The below paper focuses on recent advancements in cancer chemopreventive and anticancer efficacy of grape seed extract and other grape-based products. Overall, completed studies from various scientific groups conclude that both grapes and grape-based products are excellent sources of various anticancer agents and their regular consumption should thus be beneficial to the general population.
As you may have found out from a Google search, there are claims that these have a curative effect. However, as far as I know, there’s never been a rigorous scientific check of this.
I have been using this extract on and off for several years, and check it against the cancer biomarkers I use to track my own cancerousness. From this, I can tell you that there are confusing results. For my main cancer, bladder cancer, the correlation is positive - that is, the more grape seed extract I take, my bladder cancer marker shows my cancer is worsening; for my ancillary pancreatic marker, the relationship is the opposite. Note tho’: correlation does not mean causal - this is, one may not actually cause the other, tho’ it may. In this case, as the relationships are in opposite directions, then for me at least, grape seeds do not appear to be helpful.