here is a link from wiki:

http://en.wikipedia.org/wiki/Grape_seed_extract

from that article:

Side effects

In clinical studies, taking grape seed extract has been associated with mild side effects such as:

• Abdominal pain
• Cough
• Headache
• Nausea
• Sore throat

As with any dietary oil, taking large amounts of grape seed oil may lead to soft stools or oily leakage from the gastrointestinal tract.