Update: New research shows that the effects of pot are even worse for teens than we previously thought: Scientists now believe that marijuana causes developmental changes to the part of the brain that controls motivation and decision-making, and that the impact is long-lasting. Even scarier, studies have also shown that teens who are dependent on pot before the age of 18 and continue to use the drug lose an average of eight I.Q. points by the time they're 38.

In Lea's* world, pot is practically inescapable. "Kids come to school high. It's at every party," says the seventeen-year-old from New Hampshire. "I've seen guys weighing and packaging weed on dining room tables." These days, everyone else's world is looking a lot more like Lea's: Recent studies show that almost 23 percent of high school seniors used marijuana in the past month, and nearly half of all teens have tried it—a staggering 21 percent increase from just five years ago.

Maybe it's in the air. State after state has legalized weed for medical use, and now they're starting to approve it recreationally—if you're 21 or older, you can light up a joint, sans permit, in Colorado and Washington. And then, of course, there's the star factor: Young celebrities are regularly photographed with it (Oh look, there's a blunt in her hand!), and major icons are serving as outright mascots (both Lady Gaga and Rihanna dressed as "Mary Jane" last Halloween). "There's been a sea change and a shift in people's opinions about cannabis," asserts psychiatrist Julie Holland, M.D., editor of The Pot Book. It's no longer just the burnouts who smoke bowls—it seems like everybody's doing it. So how bad can it really be?

Honestly, it's probably worse than you think. Sure, you've heard that getting high can increase your appetite (and who needs more help polishing off a bag of chips?), but it can also have seriously scary effects on young brains. THC, the main psychoactive chemical in marijuana, may interfere with a process called pruning that happens in your teens and early twenties—a crucial time of development when experts say the brain is most vulnerable. If you're smoking pot, the process can be disrupted, and your brain may not form in the optimal way, meaning you're setting yourself up for decreased cognitive function and impaired memory. For Beth*, a nineteen-year-old from Missouri, the changes after she started smoking regularly were noticeable: "I had declined awareness, shabby coordination, and a loss of the sense of time," she shares. "I lacked motivation and made poor grades in school."

Aside from possibly disrupting brain development, there's a chance of bringing on mental illness, too. "People who have blood relatives that are prone to psychosis can have a more extreme reaction to cannabis than other people," warns Dr. Holland. "It's rare, but you don't always know if you're one of those at risk." Even scarier? There's also a possible link between teens who smoke pot and an increased likelihood of developing schizophrenia, says David Sack, M.D., CEO of Promises Treatment Centers, where Britney Spears and Lindsay Lohan have reportedly gone to rehab.

Pot overdoses may be essentially unheard of, but getting high does send people to the emergency room. "Marijuana can cause panic attacks and a racing heartbeat—sometimes people are afraid they're having a heart attack," explains Dr. Sack. Lea experienced that fright firsthand: "I tried pot one time at a party, and I started to feel anxious and paranoid. My heart literally felt like it was about to explode; it was beating way too fast." What happened next is yet another downside to the drug: It can put your sexual safety in jeopardy. "Seeing that I was a total novice, one of the guys I was with tried to make a move on me," Lea reveals. She's not alone: One study from the University of Southern California found that 30 percent of sexual assault victims at rape treatment centers had marijuana in their systems.