No wonder marijuana is often called "dope." A new study of 1000 people shows those who started smoking weed before age 18 - when their brains are still developing - had a considerable decline in their IQ.

This is worrisome because heavy marijuana use among teens is up 80 percent since 2008. No one's sure why kids are smoking more. But in recent years, as kids' perception of the risks associated with marijuana has declined, their use has increased. Some speculate that all the talk about decriminalization and medical use for marijuana sends the message to kids that weed is safe.

Take Andy and Art, 17-year-old twins who live outside of Boston. They started smoking four years ago, after their mother died of alcoholism. "When it wears off, all the anger and sadness is right back in my head. That's why I smoke again," Andy says.

For th twins, weed's both a crutch and an obstacle. They share a dream of playing in the NFL. Last fall, their football team lost the play-offs by a field goal. Andy says marijuana slowed him down. "There were tackles I'd have made if I was a little quicker."

They don't take all the blame. After all, they say, nearly everyone on the team smokes marijuana.

Weed affects their schoolwork, too. Andy says it messes with his memory. Art - his motivation.

Experts can't say for certain how harmful marijuana is. But this much is known: early marijuana use affects the developing brain.

In another study, researchers at McLean Hospital found that adults who started to smoke marijuana regularly before age 16 smoke more, and more often, than those who waited till they were older.

They also found that heavy marijuana use affects memory, problem solving and impulse control. Kids who smoke regularly face a double whammy. The part of the brain that controls these behaviors is still developing, so they're already less able to control impulses and think through consequences - this is why teens do so many things that drive us crazy! Researchers say regular marijuana use may in fact make this worse.

A pilot study at McLean also reports that earlier, heavy marijuana use during development affects the brain's wiring - so one part can't communicate with another part as well. Whether or not this change is permanent is being studied.

I told Andy and Art what scientists know about marijuana. It scared Art. He's glad he decided to smoke less and save money for college. He wants his brother to cut back. "You smoke that much marijuana, you do stupid things. He's my best friend. I don't want anything bad to happen to him."

Andy wishes an adult had warned him about marijuana. "Weed isn't a terrible drug but you shouldn't be doing it at a young age."

When kids know their parents strongly disapprove of their using marijuana, they're much less likely to try it. Still, many parents avoid the subject while the number of teen users skyrockets.

Some parents look the other way. Others don't know how or when to talk about it. Still others reason they smoked marijuana and turned out OK.

But the science is compelling. Research in hand, parents are wise to start conversations about marijuana early - and keep talking. You can say, "Early marijuana use may affect you forever. I want you to give yourself every possible chance to develop into a healthy adult."

Engage kids in conversations about what they want in life and ask what they might give up if they use marijuana while their brain is still developing.

Like many parents, you might think your child would never use drugs.

Recently at a substance abuse prevention forum, a mom Wendy was shocked when a police officer told the audience that, in their town, kids as young as 10 years old are smoking dope. Wendy told me, "My 13-year-old still plays with Barbies. But I might be kidding myself. What parent doesn't say 'not my kids?'"

*Some names have been changed to protect identities.