K2, Spice and other synthetic drugs that mimic a marijuana high sent 11,406 people - mostly teenagers and young adults - to the emergency room in 2010, according to the first report on the substances from the federal government's Drug Abuse Warning Network.
The report, the first to analyze the impact of the popular herbal incense, found that children ages 12 to 17 accounted for one-third of the emergency room visits. Young adults ages 18 to 24 accounted for another 35%.
In 59% of the cases involving patients ages 12 to 29, doctors found no other substance, differing from most emergency department visits involving illicit drugs and painkiller abuse.
Marijuana, the most popular illicit drug with 18 million regular users, sent 461,028 people to the emergency room in 2010.
"This report confirms that synthetic drugs cause substantial damage to public health and safety," Office of National Drug Control Policy Director Gil Kerlikowske said.
Spice and K2, marketed as legal, fake pot and labeled as herbal incense, emerged in 2009 as popular drugs among teenagers and college students, who could purchase the substances online and in convenience stores.
Problems quickly emerged. Doctors reported teenagers arriving in the emergency room with high fevers and strange behavior.
Police in Nebraska in 2010 arrested a teenage boy who had smoked Wicked X, herbal incense coated with synthetic cannabinoids. The teen careened his truck into the side of a house and then continued driving.
At least 18 states outlawed the substances and the Drug Enforcement Administration instituted an emergency ban. In July, Congress banned sales of K2, Spice and other synthetic drugs under the Food and Drug Administration Safety and Innovation Act.