The dietary consumption of grape and its products is associated with a lower incidence of degenerative diseases such as cardiovascular disease and certain types of cancers. Most recent interest has focused on the bioactive phenolic compounds in grape. Anthocyanins, flavanols, flavonols and resveratrol are the most important grape polyphenols because they possess many biological activities, such as antioxidant, cardioprotective, anticancer, anti-inflammation, antiaging and antimicrobial properties. This review summarizes current knowledge on the bioactivities of grape phenolics. The extraction, isolation and identification methods of polyphenols from grape as well as their bioavailability and potential toxicity also are included.
– Biological activities of polyphenols from grapes. — Xia EQ, Deng GF, Guo YJ, Li HB. — Int J Mol Sci. 2010 Feb 4;11(2):622-46.

Proanthocyanidins, extracted from grape seeds, are widely used mainly as nutritional supplements. However, there has not been a systematic report to investigate toxicological studies on proanthocyanidins, especially in oral administration. In our studies, proanthocyanidin-rich extract from grape seeds was subjected to a series of toxicological tests to document its safety for use in various foods. The results of our studies indicate a lack of toxicity and support the use of proanthocyanidin-rich extract from grape seeds for various foods.
– Safety evaluation of proanthocyanidin-rich extract from grape seeds. — Yamakoshi J, Saito M, Kataoka S, Kikuchi M. — Food Chem Toxicol. 2002 May;40(5):599-607.

Grape seed extract may cause excessive bleeding in those taking blood thinners.

Do not take if you are allergic to grapes.

May cause dizziness or nausea.

May interfere with cholesterol lowering drugs.

– J Med 2000;31(5-6):227-46 — Effects of niacin-bound chromium and grape seed proanthocyanidin extract on the lipid profile of hypercholesterolemic subjects: a pilot study. — Preuss HG, Wallerstedt D, Talpur N, Tutuncuoglu SO, Echard B, Myers A, Bui M, Bagchi D.

Can interact with other herbs changing the way they are absorbed.