"You're paying attention to details because they make a difference, just as they make a difference in the lab, " says Mr. Burns, 39, who works as a pharmacist at York Hospital in York, Pa.

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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.

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All of these drugs must have accumulated in his system during this time, said Lois Parker, senior pharmacist at Massachusetts General Hospital.

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