Fifteen people were hospitalized over the weekend after consuming artificial cannabis, Los Angeles Police Chief Charlie Beck said Wednesday. Despite the name, artificial cannabis — THC-like chemicals made in labs rather than greenhouses, which is then applied to plant matter — can be much stronger than botanical marijuana.

“They are cutting it with something lethal,” Rev. Andy Bales, who runs a local mission, told the Los Angeles Times. “We’ve seen violence and convulsions…. I saw a guy rolling into the street on Friday.”

The Los Angeles injuries are not isolated incidents. On one afternoon in November, 13 people in San Diego fell ill after ingesting synthetic cannabinoids. And last April, synthetic marijuana laid low an alarming number of New Yorkers — 160 hospitalizations in two weeks, according to New York Gov. Andrew Cuomo (D). It’s “direct proof,” Cuomo said in a statement, that “synthetic drugs are anything but harmless.” The governor’s office drew parallels between synthetic cannabis and other drugs like bath salts and PCP. Like bath salt users, people who consume synthetic marijuana report violent behavior, vomiting, seizures and other aggressive symptoms — a far cry from a typical user experience with the leafy stuff.

Also known as K2 or spice, synthetic marijuana is about 10 times more potent than naturally occurring THC, Clemson University chemist John Huffman told Livescience in 2010. He would know — Huffman synthesized the artificial equivalent of THC back in 1990s; these chemicals now bear his initials, JWH. But don’t compare him to Walter White. The compound was never intended to be ingested, Huffman told NBC Charlotte in 2015, and he has repeatedly gone on record comparing the drug to a game of Russian roulette.

The contents of a synthetic marijuana baggie are unpredictable at best. Due to spice’s manufactured origin, New York’s acting Health Commissioner Howard Zucker said in 2015 it’s impossible to know exactly what compounds are being sold as synthetic marijuana. Worse, unlike the effects of consuming too much marijuana, which dissipate fairly quickly,  overdosing on artificial cannabis mirrors a sweaty, angry amphetamine trip, New York University toxicologist Lewis Nelson told Forbes.

In a study published recently in the American Journal of Drug and Alcohol Abuse, researchers from Bowling Green State University, Ohio, specifically recruited people who had consumed artificial THC. Of 367 users of marijuana surveyed, close to half also reported consuming the synthetic version. Their perceptions of the artificial drug were not positive. The authors concluded that “synthetic cannabinoids are typically a second choice to botanical marijuana,” used only when the botanical version was not available or when worried about a drug test.

Marijuana’s health effects are the subject of vigorous national debate, though weed’s defenders point out that, by and large, the natural version is less dangerous than alcohol. No one is stepping up to sweatily bat for synthetic marijuana, however. When Huffman argued for marijuana legalization back in 2011, it certainly wasn’t on behalf of his synthetic legacy, but in support of the garden variety.