Avoid taking valerian at the same time as medication that can cause drowsiness, as the effects could be additive, adds Massachusetts General's Dr. Ulbricht.

WSJ: Can a Root Get to the Problem of Insomnia?

As for whether it works for sharpening the brain or minimizing damage from a stroke "the jury is out, " says Catherine Ulbricht, co-founder of Natural Standard Research Collaboration, a Cambridge, Mass.

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Preparations of the herb, including the amount of valerenic acid, vary widely from study to study, making it hard to draw firm conclusions, says Catherine Ulbricht, co-founder of Natural Standard and a pharmacist at Massachusetts General Hospital in Boston.

WSJ: Can a Root Get to the Problem of Insomnia?