Marijuana, a so-called "gateway drug," is the most widely used illicit drug in the United States and tends to be the first illegal drug teens use. It’s a widely held theory of drug use and abuse that there are stages of progression, from tobacco and alcohol to marijuana and from marijuana to other illicit drugs. Studies of drug abusers seem to bear the theory out: "Very few individuals who have tried heroin and cocaine have not already used marijuana and the majority have already used tobacco and alcohol," says Denise Kandel, a researcher in the Department of Psychiatry at Columbia University.

Is marijuana a gateway drug?

"I don't think so," 18-year-old Katie Falkenberg says.

"I just have known kids who have done it and they don't do anything else," adds Randy Glance 17.

"I don't think it's gonna lead them into anything bigger," 17-year-old Cody Mcguire says.

But a recent study published in the Journal of the American Medical Association suggests these teens are wrong.

The study examined 311 sets of twins, with one twin in each set having smoke marijuana before age 17.

"And what they found is that the twin who began smoking marijuana at a much earlier age had a very high increase in the probability that that twin would go on to use other drugs other than marijuana," says Dr. Robert Margolis, an addiction specialist.

"As for me, it led within about a month period to other drugs," says Kelly Crockett, 18.

Kelly says smoking pot got her closer to people who used hard drugs.

"And it's like, 'Hey, you like the way this made you feel? Try this, you know?' And I was up for it, you know, part of me was like OK, if I say no, you know they won't think I'm cool anymore," she says.

Experts say pot also releases dopamine in the brain, just like harder drugs do.

"So if marijuana triggers the release of dopamine and cocaine triggers the release of dopamine and heroin triggers the release of dopamine, it makes sense that smoking marijuana may be priming the brain, getting the brain ready for these other drugs," Dr. Margolis says.

But experts say many kids - and their parents - think marijuana is virtually harmless.

"Don't just say, 'Oh, it's only marijuana,'" Dr. Margolis says.

Instead, parents should arm themselves with information from credible sources and send a strong message to kids: Marijuana is illegal, unhealthy and could very well be a gateway to other drugs.

"I know that it is, and anyone that thinks that it isn't, it's kind of sad to say this, but wait and find out … you probably will, you know," Kelly says.