When I search on it, not coming up with much on pubmed:
http://www.ncbi.nlm.nih.gov/pubmed?t...reast%20cancer
I seem to remember looking at this a while back and found it to have estrogenic properties...perhaps explaining investigations into it helping with hot flashes. But..considering growing suggestion Her2 and ER receptor may take turns driving even ER-/her2 + BC, it seems (to me) at minimum a tough call as to whether primrose oil will help or hurt at any given point.