Whole valerian contains liver-toxic substances called valepotriates; however, valepotriates are thought to be absent from most commercial valerian products,43 and case reports suggest that even very high doses of valerian do not harm the liver.44,45
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Side effects of valerian may include headache, dizziness, itchiness, upset stomach, drowsiness during the daytime, dry mouth and vivid.dreams.
Rarely, liver damage has been associated with the use of valerian. It's not certain whether the cause of the liver damage was due to valerian itself or to contaminants in the product. Until we know more, people should use valerian only under the supervision of a qualified health care practitioner and those with liver disease should avoid it.
http://altmedicine.about.com/cs/herbsvitaminsrz/a/Valerian.htm