Over 30 people were hospitalized in New York City last month after smoking some fake marijuana that had them acting more like “zombies.” In fact, an eyewitness described the incident as “a scene out of ‘The Walking Dead.’”

They had overdosed on what is called synthetic marijuana, which is known to cause users to act erratically, experience respiratory problems and even collapse on the street.

Synthetic marijuana isn’t just one drug. It is more of a combination of elements, “a family of man-made chemicals,” all of which interact with the same cell receptors in the brain as the active ingredient in natural marijuana, THC.

Synthetic marijuana is a bunch of industrial chemicals that’s sprayed on dried leaves and lawn clippings and sold under several brand names, the popular ones being K2 and Spice. According to the National Institute of Drug Abuse, people who take artificial marijuana can experience “anxiety and agitation, nausea and vomiting, high blood pressure, shaking and seizures, hallucinations and paranoia, and they may act violently.”

It was developed in the 1990s by South Carolina chemist John W. Huffman who was trying to find a way to imitate cannabinoids, the active chemicals found in cannabis.

Synthetic marijuana is much cheaper than its natural counterpart. An ounce of natural cannabis can go up to $350 while the synthetic one can cost anywhere between $10 and $50 a bag. It’s available online and in some smoke shops or convenience stores.

Since, it isn’t made up of one single substance it doesn’t come up on drug tests either.

Synthetic marijuana is banned in several states including New York, New Hampshire and Nebraska. The ban in New York reportedly brought hospital visits related to the drug down by nearly 85 percent.

Several people including Huffman believe that making natural marijuana legal can reduce the use of synthetic marijuana. He compared smoking the drug to playing “Russian roulette.”

New York City Council member Corey Johnson is in agreement with Huffman on this. “Some people smoke K2 to avoid a positive drug test result for marijuana — a much safer drug,” he said.

“This is another reason that we should be taxing and regulating marijuana, rather than criminalizing it and driving people to alternatives like K2.”