The users are buying a product known as K2 - or "Spice," Genie" and "Zohai" - that is commonly sold in head shops as incense. Produced in China and Korea, the mixture of herbs and spices is sprayed with a synthetic compound chemically similar to THC, the psychoactive ingredient in marijuana. Users roll it up in joints or inhale it from pipes, just like the real thing.
Though banned in most of Europe, K2's key ingredients are not regulated in the United States - a gap that has prompted lawmakers in Missouri and Kansas to consider new legislation.
Special Report: Marijuana Nation
"This isn't Jerry Garcia's marijuana," said state Rep. Jeff Roorda, a Democrat from the eastern Missouri town of Barnhart. "They've used chemicals to avoid creating something that's already illegal."
Authorities in Johnson County, Kan., discovered ex-convicts on probation smoking K2, and said it is spreading to high school students.
"This has become extremely popular," said Linda Weber, owner of The Vise smoke shop in the St. Louis suburb of St. Peters, who said she only sells to adults.
She said she sells about 60 packages a week, with suppliers calling her weekly to pitch new brands. She said she's keeping an eye on what state lawmakers decide, though, because "I definitely don't want to be selling it if it comes out that it's harmful."
K2 costs between $20 and $50 for three grams - similar to the street price of marijuana - but with the key advantages of being legal and undetectable in drug tests.
The key ingredients are believed to be the unintended result of scientific research on marijuana's effects.
Dr. John Huffman, a Clemson University organic chemistry professor, was researching the effects of cannabinoids on the brain when his work resulted in a 1995 paper that contained the method and ingredients used to make the compound. That recipe found its way to marijuana users, who replicated Huffman's work and began spraying it onto dried flowers, herbs and tobacco.
"People who use it are idiots," said Huffman, referring to K2 smokers.
A proposed bill in Missouri would make possession a felony punishable by up to seven years in prison - identical to punishments given to users of real marijuana. A similar bill in Kansas would make possession a misdemeanor punishable, with up to a year in jail and a $2,500 fine, also the same as marijuana convictions.
The products are sold widely, but authorities in other states contacted by The Associated Press, including Pennsylvania, California and Michigan, said they haven't heard of their use as a drug.
Police in Missouri and Kansas said they've become aware of K2 in recent weeks.
In the rural southern Missouri city of West Plains, K2 is sold in a head shop just blocks from the high school. A botched attempt by teens to steal K2 from the shop brought the substance to the attention of police.
"A 10-year-old child could walk into a head shop and buy it," said West Plains Detective Shawn Rhoads. "It's not a tobacco, it's not regulated by anything. It would be like sending my 10-year-old son into Wal-Mart to buy potpourri."
Although it is legal, the military has banned possession of K2. The federal Drug Enforcement Administration has classified it a "drug or chemical of concern."
Conner Moore, 20, who is taking a semester off from Moberly Community College, said he and his friends started smoking K2 after reading online news articles and postings about the substance. He compares the high to smoking medical marijuana. The high, he says, is shorter.
"We just got on forums and looked it up and saw what other people said about it," he said. "Obviously if it comes out being bad, I'll obviously stop using it," Moore said. "There's really no sites out there that says what is in K2."
There is no data on the drug's toxicity or how long it stays in the body. In mice, it can lead to a lower body temperature, partial paralysis and the temporary inability to feel pain, according to the DEA.
One of the few studies of the compound's use was performed by the European Monitoring Centre for Drugs and Drug Addiction, a Portugal-based agency of the European Union, in November 2009. The study found the amount of synthetic compound varies widely between brands, and that despite being widely available, it isn't clear how many Europeans use it.
Allen St. Pierre, executive director of the National Organization for the Reform of Marijuana Laws, said K2 isn't much discussed within marijuana culture. "If government is genuinely concerned about controlling cannabis-related products, there is really only one thing that seems to have an effect: a tax stamp," St. Pierre said.