A Juul e-cigarette for sale at Fast Eddie's Smoke Shop. Shoppers must be 21 years of age. Suzanne Kreiter/Globe Staff

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A new front has opened in the never-ending game of cat and mouse between teenagers and adults — over Juuling, a discreet form of vaping that is the most widespread phenomenon you’ve likely never heard of.

In some high schools, the “Juuling in the bathroom” problem has gotten so intense that administrators are sending home e-mails warning parents about the dangers of e-cigarettes in general — and, in particular, about a brand called Juul, which makes sleek devices that are easily concealed and often mistaken for thumb drives.

In Newton, an Oct. 31 e-mail to parents showed a cop-show-style evidence photo with a dire caption: “Here is a Juul device disguised as a Sharpie Pen.”

The e-mail also schooled parents who may be unfamiliar with the whole vaping trend. “Electronic cigarettes are devices that utilize stored electricity to heat a liquid into vapors, which are then inhaled by the user,” the letter read. “The liquid can be anything from a flavored water-type mixture to liquid nicotine to THC, the principal active element of marijuana.”

The letter warned that recent studies “suggest e-cigarettes are the latest ‘gateway’ to harder drug use.”

A psychologist who sees patients in Boston’s upscale western suburbs told the Globe that every teen he treats now uses a Juul. One patient, a student at a prestigious local private school, secretly used his parents’ credit cards to buy thousands of dollars of Juuls online, and then turned around and sold the devices and flavored pods to other kids at a profit.

He was eventually expelled, said the therapist, who requested anonymity to protect his patients’ privacy.

Beyond the fact it can get them in trouble, many kids think there’s nothing wrong with vaping or using a Juul. But John Ross, a hospitalist at Brigham and Women’s Hospital who contributes to the Harvard Health Blog, said long-term safety data on e-cigarettes do not yet exist.

On the positive side, he wrote in a 2016 post, “E-cigarettes are almost certainly less lethal than conventional cigarettes. Now the bad news. Nicotine in e-cigarettes may have several negative health effects. Chronic nicotine exposure may lead to insulin resistance and type 2 diabetes. . . . Inhaled nicotine increases heart rate and blood pressure. Nicotine is highly addictive in its own right, and it may lead to changes in the brain that increase the risk of addiction to other drugs, especially in young people.”

A Juul “starter” kit costs $49.99 if you buy it online from the company. It includes a re-chargeable Juul device, a USB charger, a warranty, and a four-pack of the flavored Juul pods. On its website, the company promises a “powerful vapor experience” and says the nicotine in one pod is approximately equivalent to a whole pack of cigarettes, “or 200 puffs.”

A banner across the top of the website clearly states that its products are for those 21 and older, and would-be purchasers are alerted that an adult must sign for delivery.

Juul pods come in kid-friendly flavors, but in a statement e-mailed to the Globe, the company emphasized its position that its products are intended for those 21 and older.

“JUUL Lab’s mission is to eliminate cigarette smoking by offering existing adult smokers with a better alternative to combustible cigarettes,” the statement read. “We strongly condemn the use of our product by minors, and it is in fact illegal to sell our product to minors. No student at any high school should be in the possession of a Juul product.

Even so, kids are managing to score what they need — one high schooler says Juul starter kits can be purchased on the resale market for around $80. Students are Juuling in the boys and girls rooms, hallways, and even in class, where they take a hit and then swallow the vapor or exhale it into their hoodies when the teacher isn’t looking.

One high schooler described why teens like it: “It gives you a head rush,” he said. “The flavors are good. You can do tricks with it. You can ghost it” — breathe the vapor out and then back in. “It looks cool.”

Although many parents have never, or only recently, heard of Juuling, every student approached by a Globe reporter in multiple suburbs not only was familiar with the product, but had a story.

A Juul e-cigarette for sale at Fast Eddie's Smoke Shop, demonstrated by shop manager Cathleen McCarthy. Shoppers must be 21 years of age. Suzanne Kreiter/Globe Staff

“In history class today, a kid pulled something out of his pocket and a Juul fell out,” said a junior girl in Needham.

When you walk into the bathrooms, she added, you can hear the faint but distinct crackling sound the liquid makes. “It’s usually fairly obvious.”

It was the same deal in Newton. “People are [openly] charging them at school,” said a freshman girl hanging out at Rancatore’s ice cream after school.

“On Snapchat, I’ve seen people Juuling in class,” said her friend.

In Needham, high school principal Aaron Sicotte said the devices are so benign-looking that “we’ve had situations where [a Juul] has fallen out of a bag and a teacher has handed it back to them not knowing what they were giving back.”

He described the challenges of catching a student in the act: Because Juul pods come in flavors (including mango and crème brulee), the sweet odor “can almost be attributed to a light perfume,” Sicotte said.

“It can happen so fast,” he added. “A student can step in a bathroom stall or in a hallway and take a quick hit. It’s opening up a whole new set of options that students haven’t had in the past.”

Wellesley High School catches about two kids a month vaping, most using Juuls, according to principal Jamie Chisum. “We have a lot of suspicions about students selling cartridges here too.”

The school is planning an educational night in March for parents, and in the meantime, officials have increased monitoring of bathrooms.

In Braintree, Juuling has gotten so popular that two broadcast journalism students made a news-style video about it, complete with a female Juuler whose voice is changed to conceal her identity as she describes where she partakes.

“In the girls bathroom,” she says in a strange techno-voice, “and sometimes in class,

and I blow it into my sweat shirt.”

Like a lot of teenagers, Emily Linskey — the video’s cocreator and on-screen reporter — was surprised to learn that Juuls contain so much nicotine.

“I thought it was a way healthier choice than cigarettes — it won’t give you as much cancer,” she told the Globe. “But finding out that one pod is equivalent to one pack of cigarettes was shocking.”

Meanwhile, even as kids are Juuling to be cool, doing it the wrong way can backfire, at least that’s the word from two Needham High athletes.

Seniors confident in their social rank, they sat parked outside the school and criticized freshmen who are so inexperienced they huddle around a Juul and then try to look innocent when someone walks in on the action.