Like zombies in an episode of “The Walking Dead.”

That’s how witnesses described the scene in Brooklyn this week, where dozens of dazed men and women staggered in the streets after smoking K2 – a popular brand of synthetic marijuana.

Some victims collapsed on the sidewalk. Others were twitching, vomiting or screaming at passerby. In just a two-block area, 33 people were taken to New York hospitals with a suspected K2 overdose (all were treated and are expected to recover).

The gritty scene is a reminder of the wildly unpredictable contents of “fake weed,” an herb mix laced with synthetic cannabinoids.

It’s easy to get a bad batch, according to a recent interview that we conducted with Dr. Eric Wish, one of the nation’s top experts on synthetic cannabinoids.

“Basically the only person who knows what’s in a particular batch is the chemist who created that batch,” says Wish, Director of the Center for Substance Abuse Research (CESAR) at the University of Maryland.

“People who take it are playing Russian Roulette because they don’t know what they’re taking and how it will affect the body,” Wish says.

Known by more than a hundred street names – including K2, Spice, and No More Mr. Nice Guy – synthetic marijuana claims to mimic the effects of real marijuana.

But experts say it’s a poor replica.

“Scientists and public health experts will not use the term ‘synthetic marijuana’ because it implies that the substance mimics marijuana,” Wish says. “The new chemicals are so different from marijuana that nothing could be further from the truth.”

The recent mass overdose in Brooklyn, the most serious incident involving K2, was traced to an infamous local deli that sold the drug. But the danger goes well beyond New York.

So far this year, every state has reported calls to poison control centers involving exposure to synthetic cannabinoids, with Texas reporting the most calls (159), according to the American Association of Poison Control Centers.

Through June 30, 2016, poison control centers fielded 1,462 calls regarding exposure to synthetic cannabinoids. That’s actually a downward trend, since there were 7,794 exposure calls last year overall, and at least 15 deaths related to synthetic cannabinoids in 2015, according to the CDC.

Whether or not we’ll see more zombie-like tragedies from the effects of K2 is unknown.

Vendors who sell the drug – typically some bodegas, head shops and corner stores – will continue to skirt the law by marketing synthetic cannabinoids as “herbal products” with the disclaimer “Not for Human Consumption.” Chemists will keep changing the formulation to avoid liability and prosecution. And users will continue to flock to a dirt-cheap high ($5 or less a pop).

The health risks, however, remain a growing concern. Synthetic cannabinoids “may affect the brain much more powerfully than marijuana; actual effects can be unpredictable and, in some cases, severe or even life-threatening,” reports the National Institute on Drug Abuse.

“The symptoms can be anything from coma to anxiety to drowsiness to stimulation,” Wish says. “It all depends what was in that batch.”

If you or someone you know has a bad reaction to synthetic cannabinoids, call your local poison control center at 1-800-222-1222 for help. Dial 911 immediately if someone stops breathing, collapses or has a seizure.