St. John's wort, an herb commonly used to treat mild depression, may interfere with other medications, according to a new study.

All About St. John's Wort Other Herbs Do You Use It?

Researchers from the Medical University of South Carolina in Charleston looked at 12 healthy volunteers to see how St. John's wort interacted with an enzyme commonly involved in drug metabolism.

They found that the herb diminished the effectiveness of drugs with the cytochrome P450, which includes more than half of all marketed drugs, said study leader John Markowitz.

"These findings underscore the potentialinherent problems associated with the widespread practice of using herbal products [at the same time] with conventional medications," the researchers wrote.

A study presented Thursday at a meeting of the Association of Reproductive Health Professionals suggested that St. John's wort may allow women to ovulate even though they're taking birth control pills -- leading to pregnancies.