More Americans now think marijuana is harmless than ever before - at the same time as scientists grow more aware of the dangers it poses

• In 2004 half of Americans said weed is dangerous; only a third did in 2014
• And usage of marijuana has increased from 21.9m people to 31.9m
• But more research is linking weed to mental impairment and psychosis
• The belief that marijuana isn't harmful has been linked to medical usage
• People forget that there are real risks involved, say experts 
• However, the number of reported disorders has not increased since 2004 

By Associated Press

More Americans than ever believe that marijuana poses little to no risk to their health - just as scientists are discovering it's more dangerous than they previously believed, a report said.

From 2002-2014, the number of Americans who think that smoking marijuana once or twice a week poses a 'great risk' dropped from one half to one third, according to an article in Lancet Psychiatry.

That runs counter to scientific research about pot, said Dr Wilson Compton, lead author of the study, which looked at the views of more than 500,000 US adults.

'If anything, science has shown an increasing risk that we weren't as aware of years ago,' said Compton, deputy director of the National Institute on Drug Abuse.

Other research has increasingly linked marijuana use to mental impairment, and early, heavy use by people with certain genes to increased risk of developing psychosis, Compton noted. 

And the government survey also says that more are using marijuana - and using it more often: 13 per cent, or 31.9million, up from ten per cent, or 21.9million.

Relaxed attitudes towards pot use have occurred in line with the loosening of laws around the drug.

Dozens of states now allow medical marijuana and four states have recently legalized pot for recreational use. 

That has led to a misunderstanding by the public that marijuana is completely harmless, Roger A Roffman, a professor emeritus at University of Washington, School of Social Work told ABC News. 

'The drum is beating and has been now for a number of years to remove criminal penalties for possession and make it legal to be grown and sold to adults,' he said.

'With that movement comes some claims that marijuana is not dangerous enough to justify there being severe criminal penalties. 

'For many people who hear these claims the message translates to them that "there is nothing to worry about."'

Herbal medicine: The relaxed attitude is likely informed by states passing medical marijuana laws, experts say, and people not understanding that the drug can still be dangerous

Dr Patrick Fehling, an addiction psychiatrist at the University of Colorado Hospital Center for Dependency Addiction and Rehabilitation, said the growing amount of research into marijuana is 'indicating potential for harm.'

'There is a very big difference between recreational use and "addictional" use,' Fehling said. 

He elaborated that 'signs of addiction include tolerance and withdrawal, loss of control around your use, and consequences and problems in your life around your use.'

Some highlights of the report, which compared 2002 to 2014:

• About 1 in 8 adults said they used marijuana in the past year, up from 1 in 10. The number of marijuana users grew to about 32 million.

• Daily use doubled, to 3.5 per cent or about 8.4 million U.S. adults

• Changes in marijuana use and perception began to really climb in 2006-2007.

• No increase was seen in reported marijuana use disorders, like impaired memory, difficulty thinking and withdrawal symptoms like cravings, sleeplessness and depression.

The lack of an increase in reported disorders is surprising as law enforcement officials say marijuana is more potent than in the past, wrote Australian researcher Wayne Hall in an editorial in the journal.

More use should mean more reports of marijuana-related disorders. Another U.S. survey did find such an increase in recent years, Hall noted.

'I agree that this is a puzzle,' and needs to be researched further, Compton said.

Marijuana use remains illegal under federal law, with 25 states and the District of Columbia having medical marijuana laws.

And starting in 2014, Colorado and Washington began allowing recreational sales. Alaska and Oregon now also allow sales without a doctor's note.

Hall said it's likely those changes will increase the use of marijuana and perhaps reports of disorders.

The study didn't report on kids, only those 18 and older. But research drawn from another large survey has shown marijuana use among high school students has been falling. Over two decades, it dropped from 25 per cent to about 22 per cent.

Why are fewer kids using pot at a time more and more adults are?

There could be a lag. Youths have said in surveys that it seemed to be getting harder in the last decade to get marijuana.

But that may change as more states legalize the drug, more adults use it, and if teens get into less trouble if caught with the drug, experts said.