Last year in Brooklyn, 33 people from a single neighborhood wound up in the hospital after smoking synthetic marijuana. Those afflicted could barely hold themselves up. Some were outright sprawled across the pavement. Bystanders said it looked like a zombie flick. Cops and newspapers called the culprit K2.

Maybe the people using it used the same slang, but then again, maybe not. Spice, Scooby Snax, Black Mamba, Bliss. Not that it matters. Synthetic marijuana is a catch-all term for a huge variety of chemicals engineered to mimic weed. Problem is, these chemicals are way too good at their job. Another problem: The people using them usually have no idea what they’re actually smoking, so figuring out the right dose can be difficult—and dangerous.

Synthetic marijuana has its origins in the early 1990s, when a Clemson University chemist named John Huffman synthesized a group of chemicals that interacted with cellular mechanisms in the same way as the main psychoactive ingredient in marijuana—tetrahydrocannabinol, or THC. Huffman wasn't trying to get anyone high. He was trying to help other researchers figure out how the brain worked. Specifically, the network of neural receptors activated by cannabinoids like THC.

But people love drugs, so people took Huffman's work (published openly in academic journals; he even wrote a book) and started manufacturing. And for many years, it was legal. That's partly because even though Huffman's synthetics activated the same cannabinoid receptors as THC—and users now spray the stuff on plant parts to smoke just like weed—chemically, they looked nothing like it. Not until March of 2011 did the DEA classify synthetic marijuana as a Schedule 1 narcotic.

By definition, a Schedule 1 drug has a high potential for abuse, has no medical value, and is considered dangerous. Natural marijuana probably does not deserve the classification. "Calling it synthetic marijuana does a disservice, because people tend to think of marijuana as not very harmful," says Jenny Wiley, a senior fellow at RTI International, in North Carolina1.

Part of the problem is synthetic marijuana is much better at being a drug than natural marijuana. When pharmacologists are testing drugs, they place radioactive labels on top of a cell's receptors. Then they watch and see how good the chemical is at displacing that label and binding to the receptor site. "The chemicals used in various synthetic marijuanas are able to bind to the receptor site at lesser concentrations than THC," says Wiley.

After a drug binds to a receptor, a cascade of effects amplify the signal into a bodily response. That could be a nice, gentle high. But synthetic compounds like K2 prompt way stronger responses than weed. "You have a double whammy," says Wiley. "Not only does it bind better, it also activates that receptor better, and more efficiently, than THC."

In low doses, the effects of synthetic marijuana are roughly analogous to those of marijuana. But because those synthetic chemicals are so much stronger, things tend to get out of hand quickly. Then you get people freaking out, passing out, turning into zombies.

And it’s pretty easy to overdose. Imagine yourself in a synthetic marijuana lab. You've just whipped up a batch of K2, and you’re spraying it on some innocuous plant leaves. Depending on the age and moisture content of those leaves, the concentration of the stuff is going to be different in different areas of the plant. Once you crumble the stuff up, it's nearly impossible to make sure that each alien face-embossed plastic baggy is going to have the same dose.

Plus, says Wiley, drug manufacturers are coming up with new chemical combinations all the time. "There's no quality control on what they are doing," says Wiley. Users have no idea if the stuff they are smoking this month is the same as from last month, or even if the stuff they are smoking is synthetic marijuana at all. "They don’t really know what’s in those packages,” she says. “They could be getting bath salts."

So what happened in Brooklyn? Wiley suspects it was some new formulation, rather than a case of people smoking a high-concentration batch of the same old K2 (whatever that means). The fact that all these people in such a small geographic area got sick indicates that this is something new. But, she says she'll have to wait until the DEA recovers some samples for her, or some other pharmacologists, to sample. No wait, not sample like that. They're just going to analyze the stuff.

1 Update 07/14/16 12:22pm ET — This article has been updated to note Dr. Wiley's current affiliation.