A molecular lookalike of the intoxicating compound found in marijuana has become popular among teens in the United States, especially in areas where marijuana use is illegal, a new study shows.

More than 11% of high school seniors who took part in a national survey last spring said they had used delta-8 THC, a psychoactive compound typically derived from hemp, in the last year.

That figure surprised researchers at USC and the University of Michigan, who published their findings this week in the Journal of the American Medical Assn.

Products containing delta-8 “have really only been on the market since 2018,” said study leader Alyssa Harlow, a USC epidemiologist and faculty member of the school’s Institute for Addiction Science.

Gummies, vapes and other products containing delta-8 are available online and in gas stations and convenience stores. They are often marketed as a federally legal substitute for marijuana — and often without solid measures for age verification.

“We don’t know enough about these drugs, but we see that they are already extremely accessible to teens,” said Dr. Nora Volkow, director of the National Institute on Drug Abuse, which helped fund the research. “Cannabis use in general has been associated with negative impacts on the adolescent brain.”

Delta-8 looks much like delta-9 THC — the more common molecule in cannabis that makes people feel high — but has a slightly different structure, with a double bond located between a different set of carbon atoms. Although both are naturally found in cannabis, delta-8 is less abundant.

In a 2021 survey, delta-8 users reported it to have less intense effects than delta-9, on average. But the Food and Drug Administration has warned consumers that it has not evaluated the safety of delta-8 products. Chemicals used to convert cannabinoids found in hemp into delta-8 may include harmful contaminants, the agency cautioned.

Products containing delta-8 exploded in popularity after the 2018 passage of an agricultural bill that eased federal restrictions on hemp and created a legal loophole for the compound.

The Agriculture Improvement Act allowed for the broader production of hemp, “which is cannabis that has only a very small amount of delta-9 THC,” said Ziva Cooper, director of the UCLA Center for Cannabis and Cannabinoids. That opened the door for people to extract chemical compounds from hemp, such as CBD, and convert them into other substances that can produce intoxicating effects, she said.

The result was that “delta-8 THC now seems to be legal, because it’s not specifically banned,” Cooper said. However, another federal law restricts a broader category of compounds that includes delta-8, she said, making things “very, very confusing.”

Some states have restricted or banned delta-8. But as of November, the National Cannabis Industry Assn. described it as “de facto legal” in nearly half of states.

The new findings on teen use of delta-8 come from the annual Monitoring the Future survey, a NIDA-funded project that asks U.S. adolescents about drug and alcohol use.

Some were using delta-8 fairly frequently: Among high school seniors who said they had used it in the previous year, more than a third said they had done so 10 or more times during that period.

It’s unclear if delta-8 affects teens differently than other forms of THC, but “even if it’s the same as other forms, we’re not in favor of 13-, 14- and 15-year-olds using cannabis,” said Dr. Wilson M. Compton, NIDA’s deputy director.

For teens, “we’d be concerned about its impact on learning and memory and day-to-day brain function,” Compton said. “There are also concerns about its use being associated with development of psychiatric illness, particularly psychotic disorders in those that are using cannabis particularly earlier in life,” he added.

Harlow and her colleagues found that in states where marijuana use is illegal for adults, 14% of high school seniors said they had used delta-8 in the last year. However, in states where marijuana was legal for adults, 8% of high school seniors said they used delta-8 in the previous year.

There was no clear difference based on state policies when it came to marijuana use, with more than 30% of the seniors overall saying they had used the drug in the last year. Among teens who reported using delta-8, nearly 91% said they had used marijuana as well.

The survey also revealed that teen use of delta-8 was more common in states where the psychoactive compound was not regulated.

“The alarm bells go off for me that these products are being marketed with claims of being a completely legal substitute for marijuana and they seem to be proliferating in areas where there’s really no regulation,” Harlow said. That lack of oversight can mean no required testing for potential contaminants, she said.

“We don’t necessarily know what is in the product that you are using — and you don’t know either,” Cooper said. “People are not making sure that there’s quality control or that there’s accurate labeling. … That in itself should make people who are thinking about experimenting pause.”

Other researchers have warned in the American Journal of Emergency Medicine that because of the lack of federal regulation, “products may be packaged in brightly-colored containers featuring cartoon characters, sweet or fruity flavors and candy-themed images that may be attractive to young children.”

In California, industrial hemp products may not legally contain more than a 0.3% concentration of THC — including delta-8 THC — nor can they include versions of the cannabinoid created through chemical synthesis, according to the state’s Department of Public Health. Harlow said that for delta-8, “most of the products that we’re seeing on the market are illegal in California.”

But “there is widespread flouting” of the state rules and “enforcement is practically nonexistent,” said Dale Gieringer, director of California NORML, an advocacy group focused on the rights of cannabis consumers. The result is “an enormous amount of delta-8 available by internet and convenience stores.”

Last year marked the first time the Monitoring the Future survey included a delta-8 question, which was posed to more than 2,000 high school seniors. In the coming years, researchers plan to query younger teens as well.

Compton said the numbers show that in a typical high school classroom, a handful of teens are likely using delta-8. In light of that, he said, “it’s incumbent on us to expand our research to understand the health impacts.”