Not reveling the use of herbal supplements prior to surgery can lead to some very serious complications as the herbs and anesthesia interact within your body.
For example, ginkgo biloba and garlic can cause bleeding. Kava Kava, which is used for a sleep aid, can prolong sedation with some of the agents used in the operating room. St. John's Wort may cause instability of pulse and blood pressure during general anesthesia. Complications from herbal supplements can come from high doses, so it is important to discuss the dosage with your doctor as well.
Be smart and prepared before heading into the operating room. The American Society of Anesthesiologists recommends that you discontinue herbal remedies at least two weeks before surgery.