Vape pens look just like e-cigarettes, but they're for vaporizing marijuana. They're smoke-free and very popular among marijuana users. But it can be hard to know just how strong a dose they're getting. Reporter Miles Bryan explains.

MILES BRYAN, BYLINE: It's a hot, windy afternoon outside the 420 Treehouse Medical Marijuana Lounge in Los Angeles. And Aggie Karmelita and Cesar Perez are getting stoned. The vape pen that Karmelita and Perez are sharing doesn't seem like something you would use to get high. It's sleek and black, and looks more at home in an Apple Store than a head shop. Karmelita says that's kind of the point.

AGGIE KARMELITA: You know, it's convenient for something to take to, like, a movie theater, or a concert - where you're not necessarily supposed to be smoking a joint or having a pipe.

BRYAN: On the website Leafly, the Yelp of the pot world, one out of three reviews are about vaping marijuana. And while vape pens are discreet, they can also be strong, like, really strong. Here's Perez.

CESAR PEREZ: I tried it about four years ago, and it was life-changing.

BRYAN: Vape pens are usually loaded with hash oil, which can have many times more of the psychoactive chemical THC than the average joint. A battery-powered heating element vaporizes the oil, which is then inhaled. Perez says he first tried hash oil with a friend who worked at a pot dispensary.

PEREZ: She was like, hey, come here. You want to try some concentrates? I'm like, yeah. After taking the hit, I felt like I smoked 20 joints.

JOHN LOVELL: Hash oil is correctly referred to as the crack cocaine of marijuana.

BRYAN: That's John Lovell. He's the spokesperson for the California Narcotic Officers Association, which is supporting a bill here in the Golden State that would ban medical hash oil. Lovell says that the stealthiness of vape pens can be dangerous.

LOVELL: All you see is someone smoking on an e-cigarette. You can put yourself at risk.

BRYAN: But not everyone is convinced that a future of vaping marijuana is such a bad thing.

MARK KLEIMAN: If we're going to deliver cannabis by lung, it's going to be vaporization, not smoking.

BRYAN: Mark Kleiman teaches marijuana policy and law at UCLA. He says that vaping will probably replace smoking marijuana in the long run. But Kleiman says that right now, there's a problem.

KLEIMAN: A puff of cannabis is not standard the way a drink is standard, right? If you've had three beers, you know how much beer - alcohol you've had. If you've had three hits, you don't know anything about how much cannabis you've had.

BRYAN: Which means, Kleiman says, you might get more stoned than you want to be. Back outside the 420 Treehouse Lounge, Perez and Karmelita say that whether it's alcohol or marijuana, it's really just about knowing your tolerance.

PEREZ: Some people just go all out and take 50 shots, you know.

KARMELITA: Yeah.

PEREZ: And then, some people take one or two shots, and then that's how they learn. Some people take one or two hits, and some people just smoke the whole pen, you know.

BRYAN: But vaping might not always require such trial and error. Colorado and Washington will both soon require the testing and labeling for potency of all marijuana products. For NPR News, I'm Miles Bryan.