Online cannabis culture 'a concern'
Experts have raised concerns about a thriving Twitter cannabis culture.
During a single month, researchers identified more than seven million tweets referring to marijuana, with "pro-pot" messages outnumbering those opposed to the drug by 15 to one.
Most of those sending and receiving "pot tweets" were under the age of 25, and many in their teens, said the team.
US psychiatrist and lead author Dr Patricia Cavazos-Rehg, from the Washington University Institute for Public Health, said: "It's a concern because frequent marijuana use can affect brain structures and interfere with cognitive function, emotional development and academic performance.
"The younger people are when they begin using marijuana, the more likely they are to become dependent. A lot of young people will phase out of marijuana use as they get older, but unfortunately, we're not good at predicting who those individuals are."
The findings, reported in the Journal of Adolescent Health, follow a computer search of tweets conducted between February 5 and March 5 last year.
Working with social media analytics company Simply Measured, the researchers looked for every tweet that mentioned marijuana.
Using search terms such as "joint", "blunt", "weed", "stoner" and "bong", the team turned up more than 7.6 million tweets referencing the drug.
An examination of a random sample of almost 7,000 tweets from these accounts revealed that 77% were pro-marijuana, 5% against, and 18% neutral.
Dr Cavazos-Rehg added: "Many people believe marijuana use is harmless, and social media conversations almost certainly drive some of those opinions, making the drug appear socially acceptable.
"Although we can't yet link pro-pot tweets to actual drug use, we should be worried because many people are receiving these messages are at an age when they are most likely to experiment with drugs and develop problems with substance use."
People tweeting pro-marijuana messages had more than 50 million Twitter followers - around 12 times more than those tweeting anti-marijuana messages.
Pro-pot tweets were most commonly aimed at encouraging use of cannabis and its legalisation, and made claims about the drug's health benefit.
And 10% of the pro-marijuana tweets were sent by individuals who said they were taking the drug or high at the time.
Anti-marijuana tweets often states that the drug's users were "losers" or unproductive, or that taking cannabis is unattractive.
They also stressed that the drug was harmful, or that the person tweeting was against legalisation.
The scientists focused their analysis on Twitter accounts with more than 775 followers, as well as those with "Klout scores" of 44 and above.
A Klout score measures social media influences on a scale of one to 100.