When used properly, herbs hold great health potential, says James Duke, PhD, author of The Green Pharmacy (Rodale) and one of the nation's foremost experts on herbs. And it's not hard to find believers: A multibillion-dollar industry has sprung up as more and more people turn to herbs in search of fewer side effects or products that are more "natural."

But what's green isn't always groovy. "Herbs are pharmacologically active" and should be used under the watchful eye of a health-care practitioner who knows about possible interactions with other herbs or drugs, warns Mark Blumenthal, executive director of the American Botanical Council in Austin, TX. In fact, several herbs have raised eyebrows ‑- and the concerns of the FDA ‑- because of their severe side effects or potentially harmful interactions with medications. When it comes to the following five botanicals, it's buyer beware:

Belladonna
Used for: Spasms in the bile duct, colic, gallbladder and liver problems and irregular heartbeat
How it works: It contains a sedating chemical called atropine, along with chemical compounds that have narcotic and sedative properties.
Beware: Also known as deadly nightshade, this herb can knock you out and can cause a variety of side effects, including mental confusion, constipation, cramps and delirium.

Comfrey
Used for: Bruises, joint sprains and ulcers
How it works: The allantoin in it boosts the immune system, speeds wound healing and spurs new skin and cell growth.
Beware: While comfrey is safe when used topically for bruises and other skin problems, it should not be ingested because it contains alkaloids that have been linked to severe, even deadly, liver toxicity.