cancer grow.
Studies have shown that the drug Herceptin (traztuzumab) cuts the risk of tumours returning in women with early stage Her-2/neu-positive breast cancer by 50%.
UK ministers have already decided that all women in England with early stage breast cancer should be tested to see if they could benefit from the drug.
However, Herceptin is currently licensed only for use in women with advanced breast cancer, although the government's drugs watchdog NICE has been asked to fast track its assessment of the wider use of Herceptin.
Inexpensive therapy hope
The latest work by Dr Javier Menendez and colleagues at Northwestern University suggests evening primrose oil might be a cheaper alternative, or add extra protection.
In their tests, treating Her-2/neu-positive breast cancers will both GLA and Herceptin led to a synergistic increase in death of the tumour cells and reduced cancer growth.
Earlier work by the same team showed that GLA also enhanced the efficacy of other breast cancer treatments, including chemotherapy and the anti-oestrogen drug tamoxifen.
"Since over-expression of Her-2/neu generally confers resistance to chemotherapy and endocrine therapies, our current findings can explain why GLA increases the efficacy of breast cancer treatments," said Dr Menendez.
He said the work showed that "an inexpensive herbal medicine" might regulate breast cancer cell growth and help control cancer spread.
Hazel Nunn of Cancer Research UK said: "A lot more work still needs to be done, but gamma-linolenic acid or GLA does seem to be a worthy candidate for further investigation in clinical trials.
"However, there is no evidence to suggest that taking GLA supplements can help prevent cancer. The best ways to reduce the risk of cancer are to avoid smoking and keep a healthy body weight."
Dr Sarah Rawlings of Breakthrough Breast Cancer said: "These early findings are interesting, however, this research does not suggest that women with breast cancer should routinely take evening primrose oil and any woman with questions about treatment should discuss them with her doctor."