There's an unsettling side effect for women of St. John's wort, a popular over-the-counter herbal supplement, according to a warning issued Thursday by the British Medicines and Healthcare products Regulatory Agency.

This supplement, derived from the Hypericum perforatum plant, can stop emergency contraceptives from working, the BBC reported.

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The warning said that St. John's wort can interfere with levonorgestrel — the active ingredient in Plan B One-Step and other so-called morning after pills. The warning was issued after the discovery of about 400 unwanted pregnancies, The Telegraph reported.

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So what kind of wort is this?

St. John's wort is often used to treat mild depression, sometimes in combination with prescription SSRI antidepressants, the University of Maryland Medical Center said.

From UMM:

"Researchers aren't exactly sure how St. John's wort works. Some have suggested that the herb acts similar to an SSRI, increasing the availability of the brain chemicals serotonin, dopamine, and norepinephrine. These neurotransmitters help improve mood. Scientists thought that hypericin was responsible, but now they believe that other chemicals in St. John's wort may help."

Research has found that the herb helps depression, but a recent study conducted by the National Institutes of Health found it was no more effective than a placebo.

Sweden's Medical Products Agency issued a similar warning in February, ABC News reported.

Doctors and medical researchers have been aware of the supplement's interactions for quite some time.

But consumers aren't required to consult them to buy the pills and may take them while they're also on an emergency contraceptive — unaware of the risks of unwanted pregnancy.

"Although this effect is not common, it is certainly pertinent to the U.S. population and should not be dismissed lightly," Steve Hall — a professor of medicine, pharmacology, and toxicology at Indiana University School of Medicine — told ABC News. "Unfortunately, the herbal medicines lobby and lack of FDA jurisdiction over these herbal medicines has completely prevented any precautionary labeling in the U.S."

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Medical professionals have observed other dangerous interactions and effects of the supplement.

Interfering with oral contraceptives "is but one of several serious interactions that are now known, and there are many others that are predictable," Raymond L. Woosley — vice president for health sciences at the University of Arizona — told ABC News.

"St. John's wort has caused people to reject transplanted organs when it caused their levels of cyclosporine to fall," Woosley added. "AIDS patients have lost control of HIV because St. John's wort caused them to eliminate their protease inhibitors [antiviral drugs] too quickly. Unfortunately, about 40 percent of patients refuse to tell their docs that they are taking herbals, even when asked. Most are not asked."

Thursday's warning brings up a larger issue with herbal supplements in the United States.

Because such over-the-counter pills aren't regulated as drugs or food by the FDA, researchers often don't know exactly how they work and interact with other medications — and that's a problem.

"Manufacturers don't have to seek FDA approval before putting dietary supplements on the market," the Mayo Clinic said.

The Medicines and Healthcare products Regulatory Agency warning recommends that women taking St. John's wort use double the normal dose of an emergency contraceptive pill and consult their doctors.