From the NIH:

What the Science Says
Although scientific studies provide some evidence that kava may be beneficial for the management of anxiety, the U.S. Food and Drug Administration (FDA) has issued a warning that using kava supplements has been linked to a risk of severe liver damage.
Kava is not a proven therapy for other uses.
NCCAM-funded studies on kava were suspended after the FDA issued its warning.

National Center for Complementary and Alternative Medicine (NCCAM)

http://nccam.nih.gov/health/kava

MedlinePlus Trusted Health Information for you A service of the U.S. National Library of Medicine
From the National Institutes of HealthNational Institutes of Health

How does it work?
Kava affects the brain and other parts of the central nervous system. The kava-lactones in kava are believed to be responsible for its effects.

Are there safety concerns?
Kava is POSSIBLY UNSAFE when taken by mouth. Don’t use it. Serious illness, including liver damage, has occurred even with short-term use of normal doses. The use of kava for as little as one to three months has resulted in the need for liver transplants, and even death. Early symptoms of liver damage include yellowed eyes and skin (jaundice), fatigue, and dark urine. If you decide to take kava, despite warnings to the contrary, be sure to get frequent liver function tests.

Using kava can make you unable to drive or operate machinery safely. Do not take kava before you plan on driving. "Driving-under-the-influence" citations have been issued to people driving erratically after drinking large amounts of kava tea.

Special precautions & warnings:

Pregnancy and breast-feeding: Don’t use kava if you are pregnant or breast-feeding. Kava is POSSIBLY UNSAFE when taken by mouth. There is a concern that it might affect the uterus. Also, some of the dangerous chemicals in kava can pass into breast milk and might hurt a breast-fed infant.

Depression: Kava use might make depression worse.

Liver problems: Kava is hard on the liver, even healthy ones. Taking kava if you already have liver disease is taking a risk.

Surgery: Kava affects the central nervous system. It might increase the effects of anesthesia and other medications used during and after surgery. Stop using kava at least 2 weeks before a scheduled surgery.

http://www.nlm.nih.gov/medlineplus/d...tural/872.html

Best wishes, Elaine