Marijuana use among adults in the United States more than doubled between 2001 and 2013, according to a new study.

Researchers also found a similar increase in the number of people who abused or were dependent on marijuana.

These results come as marijuana legalization continues to move forward in the United States.

The researchers used data from two large national surveys of alcohol, tobacco, and illegal drug use called the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC).

The surveys also asked questions meant to identify marijuana use disorders such as abuse or dependence.

More than 79,000 adults total responded during the two survey years — 2001 to 2002 and 2012 to 2013. The results were published online today in JAMA Psychiatry.

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Who’s Using Marijuana

The survey revealed that the percentage of people who had used marijuana during the past year more than doubled from 4.1 percent in 2001 to 9.5 percent in 2013.

These increases were most prominent in women, African-Americans, and Hispanics as well as people who had the lowest income, lived in the South, or were middle-aged or older.

The rate of marijuana abuse and addiction has actually gone down as the numbers of active pot users goes up. In addition, the percentage of marijuana users who had a marijuana use disorder decreased. The 6.8 million adults who abuse or are dependent on marijuana represent around 30 percent of all marijuana users. A decade ago, more than 35 percent of pot users abused the drug.

“As is the case with alcohol, many individuals can use marijuana without becoming addicted,” wrote the authors. “However, the clear risk for marijuana use disorders among users — approximately 30 percent — suggests that as the number of U.S. users grows, so will the numbers of those experiencing problems related to such use.”

The Science of Medical Marijuana »

Shifting Attitudes Toward Marijuana

The surveys don’t indicate why more people are using marijuana, but it’s clear that attitudes toward marijuana have been shifting in recent years.

Currently, 23 states, the District of Columbia, and Guam allow the use of marijuana for medical purposes.

Four states — Alaska, Colorado, Oregon, and Washington — and the District of Columbia have passed recreational marijuana laws since 2012. In 2015 similar bills were introduced in 21 state legislatures.

A 2013 study in the journal Drug and Alcohol Dependence found that states with legalized medical marijuana had higher overall rates of marijuana usage and marijuana abuse or dependence.

However, researchers can’t say for certain that medical marijuana laws lead to higher usage. To understand that, more research is needed.

“One of the things that will be important to do is conduct studies designed to examine the reasons for the increases we found. These could be laws, but there could also be other factors” study author Deborah Hasin, Ph.D., a professor of epidemiology at Columbia University, said in an email to Healthline.

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Health Effects of Marijuana Are Real

One of the difficulties in monitoring the effects of marijuana use is that no single survey tracks everything. And different surveys may give different results.

According to the researchers, the National Survey on Drug Use and Health (NSDUH) showed that marijuana use among American adults increased only 12 percent between 2002 and 2012. This survey also indicated that marijuana use disorders didn’t change during that time.

The researchers are not certain why there’s a discrepancy. It could be that the NESARC, which includes more detailed questions, may be better at identifying marijuana use trends in adults. The results of the new study, though, line up with other research.

“Compared to NSDUH findings,” said Hasin, “our results are more consistent with increases over the same general time period in other marijuana-related consequences, including fatal motor vehicle crashes and emergency room visits involving marijuana.”

In addition to these consequences, marijuana can also have long-term effects on the brain, lead to breathing problems, increase heart rate, and contribute to mental health problems.

Marijuana research is further complicated by the tendency of some marijuana users to cut back on certain unhealthy behaviors.

Medical marijuana users may use marijuana instead of alcohol, illegal drugs, or prescription medications.

Another study found that when people turn 21 they drink more alcohol and use less marijuana.

With recreational marijuana legalized for the first time in the United States only a few years ago, it’s still too soon to know exactly how this will shape the health of marijuana users.

This has prompted some health officials to call for a more measured approach to legalization.

“While many in the United States think prohibition of recreational marijuana should be ended,” wrote the authors, “this study and others suggest caution and the need for public education about the potential harms in marijuana use, including the risk for addiction.”

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