The "incense" is packaged to look like pot and sold at tobacco shops and online. Users typically smoke the contents but the chemicals used are extremely unpredictable and produce highs that can induce convulsions, seizures, hallucinations and even death.

Other synthetic drugs meant to mimic cocaine, LSD or meth are sold as "bath salts" and can produce equally unpredictable highs. Users of both synthetic compounds have committed suicide and attacked their children. "The recurring theme is monsters, demons and aliens. A lot of them had suicidal thoughts," says Dr. Mark Ryan, director of the Louisiana Poison Center.

Marketed as "water-softening" products, the "bath salts" are actually crystalized chemicals that are snorted, swallowed or smoked.

Emergency room visits due to synthetic drug overdoses have skyrocketed in 2011. Poison control centers across the U.S. have received more than 6,600 synthetic drug calls this year, 10 times more than the first half of 2010. More than 20 deaths have been linked to the drugs.

The chemicals used in the majority of synthetic marijuana products were invented by John W. Huffman, a Clemson University professor who spent years studying the effects of drugs on the brain. The chemicals were never intended for human consumption.

"These products are in an unregulated, unlicensed industry. No one knows the strength of the ingredients. You don't know what you're taking," says federal Drug Enforcement Administration agent Gary Boggs. The DEA issued an emergency order to temporarily ban five synthetic marijuana compounds earlier this year, three of which were invented by Huffman.

Earlier this week, California became one of more than 20 states to ban synthetic drugs, enacting harsher punishments for the sale and consumption of "fake pot" than for real marijuana.