The Journal of the American Medical Association just came out with a report of the risks of herbal medicine while undergoing surgery. They warn that you have to tell your doctors about exactly what you are taking.
In particular, this study looked at echinacea, ephedra (also known as ma huang), garlic, ginkgo, ginseng, kava, St. John's wort, and valerian.
Echinacea may cause problems with immunity. No data on when to stop taking it
Ephedra increases heart problems and stroke. Stop at least 24 hours before surgery.
Garlic increases blood loss. Stop 7 days before surgery
Ginkgo increases blood loss. Stop at least 36 hours before surgery
Ginseng increases blood loss, low blood sugar. Stop at least 7 days before surgery.
Kava increases the effect of anesthesia unpredictably. Stop at least 24 hours before surgery
St. John's wort increases the effect of anesthesia and interferes with many/most other drugs. Stop at least 5 days before surgery.
Valerian increases the effect of anesthesia unpredictably and causes withdrawal symptoms. No data on when to stop.
The full article is at
SOURCE: The Journal of the American Medical Association 2001;286:208-216.