"Plenty believe that they can't get addicted or hold on to the idea that it's only psychologically addictive and 'I can think my way out of it,'" said Massella. "But once you develop a dependency, there is always a dependency."
Such was the case with Alex, a 55-year-old Los Angeles lawyer whose life fell apart after a 39-year dependency.
"My life crashed around it," said Alex, who did not want to use his real name. "Early on, I used it to escape and have fun, but it turned into a self-medication sort of thing. Ultimately, I was using it morning, noon and night."
He became sober 12 years ago after hitting bottom: His career had fallen apart, a relationship failed and he got a diagnosis of attention deficit disorder. "I had a lot of inattention, lack of focus and difficulty completing projects. Early on the marijuana helped, but then I used it more and more."
"I have a lower bottom than a lot of people," said Alex, whose parents encouraged him to seek help. He spent 11 months in rehab and joined Marijuana Anonymous. "I had my car repossessed, I was thrown from my home and I was living on a guy's sofa and I was no longer able to practice law."
"Whether it's a physical addiction or not, I was behaving like an addict," he told ABCNews.com.
"It's a complex issue," said Paul L. Doering, a professor of pharmacy practice at the University of Florida. "Many can smoke once or twice a week and not have any problems."
According to the American Psychiatric Association, a person is considered dependent if he or she meets three of seven criteria. Only two of those criteria are associated with physiological symptoms like tolerance and physical withdrawal.
All addictive drugs have a "common signature," according to NIDA director Nora D. Volkow. "They increase dopamine levels in the brain's pleasure center and produce repetitive behavior. Marijuana appears do both, though at intermediate levels compared to other drugs.
"Social variables play an important role in humans -- we are not lab animals," she ABCNews.com. "We live in a complex social network system. Nicotine and alcohol produce lower increases in dopamine, but greater levels of addiction in society because they are legal and widely available."
Why some like Vicki and Alex become dependent and others can smoke socially, is still a mystery.
"There is tremendous variability and vulnerability," Volkow said. "It's like the H1N1 flu. Some people don't even get the symptoms and others die. It's the same thing with drugs."
"There is a tendency to try to minimize the risks associated with drugs," she said. "The one that is targeted the most in marijuana. For some, the willingness to experiment may be affected by how dangerous their perception of the drug is."
Addiction experts agree that marijuana's dangers pale in comparison to harder drugs. But distorting its risks may encourage more young people to experiment with a drug that can cause dependency.
"Marijuana has a fascinating history in part because of the hyperbole and exaggeration to either scare people so they don't use it or to justify more liberal laws," said Roger A. Roffman, a professor of social work at University of Washington, who for 20 years has studied marijuana dependency counseling. "The misrepresentations and half-truths and outright lies are really incredible."