A change in laws about marijuana use across the United States is leading to a change in behavior.  The number of marijuana use grows as Americans partaking increased by 10 million between 2002 and 2014.

That’s the finding of a new study published in The Lancet Psychiatry. The study uses statistics from the annual U.S. National Survey on Drug Use and Health.

The study reports that the number of marijuana users in the U.S. increased from 21.9 million in 2002 to 31.9 million in 2014.

Other findings from the study:

• About 1.4 million adults first used marijuana in 2014, far more than the 823,000 who first used marijuana in 2002.
• The percentage of U.S. adults who use marijuana rose from 10.4 percent in 2002 to 13.3 percent in 2014.
• The number of adults who use marijuana daily or almost-daily rose from 3.9 million in 2002 to 8.4 million in 2014.

“This translates to a more than doubling of the number of adults using marijuana daily or near daily,” Dr. Wilson M. Compton of the National Institute on Drug Abuse, who worked on the study, told CBS News.

Marijuana Use Grows with Changes in Attitude

The study results come against the backdrop of changing attitudes about marijuana use in large portions of the U.S. Voters in four states (Alaska, Colorado, Oregon and Washington) and the District of Columbia have legalized recreational use of marijuana. And almost half the states have legalized medicinal marijuana.

More legalization amendments are on the ballot this fall, including making marijuana legal for recreational use in California (where it is already legal for medicinal use).

However, the study itself was conducted before recreational use became legal. For the study, researchers looked at the data from an annual survey of 596,500 adults aged 18 and up.

Marijuana Found Less Harmful

One of the key findings is that there is a decrease in the number of adults who perceive marijuana use as posing “great risk of harm” if used once or twice a week. In 2002, about 50 percent of those surveyed thought that was true, but in 2014 that number had dropped to 33 percent.

The survey also revealed important information of healthcare providers, finding that a dependence on marijuana is more likely to develop among young males, particularly those who have lower levels of educational attainment, unemployed, depressed and used tobacco.

However, despite the increase in marijuana use, researchers did not find a proportional increase in those who abuse marijuana. That number stayed at about 1.5 percent.