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Marijuana use has more than doubled in the U.S. since the beginning of the century — but so have problems for users, including addiction, researchers reported Wednesday.

They found 9.5 percent of U.S. adults used marijuana in 2013, up from 4.1 percent in 2001-2002.

More Americans than ever approve of marijuana use. Just this week, a new Gallup survey showed that 58 percent of Americans favor the legalization of cannabis — the highest number ever.

Bridget Grant of the National Institute on Alcohol Abuse and Alcoholism (NIAAA) and colleagues got the data from two giant face-to-face surveys of nearly 80,000 U.S. adults.

“The prevalence of marijuana use more than doubled between 2001-2002 and 2012-2013, and there was a large increase in marijuana use disorders during that time,” they wrote in the Journal of the American Medical Association’s JAMA Psychiatry.

"Young adults were at highest risk for marijuana use disorder."

“While not all marijuana users experience problems, nearly three of 10 marijuana users manifested a marijuana use disorder in 2012-2013.” That adds up to 6.8 million Americans, they said.

This is a big issue for younger users, they said.

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“When examined by age, young adults were at highest risk for marijuana use disorder in both surveys. Clearly, concerns about this age group continue, with prevention and intervention efforts for this group critically needed.” Blacks are now more likely to use cannabis than whites, they added.

It’s clear that legal changes reflect a change in attitude about marijuana and cannabis products, Grant’s team wrote. Four states have legalized recreational marijuana use and 23 states have medical marijuana laws. People often don’t realize that marijuana use is not always safe.

“Further, fewer Americans view marijuana use as risky, although studies have shown that use or early use of marijuana is associated with increased risk for many outcomes, including cognitive decline, psychosocial impairments, vehicle crashes, emergency department visits, psychiatric symptoms, poor quality of life, use of other drugs, a cannabis-withdrawal syndrome, and addiction risk,” they wrote.

One recent study showed that the brains of people who tend to use marijuana may be smaller to start with, while another found that marijuana appears to change the brain structure of young men with a high genetic risk of schizophrenia.

Studies also have shown that teenagers who use marijuana heavily grow up to have poor memories and also have brain abnormalities.

"As marijuana and alcohol are frequently used together, more research is also needed to understand the effects of combined use. Studies suggest that using marijuana and alcohol together impairs driving more than either substance alone and that alcohol use may increase the absorption of THC, the psychoactive chemical found in marijuana," the NIAAA said in a statement.

“Our study clearly cannot predict the impact of further legalization,” Grant’s team wrote. “As is the case with alcohol, many individuals can use marijuana without becoming addicted.”

But they said minimum legal drinking age and smoke-free laws have changed the use and abuse of alcohol and tobacco.

"Marijuana is associated with increased risk for many outcomes, including cognitive decline, psychosocial impairments, vehicle crashes, emergency department visits, psychiatric symptoms, poor quality of life, use of other drugs, a cannabis-withdrawal syndrome, and addiction risk."

“If the prevalence of marijuana users in the adult general population continues to increase, then the numbers of those with marijuana use disorders may increase as well. As is the case for addiction to other substances, most individuals with marijuana use disorders in the general population go untreated.”

And it’s likely to get worse. The Gallup poll shows approval of marijuana use has gone up steadily.

“When Gallup first asked the question, in 1969, 12 percent of Americans thought marijuana use should be legal,” the polling group said in a separate statement.

“By the late 1970s, support had increased to about 25 percent, and held there through the mid-1990s. The percentage of Americans who favored making use of the drug legal exceeded 30 percent by 2000 and was higher than 40 percent by 2009.”

Grant’s team said as the number of marijuana users grows, so will the number of addicts and people having other problems related to marijuana use.