Yet, many U.S. adults spend hundreds and even thousands of dollars on the supplement each year, hoping that the herb could be an easy over-the-counter cure for their mood disorder.
"This study showcases the fact that the American consumer uses dietary supplements in a way that is inconsistent with the scientific indications which have been studied," said Dr. Nicole Nisly, director of the complementary medicine program at the University of Iowa. "The average U.S. user uses dietary supplements based on advertisements, and is unfamiliar with the reliable information on supplement use."
In fact, a study published in the journal Mayo Clinic Proceedings in 2007 found that roughly two-thirds of adults who use popular herbs and herbal supplements do not do so in accordance with evidence-based indications.
Moreover, herbal supplements such as St. John's wort are not as highly regulated as FDA-approved prescription drugs, which are subject to strict quality control measures before entering the market. For this reason, experts urge consumers to use the herb only for those indications that it has been proven to treat.
"Much less is known about using wort than using the FDA-approved medicines. Its adverse events are not well understood -- thus, caution is advised," said Michael Manos, head of the Pediatric Behavioral Health Center at the Cleveland Clinic's Children's Hospital.
According to the National Institutes of Health, the most common modern-day use of St. John's wort is for the treatment of depression, regardless of the conflicting evidence on whether the herb actually decreases symptoms of mood disorders at all.
And despite the fact that the NIH says there is not sufficient scientific data to recommend St. John's wort for children under the age of 18, more and more parents each year buy the supplement in hopes that it will help their child's mood disorder.
"Parents of children with ADHD are desperate to find something to help their children," Manos said. "This is one less place to look."