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(Reuters) - With U.S. backers of legalized marijuana emboldened by victories in two states during the November elections, foes are ramping up efforts and honing their message to focus on risks they say the drug poses to mental health and intellectual development, especially for the young.
A newly formed group, Project SAM, launched this month in Denver and headed by former Congressman Patrick Kennedy, has emerged as one of the main adversaries of legalizing pot.
"I believe that drug use, which is to alter the mind, is injurious to the mind ... It's nothing that society should sanction," Kennedy, a Democrat from Rhode Island who spearheaded legislation in Congress to expand access to mental health care, said in a phone interview. Kennedy has admitted to past drug use and has been treated for addiction to prescription painkillers.
In November, voters in Washington state and Colorado approved measures to allow adult recreational pot use, despite a federal ban on the drug. Both states are creating frameworks to tax and regulate marijuana sales.
Leaders of Project SAM say the group has no plans to push for repeal of legalization in Colorado and Washington, but will challenge any future legalization efforts.
Kevin Sabet, a co-founder of Project SAM, said warning of the mental health dangers of pot would be a "central part" of his group's work. At the same time, the group is developing model legislation to take to lawmakers to achieve such goals as improving access to drug treatment programs, he said.
Leaders of Project SAM, which stands for Smart Approaches to Marijuana, have highlighted studies that show a link between marijuana and schizophrenia and suggest heavy use of the drug among teenagers can lead to an IQ drop.
But the science around many aspects of marijuana's effects on the brain remains unsettled. A pair of papers published this month called into question any harm to intelligence and the possibility that marijuana causes schizophrenia.
One Project SAM board member, Dr. Paula Riggs, a psychiatry professor at the University of Colorado at Denver, said she fears that legalization will lead to more teenagers in Washington state and Colorado getting marijuana second-hand from adults.
For teens who use marijuana, it will "affect their development for a very long time, if not forever," Riggs said.
Project SAM's experts cite a 2012 study of over 1,000 New Zealanders by researchers from Duke University and King's College London. It found those who used pot heavily before age 18 and continued for years afterward had an average IQ decline of eight points from age 13 to 38.
But that study's conclusions came under fire this month, as a statistical analysis in Proceedings of the National Academy of Sciences questioned the link between pot use and lower intelligence. The paper's author, researcher Ole Rogeberg of Norway, argued socioeconomic factors rather than marijuana use could also have produced any drop in IQ.
Project SAM researchers have also pointed to studies showing an association between marijuana use and schizophrenia.
Dr. Sharon Levy, a Harvard Medical School assistant professor of pediatrics and a leader with Project SAM, cited a study of Swedish conscripts in 1969 that found heavy users of cannabis were six times more likely to develop schizophrenia.
The study by Sven Andreasson and other Swedish researchers was published in 1987 in The Lancet.
"The hypothesis is that there are people who are more susceptible to developing schizophrenia," Levy said. "For whatever combination of genes they have, they're more susceptible. These people still may not develop schizophrenia, but if they are stressed enough they will develop it."
"Cannabis is on the list of those kinds of stresses, that's the hypothesis," she said.
But the link between pot and schizophrenia was challenged by a paper this month in F1000 Medicine Reports from researchers Suzanne Gage, Stanley Zammit and Matthew Hickman. They said stronger evidence was needed to show marijuana plays a key role in causing schizophrenia in the general population.
These researchers said pot use has increased in recent decades in the United Kingdom and Australia, but those countries have not seen a higher incidence of schizophrenia.
Project SAM leaders also say the science shows that for those who have mental illnesses or are predisposed to them, using marijuana can worsen their condition. Some supporters of pot legalization agree those people should avoid the drug.
Levy also cited a 2010 paper in the American Journal of Epidemiology that used data from more than 50,000 adults taking part in a World Health Organization mental health study. It found pot use before age 17 was linked to a 50 percent increase in the risk of later developing a spell of depression.
Reporting by Alex Dobuzinskis; Editing by Cynthia Johnston and David Gregorio