More than 10 percent of high school seniors recently admitted to using synthetic marijuana, also known by nicknames like Spice, K2, fake weed and Moon Rocks. And among that age group, the drug is second in popularity only to the old-fashioned pot.

But despite its easy accessibility and reputation as “natural,” Spice sends thousands of people to emergency rooms each year with side effects ranging from seizures to hallucinations to heart attacks.

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In a new study, scientists have come closer to understanding why chemical versions of marijuana can be so toxic, with some evidence that some people may be more susceptible to those dangers than others.

The researchers have also developed a test to detect this new class of drugs in urine, which is an important step toward discouraging their use.

“You have individuals that may be on probation or workplace drug monitoring and they can migrate to these drugs because we don’t have tests for them,” said Jefferey Moran, a toxicologist and analytical chemist at the University of Arkansas for Medical Sciences in Little Rock and the Arkansas Department of Health. “If they know a test is available, they may be encouraged not to take the drug.”

“I want people, kids especially, to understand that these things are not safe,” he added. “They are not marijuana. They’re toxic. They’re dangerous. They can kill you.”

Spice is just one of many emerging designer drugs that are made to mimic well established but harder-to-get substances. Some synthetic drugs act like stimulants, others like opiates, others like hallucinogens.

Synthetic marijuana works by stimulating the same receptors in the brain that react to THC, the active ingredient in cannabis. The intended result is to deliver a pot-like high.

But one difference between the nature-made and manmade versions, Moran said, is that the plant activates receptors only partially, while drugs like Spice fully stimulate receptors, making them far more potent.

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Because designer drugs are new and constantly being reformulated, they manage to skirt regulation, often showing up in gas stations and convenience stores with flashy marketing that appeals to kids. Users get the message that they’re fun and safe, but the reality is often much darker.

To better understand how Spice works in the human body, Moran and colleagues developed a new method to analyze urine samples taken from 15 people who visited emergency rooms after taking the drug.

The researchers were able to reliably detect metabolites -- or substances produced by the body as it breaks down the drugs -- in all of the samples, the team reported in the journal Analytical Chemistry.

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Preliminary results also suggested that there were differences in how people metabolized the drugs. Some appeared to more effectively detoxify Spice. Others were worse at detoxification, making them more likely to react poorly.

Because the researchers didn’t know how long it had been since each individual had taken the drugs or how much they had smoked, it’s possible that those differences were irrelevant.

But biological confirmation that different people process Spice differently would explain repeated anecdotal evidence that some kids react horribly to the same drug that their friends tolerate just fine, said Steve Pasierb, president and CEO of The Partnership at Drugfree.org in New York.

Figuring out which groups of people are more vulnerable could help public health officials target their messages to people who are most susceptible.

“Right now, we have the problem that this is kind of like Russian roulette,” Pasierb said. “You just don’t know if you’re the kid or this is the time you’re going to have a bad reaction.”

Compounding those unknowns is the fact that unregulated designer drugs are made with questionable materials that are constantly changing, he added. They may contain wood shavings or unidentified herbs. Even two batches of drugs from the same brand can contain wildly different amounts of active ingredients.

One batch may be five times more potent than marijuana and deliver a positive experience, but the next one could have 250 times the potency and be deadly.

“I’m always impressed that kids come in to focus groups drinking bottled water instead of tap but they smoke something like this,” Pasierb said. “It’s like lawn chemicals put together to have the desired impact. This is a particularly dangerous way to get high.”