A teen who consumes alcohol is likely to have reduced brain tissue health, but a teen who uses marijuana is not, according to a new study.

Researchers scanned the brains of 92 adolescents, ages 16 to 20, before and after an 18-month period. During that year and a half, half of the teens -- who already had extensive alcohol and marijuana-use histories -- continued to use marijuana and alcohol in varying amounts. The other half abstained or kept consumption minimal, as they had throughout adolescence.

The before-and-after brain scans of the teens consuming typically five or more drinks at least twice a week showed reduced white matter brain tissue health, study co-author Susan Tapert, neuroscientist at University of California, San Diego, told HuffPost. This may mean declines in memory, attention, and decision-making into later adolescence and adulthood, she said.

However, the level of marijuana use -- up to nine times a week during the 18 months -- was not linked to a change in brain tissue health. The researchers did not test performance; they only looked at brain scans.

The study was conducted by researchers at UC San Diego and is scheduled to be published in the April issue of the journal Alcoholism: Clinical & Experimental Research.

The damage occurs because white matter brain tissue develops throughout adolescence and into a person's 20s, Joanna Jacobus, postdoctoral fellow at the UC San Diego, and co-author of the study, told HuffPost.

Part of that still-developing brain tissue is where decision-making ability comes from, which can exacerbate substance use. "It becomes a cycle. If teens decrease their tissue health and cognitive ability to inhibit themselves, they might become more likely to engage in risky behavior like excessive substance use," Jacobus said.

While studies showing the deleterious effect of alcohol on adolescents and adults have been more consistent, studies of the effect of marijuana have not, Tapert said. "One reason is that marijuana can really vary. Different strains contain different levels of THC and other marijuana components. For example, some studies have suggested one component, cannabidiol, may actually have neuroprotective effects," she said.

The researchers are not sure why alcohol had an effect and marijuana did not. They said the study results cannot be considered definitive without more research. They also said they do not know if the reduced brain tissue health is permanent.

Still, Duncan Clark, a medical doctor who was not involved in the study and who studies teen substance abuse, said the study is valuable because similar research has only conducted a one-time test instead of tests before and after a period of substance use.

Clark, associate professor of psychiatry at the University of Pittsburgh Medical Center, said that because white matter brain tissue development may be the basis for greater self-control, researchers hope to be able to eventually use brain scans like those in this study to predict young individuals' likelihood of substance abuse.

Because the researchers followed the subjects for 18 months, they were able to at least partially monitor preexisting differences in the two groups. But Jacobus conceded that eliminating other factors -- such as genetics, home environment, and even minimal use of other drugs -- is very difficult.

Each teen in the study received brain imaging, a detailed substance-use assessment, and toxicology screening at the beginning of the study and at the end of the study -- as well as substance-use interviews every six months.

Tapert led another study published in 2009 that looked at people ages 12 to 14 before and after they started to drink. Tapert's team found poorer performance on tests of thinking and memory in the teens who had begun to drink. The researchers reported that alcohol particularly compromised boys' attention span, and girls' comprehension and interpretation of visual information.

While this latest study examines marijuana's effect on physical brain tissue, a Duke University study earlier this year examined the drug's effect on intelligence and performance. That study found that teens who routinely smoke marijuana before turning 18 risk a long-term drop in their IQ.

Teen use of marijuana continues to be high, while teens' perception of the drug's harmfulness is down, according to a University of Michigan study published Wednesday. Nearly 23 percent of high school seniors polled in the study said they had smoked marijuana in the month prior.

"It is clear that more research is needed into the long-term effects of marijuana on the brain," Tapert said. "Especially because use is up."

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  • Louis Armstrong, the jazz marvel, gained fame initially as a horn player and later as a vocalist, a musical ambassador, and a character of epic proportions. Known as "Satchmo" and "Pops" to millions of fans, the trumpet maestro swore by "gage," one of the preferred nicknames for cannabis in jazz circles. Satchmo, a notorious pot smoker, often touted the benefits of the herb. "We always looked at pot as a sort of medicine," he stated. Armstrong, the grandchild of a slave, said he used reefer to unwind, to relieve stress, to ease the chronic pain of racism: "It makes you feel good, man. It relaxes you, makes you forget all the bad things that happen to a Negro."

  • The Hotel Delmonico on Park Avenue, NYC, where Bob Dylan turned the Beatles on to marijuana on August 28, 1964. Beatlemania was then at its peak, and twenty police stood guard outside the Beatles' sixth-floor suite, while behind closed doors Dylan lit up and handed the Fab Four their first joint. A few minutes later everyone was laughing uproariously. "We were kind of proud to have been introduced to pot by Dylan," Paul McCartney remarked. Just as the jazz vipers of an earlier era had portrayed reefer as a kick not to miss, rock stars promoted a similar message to a huge audience. Cannabis opened the door to new dimensions of popular music, and Dylan and the Beatles carried the youth of the world with them across the psychoactive threshold.

  • Marijuana legalization was not on America's political radar when Allen Ginsberg, the famous beat poet, led a pro-marijuana march outside the Women's House of Detention on Sixth Avenue in lower Manhattan on January 10, 1965. A dozen demonstrators chanted slogans and waved placards, resulting in one of the quintessential images of the Sixties: a photograph of Ginsberg, snowflakes on his beard and head, holding a sign that said, POT IS FUN. Another picket sign read: POT IS A REALITY KICK. The pro-pot protest was the inaugural event of the Committee to Legalize Marijuana (LEMAR) in New York City. It was the first organization in the United States to publicly agitate for marijuana law reform.

  • Israeli scientist Raphael Mechoulam and his colleagues at the Hebrew University of Jerusalem first synthesized and elucidated the molecular structure of delta-9-tetrahydrocannabinol (THC), marijuana's principal psychoactive ingredient, in 1965. Although he didn't know it at the time, Mechoulam had lit a slow burning fuse that would detonate a revolution in medical science. In the early 1990s, Mechoulam's team discovered a natural THC-like substance, an endogenous compound, in the brain and body (our "inner cannabis"), which protects neurons, stimulates adult stem-cell growth, and regulates a broad range physiological processes, including glucose metabolism, blood pressure, bone density, intestinal fortitude and pain perception. Within the scientific community, the discovery of the "endocannanbinoid system" is increasingly recognized as a seminal advancement in our understanding of human biology.

  • Inside Edgewood Arsenal, Maryland, headquarters of the U.S. Army Chemical Corps, where American soldiers were given an array of mind-bending drugs, including an exceptionally potent synthetic marijuana derivative. These secret experiments took place in the 1960s, when U.S. national security strategists were high on the prospect of developing a so-called humane weapon that could knock people out without killing them. Top military officers hyped the notion of "war without death," conjuring visions of aircraft spewing clouds of "madness gas" over enemy territory that would disorient the bad guys and dissolve their will to resist. In pursuit of this pipe dream, army medical officers inadvertently discovered marijuana's powerful anticonvulsive properties. Cannabis, an army chemical warfare specialist concluded, "is probably the most potent anti-epileptic known to medicine."

  • At a time when the therapeutic use of marijuana had been abandoned in the United States, Dr. Tod Mikuriya fought to restore cannabis to its proper place in the Western pharmacopeia. While serving as director of marijuana research for the National Institute of Mental Health in1967, Mikuriya rediscovered the forgotten medical literature on cannabis and brought it to the attention of physicians and scientists. Almost singlehandedly, he kept the issue alive while very few Americans--even pot smokers--were aware of marijuana's medicinal history. Mikuriya would play a crucial role in drafting the language of Proposition 215, the 1996 ballot measure that legalized medical marijuana in California not just for a shortlist of specified diseases but also for "any other illness for which marijuana provides relief."

  • Robert Randall, the Rosa Parks of the medical-marijuana movement, made legal and medical history in 1976 when he successfully sued the U.S government for access to cannabis, the only remedy that controlled his glaucoma. Rather than copping a plea after he was busted for growing pot, he chose to fight the criminal charges on the basis of "medical necessity." Randall argued that any sane person would break the law to save his eyesight. DC Superior Court Judge James A. Washington agreed and Randall was acquitted. This landmark verdict compelled the Food and Drug Administration to establish a "Compassionate IND [Investigational New Drug] Program," which continues to distribute government-grown marijuana to a handful of medical necessity patients - while U.S. officials allege that cannabis lacks therapeutic value.

  • Jack Herer, the patron saint of hemp, promoted a unified field theory of cannabis as a multifaceted sustainable resource, an eco-friendly source of food, fiber, medicine and recreation. Herer was instrumental in catalyzing a renewed interest in the many forgotten industrial uses of a plant once prized by America's Founding Fathers. He maintained that hemp possessed a near limitless potential for replacing petrochemical and timber products and phasing out environmentally destructive industries. Herer's boisterous marijuana evangelism broadened the scope of the drug policy reform movement and set the stage for the reefer resurgence of the 1990s. "It's the safest, smartest, best medicine on the planet. You'd be stupid not to use it!" he declared.

  • More than any other factor, it was the AIDS epidemic that made medical marijuana an urgent, cutting-edge issue in America. Mary Jane Rathbun baked as many as 15,000 marijuana brownies a month and distributed them free of charge to HIV-infected patients in the AIDS ward at San Francisco General Hospital. Sickened gays found that cannabis, an appetite stimulant, was the most effective and least toxic treatment for HIV-associated anorexia and weight loss. Without marijuana many AIDS patients would not have been able to tolerate the harsh side effects of potent, life-saving protease-inhibitor drugs when they became available in the late 1980s. Brownie Mary's repeated run-ins with law enforcement generated national media coverage of San Francisco's burgeoning medical marijuana underground.

  • Dennis Peron, California's most vocal, effective, and controversial marijuana activist, standing outside the entrance to the San Francisco Cannabis Buyers' Club, at 1444 Market Street near the Civic Center. Founded by Peron with the tacit approval of City Hall, the Cannabis Buyers' Club openly defied federal law by providing marijuana to thousands of people with AIDS and other ailing Bay Area residents. More than anyone else, Peron was responsible for catalyzing the grassroots social movement that culminated in the passage of Proposition 215, California's landmark ballot measure that legalized cannabis for therapeutic purposes in 1996. More than a dozen other states would soon break ranks from America's drug war juggernaut and legalize medical marijuana.

  • Although federal officials maintain that marijuana is properly classified as a Schedule I substance of abuse with no therapeutic value, the federal government awarded the U.S. Department of Health and Human Services a patent titled "Cannabinoids as Antioxidants and Neuroprotectants" in October 2003. The patent was based on research sponsored by the National Institute of Mental Health, which found that THC and cannabidiol (CBD), two biologically active compounds unique to the marijuana plant, protect the brain against neurological insults, inflammation, and oxidative stress. Subsequent research by investigators at the Scripps Research Institute in La Jolla, California, showed that THC inhibits an enzyme responsible for the accumulation of amyloid plaque that disrupts communication between brain cells, the hallmark of Alzheimer's-related dementia.

  • Oaksterdam University, a trade school for the cannabis industry in downtown Oakland, was raided by federal agents on April 2, 2012, as part of a stepped-up campaign to roll back the gains that medical-marijuana supporters thought they had achieved through democratic means with the passage of Proposition 215. The assault on California's 1996 Compassionate Use Act would turn courtrooms into stages for show trials, send the innocent to prison, deny medication to the seriously ill, threaten doctors, and terrorize America's weakest citizens with fascistic paramilitary raids that ransacked their homes and property. Instead of abiding by California law, state and local cops conspired with federal authorities to undermine the will of the electorate.

  • The Seattle Hempfest: On the third weekend of August, several hundred thousand people converge at Myrtle Edwards Park, a mile-long strip of greenery and winding paths along the waterfront, to celebrate their favorite plant. Revelry and civic-mindedness mix as ganja lovers from all corners join a throng of locals in what is by far the biggest pro-pot gathering in the world. In keeping with Seattle's reputation as a progressive, cannabis-friendly bastion, the police refrain from arresting marijuana smokers. "Hempfest is about promoting the freedom to choose and human rights," chief organizer Vivian McPeak asserted. "No political or human rights movement in America has made it this far without eventually winning. It's just a matter of time."