The mass overdose last week in a Connecticut public park — 47 people collapsed and began convulsing — was only the most recent. Nearly 50 people in Brooklyn overdosed similarly just a few months ago. And before that, nearly 150 people in Illinois showed many of the same symptoms, stoking a nationwide alert from the U.S. Centers for Disease Control and Prevention.

The incidents were all connected to synthetic cannabinoids, more commonly referred to by the street names “K2” or “Spice.” They’re the work of chemists who constantly tweak the drugs’ makeup, and they create lab rats out of unsuspecting users in the process.

The drugs are cheap, sometimes sold in shiny packages at gas stations or head shops under names like “Scooby Snax” or “Judgement Day,” and can fly under the radar until disaster strikes. U.S. regulators have classified several compounds of the drug as Schedule I substances, putting them in the same class as heroin and other drugs that have a high potential for abuse but no accepted medical benefit. As a result, chemists are constantly making new compounds that are technically legal once they hit the market. The experimentation has had frightening consequences as chemists have turned to untested, dangerous compounds to evade drug laws.

Why that particular batch of synthetic cannabinoids knocked out so many people in New Haven, Connecticut — there were more than 100 overdoses, 47 people were sent to the hospital, and many returned to the park to overdose once again — is still a mystery a week later.

Luckily, no one died this time. But variants of K2 have been linked to some deaths in the past.

Here’s what you need to know.

The mass overdose last week in a Connecticut public park — 47 people collapsed and began convulsing — was only the most recent. Nearly 50 people in Brooklyn overdosed similarly just a few months ago. And before that, nearly 150 people in Illinois showed many of the same symptoms, stoking a nationwide alert from the U.S. Centers for Disease Control and Prevention.

The incidents were all connected to synthetic cannabinoids, more commonly referred to by the street names “K2” or “Spice.” They’re the work of chemists who constantly tweak the drugs’ makeup, and they create lab rats out of unsuspecting users in the process. The drugs are cheap, sometimes sold in shiny packages at gas stations or head shops under names like “Scooby Snax” or “Judgement Day,” and can fly under the radar until disaster strikes. U.S. regulators have classified several compounds of the drug as Schedule I substances, putting them in the same class as heroin and other drugs that have a high potential for abuse but no accepted medical benefit. As a result, chemists are constantly making new compounds that are technically legal once they hit the market. The experimentation has had frightening consequences as chemists have turned to untested, dangerous compounds to evade drug laws.

Why that particular batch of synthetic cannabinoids knocked out so many people in New Haven, Connecticut — there were more than 100 overdoses, 47 people were sent to the hospital, and many returned to the park to overdose once again — is still a mystery a week later.

Luckily, no one died this time. But variants of K2 have been linked to some deaths in the past.

Here’s what you need to know.

What is this stuff?

First, it’s definitely not marijuana.

It’s manufactured as a plant substance that gets sprayed with chemicals that contain synthetic cannabinoids, and then often smoked like pot. Synthetic cannabinoids bind to the same receptors in the brain as marijuana’s active ingredient, tetrahydrocannabinol (THC), but they can produce vastly different effects.

People who smoke them can experience vomiting, violent behavior, suicidal thoughts, heightened blood pressure, kidney damage and seizures upon ingesting the drug, according to the National Institute on Drug Abuse. In New Haven, people were passing out and rolling on the ground.

Police scooped up samples of the drug involved last week and shipped it to a Drug Enforcement Administration lab, which confirmed the samples contained fubinaca and pinaca, two types of synthetic cannabinoids, DEA special agent Timothy Desmond told VICE News. It can take a lot of effort to figure out what type of synthetic cannabinoid a person took, though, Eric Wish, director of the Center for Substance Abuse Research at the University of Maryland, said. He was part of a report published Tuesday that looked at 175 patients from local emergency departments who were suspected of overdosing on synthetic cannabinoids. Even with advanced urine tests that detected dozens of types of synthetic cannabinoids, all but one of the patients tested negative for the drug. After researchers obtained a newer version of the test that could detect up to 46 types of the drug, they found that only a quarter of the samples tested positive for synthetic cannabinoids. Many of the samples tested positive for a litany of other drugs, including PCP and opioids, and often for multiple drugs at once.

“We can’t keep up, the scientists can’t keep up with the changing drugs and the combinations of what’s in these preparations,” Wish told VICE News. “So, imagine what it is for a consumer who buys this thinking it’s ‘synthetic marijuana’ when it couldn’t be further from the truth.” “Basically, the only person who knows what’s in it is the chemist who created it … It’s Russian roulette when you take these.”

Where does it come from?

Here’s the historical perspective: John W. Huffman developed some of the earliest compounds that later stoked widespread attraction to synthetic cannabinoids. The now-retired chemist from Clemson University in South Carolina produced hundreds of compounds that interact with the cannabinoid receptor for medicinal purposes, and most famously created a compound called JWH-018 in 1993. By 2008, that compound cropped up in Germany, where it was called Spice, according to the Washington Post. Huffman did not respond to an interview request from VICE News. Federal and state authorities often identify the original source of the drugs as chemical companies in China that also manufacture fentanyl and other synthetic drugs.

In February, U.S. Customs and Border Protection authorities seized three shipments of synthetic cannabinoid compounds from a Chinese shipper that was trying to send about two pounds of bath salts, another synthetic drug that can cause delusions or death, to an address in Pittsburgh, and another three pounds of ADB-FUBINACA to an address in Swissvale, Pennsylvania. Meanwhile, police arrested two men in Schenectady County in upsate New York on Friday for charges of conspiring to distribute ADB-FUBINACA out of a local deli, according to the U.S. Attorney’s Office for the Northern District of New York. Officers seized 1 kilogram of synthetic cannabinoids in brightly-colored baggies with labels like “Geeked Up.”

What sort of impact are synthetic cannabinoids having?

Besides New Haven, synthetic cannabinoids have trickled into other cities over the past several months. Four people died in the Chicago area this pastt spring after smoking K2 laced with brodifacoum, a rat poison. And in Washington, D.C., more than 300 people overdosed over a two-week period in July. Police in Milwaukee County believe that two people died in July after smoking K2 that was contaminated with rat poison, too, according to the Milwaukee Journal-Sentinel. There isn’t an antidote for synthetic cannabinoids, so physicians are left to treat the symptoms that people present with after ingesting the drugs. And because some manufacturers sloppily spray the synthetic plant material with chemicals, users can unexpectedly fall victim to “hot spots” of concentrated potency across one batch, making it hard to control for safer dosing. “You don’t know what you’re getting, and these have the potential to cause really life-threatening effects,” said Anne-Michelle Ruha, vice president of the American College of Medical Toxicology.

What’s the latest on the New Haven situation?

After the city’s emergency services scrambled to keep up with the rapid-fire calls — 70 in the first 24 hours starting Wednesday — at the New Haven Green near the Yale campus where transients and homeless people congregate, the overdoses died down substantially over the weekend.

Three people have been arrested. John Parker and Felix Melendez were charged with possession of controlled substances and the sale of hallucinogens with the intent to sell and distribute, according to the New Haven Police Department. Melendez has been arrested a whopping 38 times, so he’s no stranger to the officers there. Both men were identified by victims and witnesses in the case, and police said Tuesday that more arrests are possible. Parker told police he was selling K2 to pay for a hotel and avoid being homeless, and could make around $200 per week, according to the New Haven Register. Meanwhile, federal agents made a third arrest last week and charged Quentin Staggers, a local advocate for the homeless, with possession with intent to distribute synthetic cannabinoids and conspiracy. However, it’s not clear whether he’s connected to the overdoses, and a spokesman for the U.S. attorney’s office in Connecticut said the criminal complaint against him is sealed, as the case is still under investigation. Staggers’ lawyer, Kelly M. Barrett, said during a detention hearing on Tuesday that Staggers has an “extensive history” with substance use and addiction, according to the New Haven Register. Barrett did not immediately return a request for comment.