Sometimes it’s the glassy look on their faces that gives them away, said Dr. Peter Antevy, an emergency room doctor at Joe DiMaggio Children’s Hospital in Hollywood.

Other times they’re brought in acting psychotic, like they’re seeing things or hearing voices. Once, a young man arrived at the emergency room after jumping out of a car as it barreled down the freeway.

These are some of the most frightening possible side effects of “designer drugs” that have become increasingly popular among teenagers and young adults.

“These don’t show up on any drug tests, unfortunately,” Antevy said. “And kids don’t like to admit they’ve taken them, especially if they’re in front of their parents. The only way I know is by asking, simply, ‘Are you on K2? Are you taking Spice?’ They’ll look at me and say, ‘Yes.’ ”

The creators and marketers of some of these new drugs sidestep government regulations by making minor changes to the drugs’ molecular composition or by advertising them for a different use. In recent years, one of the most common designer drugs used by adolescents has been synthetic marijuana. Experts say that use of so-called “bath salts” is less frequent among teens as it is among adults.

The American Association of Poison Control Centers reported just over 300 calls about exposure to bath salts in 2010. A year later, the calls numbered more than 6,000. Meanwhile, calls about exposure to synthetic marijuana jumped from nearly 3,000 in 2010 to about 7,000 in 2011.

What makes these drugs troubling is that their possible side effects and long-term damage have not been fully studied. That’s even more challenging when the labs that create them continue to make slight changes to the chemical composition to avoid prohibition, Antevy said.

“They’re able to change one molecule on this drug and then you can call it something different, so the regulators can’t disallow them,” he said. “They’re skirting the law.”

Synthetic marijuana — often called by the common brand names K2 and Spice — is essentially natural herbs sprayed with synthetic chemicals whose composition is similar to THC, the main psychoactive component of marijuana. However, the compounds in synthetic marijuana bind to the cell receptors in the human brain more strongly than TCH.

Bath salts contain chemicals such as mephedrone, which can have a similar stimulating effect to cocaine or amphetamines. Like synthetic marijuana, bath salts are labeled “not for human consumption,” which has made them difficult to regulate.

While the most common symptom for both classes of drugs is a sense of high, in some cases users develop an elevated heart rate and blood pressure. That’s when they wind up in the emergency room. Some have difficulty breathing or develop seizures.

Still others behave psychotic for as many as two or three days, said Dr. Ushimbra Buford, a child and adolescent psychiatrist from Jackson Mental Health Hospital. Buford, who directs the hospital’s intake unit, sees young patients with symptoms that appear to have been induced by a synthetic drug about two to three times per month.

“They may have perpetual disturbances, like they’re seeing and hearing things, or are paranoid that people are trying to kill them,” he said.