Herbal therapies are astoundingly popular among the American public. In 2008 statistician Patricia M. Barnes of the National Center for Health Statistics and her colleagues reported that almost 20 percent of children and adults in the U.S. had used an herbal medicine during the past year. In 1998 a team led by physician David M. Eisenberg of Harvard Medical School determined that use of herbs for physical and mental problems had risen 380 percent between 1990 and 1997.

Our enthusiasm for herbal medicine is undoubtedly fueled by the high cost of prescription drugs, the fact that these drugs do not work for everyone, and a burgeoning interest in natural remedies. Moreover, many people erroneously assume that natural substances are inherently safer than synthetic medications.

The use of plants as treatments dates to at least 3000 b.c. Today this practice is part of a broader movement known as complementary and alternative medicine. Many people turn to such treatments—which also include remedies such as acupuncture, aromatherapy and massage—for psychological problems. In a 2001 study sociologist Ronald C. Kessler of Harvard Medical School and his associates found that more than half of people with panic attacks or severe depression used some form of alternative therapy, including herbs, during the previous year, usually without medical supervision.

Nevertheless, most plants have not been studied for their therapeutic value or side effects. Studies of two herbal treatments—kava for anxiety and St. John's wort for depression—indicate that some plant-derived substances might help individuals with psychological problems, but the evidence is so far inconclusive.

Kick Back, Relax

Pacific Islanders have long used an extract of the root of the kava plant (Piper methysticum), which grows on those islands, for social, ceremonial and medicinal purposes, including relaxation and the reduction of anxiety. Introduced in the U.S. in the 1980s, kava extract is most often served as a drink, which Americans can now sample in any of various kava bars. The Purple Lotus Kava Bar in West Palm Beach, Fla., for example, offers “a popular alternative to the same old nightlife, a place to truly kick back and relax.”

For the treatment of anxiety, people generally purchase kava in drug and health food stores. Some experimental results suggest the root has antianxiety properties. In a review article published in 2010, physician Max H. Pittler of the German Cochrane Center at the University of Freiburg and physician Edzard Ernst of the Peninsula Medical School in Exeter, England, analyzed 12 well-designed studies comparing kava with a placebo for the treatment of anxiety. They found that kava was more effective than the inert substance for ameliorating general anxiety, but the difference between the two was small. Unfortunately, relatively few studies qualified for inclusion, and the investigations differed in the dosages used, strains of kava, duration of treatment and severity of the patients' anxiety.

In addition, although the herb is relatively safe, it should not be used with abandon. Its side effects may include stomach complaints, restlessness, headache and fatigue. Reports of the root causing liver damage led to a 2002 warning from the Food and Drug Administration, along with bans on kava in several countries. Further research has quieted this concern, and the bans have since been lifted. Even so, we cannot rule out the possibility that kava causes liver damage because alternative explanations for a few cases of liver problems remain unexplained by other factors.

Of more concern are interactions between kava and other medications. Kava can intensify sleepiness if taken with sedatives, sleeping pills, antipsychotics or alcohol, raising the risk of injury during activities such as driving and using heavy machinery. It may also enhance the sedating effects of anticonvulsants and worsen the side effects associated with antipsychotic medication.

Scientists have studied other herbal remedies for anxiety, too. A few studies hint, for example, that lemon balm and valerian may be calming. Still, the evidence supporting their efficacy is even more preliminary than that for kava. And their potential dangers are equally uncertain.

Weeding Out Despair

St. John's wort (Hypericum perforatum), a plant historically used to drive out evil spirits, is the most widely studied herbal medicine for depression. In a 2009 review physician Klaus Linde of the Munich Technical University and his colleagues evaluated 29 well-designed studies of St. John's wort, usually taken as a pill for major depression. Overall, the studies suggested that the herb was more effective than a placebo and just as effective as antidepressants such as Prozac yet came with fewer side effects.

Not all of the studies Linde's team analyzed, however, found an advantage for St. John's wort over a placebo. In addition, two large-scale U.S. investigations revealed scant support for the idea that St. John's wort could ease depression. One of these, a 2011 study led by psychiatrist Mark H. Rapaport, then at Cedars-Sinai Hospital in Los Angeles, indicated that the plant was no more effective than a placebo for mild to moderate depression. Thus, this herb may ease some cases of depression, but the evidence is far from overwhelming.

St. John's wort is not without some risks. It can cause stomach upset, skin rashes, fatigue, restlessness, headache and confusion. More serious is the possibility, though slight, that it can interfere with becoming pregnant, worsen dementia or trigger psychosis in vulnerable individuals. And as with traditional antidepressants, St. John's wort can trigger a manic episode in people with bipolar disorder. The herb also may interact dangerously with other drugs. If taken with certain antidepressants, it may cause potentially life-threatening increases in the neurotransmitter serotonin. It also can reduce the effectiveness of birth-control pills, heart medications and HIV treatments, among other drugs.

Approved drugs for anxiety and depression carry risks, too, but in those cases, the FDA is supposed to monitor and publicize them. In contrast, no government agency regulates herbal treatments. In addition, the fact that doctors do not prescribe herbs—and often do not even know their patients are using them—raises the risk of drug interactions. Further, relatively little is known about the purity or long-term effects of herbal remedies or what dose is optimal for a particular ailment.

Despite these concerns, advertisements for herbal remedies often make exaggerated and unsubstantiated claims. One online ad for an antianxiety pill called Seredyn containing a combination of largely untested herbs asserts that “over 93% of Seredyn users with occasional anxiety and 85% with chronic anxiety report positive results, and 80–83% of users with panic and anxiety attacks say that Seredyn helps prevent and stop their attacks.” Such testimonials are essentially meaningless if not backed by verifiable data.

Nevertheless, more safe and effective herbal treatments for anxiety and depression may still be in our future. Perhaps one day scientists will discover in nature a large medicine cabinet offering new remedies for a broad spectrum of mental disorders.