Large amounts of designer drugs are being imported into the United States legally, CBS News reports.

The drugs include synthetic marijuana, known as Spice. Drug Enforcement Administration Special Agent Doug Coleman says China is the main source of these drugs. CBS News found a Chinese manufacturer online that sells chemical compounds. The company offered to ship two pounds of synthetic marijuana for $2,500.

While Spice and several other synthetic drugs were outlawed by the federal government last year, chemists have been evading the law by continually coming up with chemical compounds that are slightly different from the ones that have been banned. Coleman says U.S. Customs authorities cannot stop imports of compounds that are still legal.

“It’s like whack a mole,” he told CBS News. “They pop their head up, we hit them, they go down and then they pop their head up in another spot. It’s always a cat-and-mouse game. This is just a more advanced type of cat-and-mouse because now we’ve got chemists manufacturing synthetic drugs as opposed to cartel members trafficking heroin, or coke, or methamphetamine.”

Another synthetic drug that has been growing in popularity is Molly, or Ecstasy. Emergency room visits related to Molly rose 128 percent among people younger than 21 between 2005 and 2011, according to a new government report.