The homeless are proving to be especially susceptible to latest version of synthetic marijuana, a man-made hallucinogen that experts say is far more dangerous and unpredictable than the real thing.

Nearly 300 homeless people became ill last month in St. Louis. Other outbreaks have occurred in New York City, Los Angeles and Austin, Texas.

Experts say synthetic marijuana is popular among the homeless for several reasons: It's cheap. It's difficult to detect in a drug test. And it's a fast escape from reality.

Dr. Anthony Scalzo of the Saint Louis University School of Medicine says the synthetic marijuana on the streets is up to 100 times more potent than real marijuana. Users often experience rapid heartbeat, high blood pressure, anxiety and hallucinations.