There was a well publicized study completed several months ago showing that kava can and does sometimes cause liver damage. Also, kava should not be taking during pregnancy or while nursing. Certain herbs, kava among them can cause thinning of blood and resistance to anesthesia so should not be taken prior to surgery. Finally kava should not be taken in conjunction with certain prescription anti depressants such as xanax as it can cause coma. This information is all available through webmd.
I would urge you to discuss any use of herbs and vitamins you are considering taking with your physician.