The reclassification of marijuana is important, according to the APA because its omission as an addictive substance then professionals might not see treatment regimens for dependence as necessary.
"They are right on about that," said Tom, a Long Beach, Calif., graphic designer who has been clean for 18 years, thanks to Marijuana Anonymous.
"I was amazed at the depth of my addiction and how pervasive it was in my life," said Tom, now 53, who smoked for 19 years. "If I started up again, I couldn't put it down. It's just like alcoholics. It doesn't matter what substance it is, it's the same disease."
About 40 percent of all Americans aged 12 and older -- about 94 million -- have tried marijuana at least once, according to a 2003 survey by the National Institute of Drug Abuse (NIDA). Of those, about 3.6 million were daily users.
As the most commonly used illicit drug in the United States, marijuana produces dependence and relapse rates comparable to other drugs some researchers believe.
About 9 percent of all those who used marijuana became dependent, compared to rates of 32 percent for tobacco, 23 percent for opiates and 15 percent for alcohol, according to the 1994 National Comorbidity Survey.
For daily smokers, that dependency rate soars to between 33 and 50 percent, say more recent studies.
The drug's popularity reached its height in the 1970s among baby boomers -- about 40 percent of all high school seniors, compared with 20 percent today -- said they had smoked marijuana in the previous month, according to NIDA.
The active ingredient in marijuana that causes the "high" is tetrahydrocannabinol, or THC. Orally, THC is used to treat nausea in cancer treatment and to stimulate appetite in AIDS patients.
Today's marijuana has far more THC than decades ago. The average potency has gone up from about 3.4 percent THC in 1982 to approximately 10 percent in 2008, according to NIDA.
With stronger pot, emergency rooms have reported more associated accidents. Just this week, seven people were killed when the driver -- drove the wrong way on a New York highway and collided head on with a pickup truck. Although the drivers family has disputed the results, toxicology tests showed high levels of alcohol and marijuana.
"The marijuana that is now out has been cross-bred like people breed flowers so what you have now is different from what you had 20 to 30 years ago," said John Massella, regional program director for the Pittsburgh-based Gateway rehabilitation center, which treats 10,000 to 12,000 patients a year.
"They develop a tolerance and need more to get the desired effect," he told ABCNews.com.
Gateway has seen an increase in number of marijuana dependency cases, mostly adults who do not come of their own volition. Many have been referred by family or have had trouble with the law or have tested positive in an employment-related urine test.
The younger a person begins smoking -- especially at today's higher potency levels -- when the brain is still developing, the greater the risk of dependence. "The progression is more aggressive," he said.
The biggest hurdle in treating these patients is that marijuana "still has a positive spin to it," he said. "People don't believe it's a problem."