An obscure hallucinogenic herb from Mexico is gaining a toehold in the world of recreational drugs, prompting law enforcement officials to increase their scrutiny of the plant, which is legal, and moving health experts to issue cautions about the drug, whose jarring effects are not fully understood.

The herb, Salvia divinorum, is a type of sage plant that can cause intense hallucinations, out-of-body experiences and, when taken in higher doses, unconsciousness and short-term memory loss. Users have also reported sensations of traveling through time and space, assuming the identities of other people and even merging with inanimate objects.

''This is a very interesting agent,'' said Dr. Ethan Russo, a neurologist in Missoula, Mont., who studied Salvia divinorum and other herbs while preparing his book, ''Handbook of Psychotropic Herbs'' (Haworth Press). ''It is really in a class by itself.''

Dr. Russo said that scientists had identified the active chemical compound that causes the hallucinations -- Salvinorin A -- but knew little else about Salvia divinorum. Scientists are still unclear about precisely how it interacts with the brain or may affect the rest of the body, and do not know if it leads to long-term side effects.

''We don't know how it works,'' Dr. Russo said. ''It doesn't work on serotonin, dopamine or any of the known neurotransmitters. People who are arbitrarily using it need to be cautious. It's totally different from anything they may have tried before.''

Salvia divinorum (pronounced SAL-vee-ah dee-vin-OR-um), which is native to Mexico, can be smoked or chewed like tobacco. Its leaves can also be boiled to make an intoxicating tea. It is different from common species of Salvia like the brilliantly colored scarlet sage or culinary garden sage. And unlike most other hallucinogenic substances, Salvia divinorum is legal in the United States, although drug enforcement officials say they are looking closely at the herb.

''It's not currently controlled and we're actually collecting information on it,'' said Rogene Waite, a Drug Enforcement Administration spokeswoman.

Precise figures about the plant -- it is also known as ska Maria Pastora and diviner's sage -- its use and proliferation are almost impossible to gather. But herbalists, users and sellers say its popularity is growing.

National drug information clearinghouses and law enforcement officials acknowledged only a passing familiarity with Salvia divinorum. The authorities said they had no reports of health problems, hospitalizations or emergency room visits that might be attributed to the plant. And researchers say they are still trying to conclusively answer such questions about the drug as its potential for addiction and tolerance.

Users dismiss such concerns, saying that no evidence of an addictive quality has been documented, and pointing out that the Mazatec Indians in the Oaxaca region of Mexico have used it, with no apparent ill effects, for centuries.

The mystery of just how Salvia divinorum works seems to be part of its appeal. It is available almost exclusively through the Internet and has spawned a small but thriving group of commercial Web sites, like the ''Sage Wisdom Salvia Shop,'' which offer dried Salvia divinorum leaves for as much as $120 an ounce.

''The Mazatec people have preserved Salvia divinorum and the knowledge surrounding its use for hundreds of years,'' reads one passage on the Web site. ''We are privileged to have them share their sacred herb with us.''

Daniel Pinchbeck, a 35-year-old freelance writer from SoHo, said that when he first tried Salvia divinorum two years ago, ''it totally freaked me out.''

''It was like you were calling in something, some presence,'' said Mr. Pinchbeck, who warned against abusing the drug. ''I had to call a friend; then I started to calm down. It's not like anything else. It's a totally unique experience.''

Despite its upper-middle-class price tag, herbalists and drug experts say that Salvia divinorum draws those from wide-ranging backgrounds -- everyone from partygoers to practitioners of transcendental meditation -- who are attracted to this year's hip herb.