Voters in nine states will have the chance this year to decide whether they want to legalize the recreational or medical use of marijuana.

Marijuana is still classified as a Schedule I drug by the Drug Enforcement Administration (DEA), meaning the national agency views it as a substance with high potential for abuse and psychological or physical dependence. As such, cannabis is illegal under federal law.

But that hasn't stopped states from legalizing marijuana within their borders. In 2013, the Department of Justice issued a memo in response to the increasing state-level trend toward legalization stating that the federal government reserves the right to police marijuana when state legalization interferes with the government's priorities, such as keeping pot out of the hands of minors or preventing the drug from crossing state lines. However, the memo made clear that if states strictly regulated their marijuana markets to prevent problems, the DOJ would not step in.

But what exactly does a vote for, or against, the legalization of marijuana mean? And what impact might this legislation have? Here is an at-a-glance guide at the states voting and the issues on the table.

Which states are voting on marijuana?

Five states are voting on whether to legalize marijuana for recreational use by adults. They are:

Arizona: Proposition 205 would allow adults 21 or older to possess up to an ounce of marijuana and grow up to six marijuana plants for personal use. The proposition would create a regulatory body to oversee commercial sellers and allow marijuana retail stores capped at 10 percent the number of liquor-store licenses in the state. A 15 percent excise tax on marijuana sales would fund enforcement of regulations, and additional tax revenue would go to school facilities, kindergarten funding and anti-drug education. Marijuana could not be consumed in public.

California: Proposition 64 would legalize limited marijuana possession and consumption for adults 21 or older and allow personal cultivation of up to six plants. The proposition would establish a Bureau of Marijuana Control to regulate the commercial marketplace for pot. Tax revenue from marijuana sales would go toward funding this bureau. Any remaining funds would be used for public health campaigns aimed at preventing youth drug use, environmental remediation of public lands used for illegal marijuana cultivation, and training of law officers to enforce the new law. Public consumption would remain illegal.

Maine: Question 1 would legalize up to 2.5 ounces of marijuana for adults ages 21 or over and allow for personal cultivation of up to six flowering plants. Public consumption would remain illegal. A 10 percent tax on sales would fund the regulation and enforcement of the commercial market, with leftover funds returned to the state legislature to use in the general budget.

Massachusetts: Question 4 would allow the possession of up to an ounce of marijuana by adults ages 21 or over and the cultivation of up to six plants for personal use. Public consumption would remain illegal. A 3.75 percent excise tax would be added to the 6.25 percent Massachusetts sales tax, and cities could choose to add up to an additional 2 percent tax on marijuana sales in their jurisdiction. Tax revenue would go into a Marijuana Regulation Fund for implementation and enforcement of the new law, and any leftover funds would go into the state's general coffers at the end of the fiscal year.

Nevada: Question 2 would legalize the possession of up to 1 ounce of marijuana or one-eighth of an ounce of marijuana concentrate for adults 21 or older. Cultivation of up to six plants would be allowed for people who do not live within 25 miles of a retail marijuana store. A 15 percent excise tax would fund enforcement and regulation of the market, with leftover money to go to public K-12 education. As in all the states voting this year, public consumption would remain illegal.

Four states are voting on medical marijuana initiatives in 2016. They are:

Arkansas: Issue 6 and Issue 7 deal with the establishment of a medical marijuana system in the state. Issue 6 would make medical marijuana legal for certain medical conditions but require local city and county votes to approve dispensaries and cultivation facilities. Issue 7 would provide a framework for this market, including 39 nonprofit medical cannabis dispensaries and limited home cultivation for people who qualify for medical marijuana but live more than 20 miles from a dispensing center.

Florida: Amendment 2 would legalize medical marijuana for specific medical conditions. The state currently allows some patients to access non-smokable marijuana low in tetrahydrocannabinol (THC), the drug's psychoactive ingredient. Amendment 2 would expand this limited system and task the state's Department of Health with overseeing the medical marijuana market.

Montana: Issue 182 likewise expands a currently limited medical marijuana system. The state approved a medical marijuana program in 2004, but state legislators repealed that initiative in favor of a program that allows a physician to prescribe medical marijuana to no more than three patients at a time. Issue 182 would reverse this limitation as well as other limitations of the current law.

North Dakota: Measure 5 would legalize medical marijuana and establish a regulatory system under the Department of Health for regulating and monitoring the market. Patients could receive up to 3 ounces of marijuana, and those who live more than 40 miles from a dispensary would be allowed to cultivate a limited number of plants.

Which states have already legalized marijuana?

Any states that legalize recreational marijuana this year would join Alaska, Washington, Oregon, Colorado and Washington, D.C., in allowing adults to use the drug.

The legal use of marijuana for medical reasons is more widespread, with the following states allowing it: Alaska, Arizona, California, Colorado, Connecticut, Delaware, Hawaii, Illinois, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Montana, Nebraska, New Hampshire, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Washington and West Virginia.

Arguments for marijuana legalization

Keeping marijuana illegal is costly: Proponents for the legalization of marijuana generally argue that prohibition of the drug is more harmful than a legal, regulated marketplace. The advocacy organization NORML cites the high number of Americans who have been arrested for marijuana-related offenses since 1985 as evidence that criminalization harms citizens while doing little to reduce the availability of marijuana. According to the American Civil Liberties Union, 52 percent of all drug arrests nationwide in 2010 were for marijuana; between 2001 and 2010, 8.2 million people were arrested on marijuana-related charges, with 88 percent of those arrests for simple possession (a legal term that means having a small amount of marijuana). The ACLU also found that blacks and whites use marijuana with equal frequency, but blacks are nearly four times more likely than whites to be arrested on marijuana charges.

Legalization doesn't start a wave a teenage use: Advocates also argue that legalization does not make teenagers more likely to use marijuana, which is often given as a reason against legalization. One study that looked at this question found that teenage use of marijuana in Colorado did not change significantly before and after the state legalized the drug. In 2009, the study found, 43 percent of teens said they'd tried marijuana at least once, and 25 percent said they'd used it in the past 30 days. In 2015, after legalization, 38 percent of teens said they'd ever tried marijuana and 21 percent said they had used it within the last 30 days. On average, 21.2 percent of Colorado teens said they were current pot users, compared with 21.7 percent nationally. There has been a shift in Colorado teens toward seeing marijuana use as less risky, from 54 percent who saw it as risky in 2013 to 28 percent in 2015, the researchers noted.

"Just from a logical standpoint, right now marijuana is very easy for kids to get because it's not regulated," said Morgan Fox, a spokesperson for the Marijuana Policy Project, an advocacy organization that promotes marijuana legalization. People who sell Illegal drugs are incentivized not only to sell to kids, Fox said, but to recruit them to sell marijuana, too. In contrast, regulated marijuana retailers are at risk of losing their license and livelihood if they fail to check IDs.

Arguments against marijuana legalization

The drug may affect people's brains: Opponents of marijuana legalization argue that marijuana is, in fact, dangerous. Opponents commonly cite a 1996 study from the Journal of the American Medical Association (JAMA) that compared 65 college students who smoked marijuana nearly every day to 64 who smoked only a few days a month and those who did not smoke at all. Heavy users were more impaired than light users a day after smoking on tasks that required attention and executive function. The finding persisted even after controlling for estimated levels of pre-pot cognitive functioning, the researchers reported, though it was not clear whether the impairment was due to residual effects of marijuana, withdrawal from the drug or neurotoxicity.

Driving while high: Opponents also cite the dangers of driving while high as reason not to legalize marijuana. According to the National Institute on Drug Abuse, marijuana intoxication does impair driving ability, and marijuana is the substance most frequently found in the blood of drivers who have had accidents. It is not always clear what role marijuana plays in accidents, in part because users often mix it with alcohol. However, the combination of the two appears riskier than either substance alone.

What medical groups say

The American Association of Pediatrics opposes legalization of marijuana for both recreational or medical use, but supports the decriminalization of marijuana to reduce the penalties for marijuana offenses.

"We know marijuana can be very harmful to adolescent health and development," Dr. Seth D. Ammerman, a member of the AAP Committee on Substance Abuse and an author of organization's 2015 policy statement, said in a news release. "Making it more available to adults – even if restrictions are in place – will increase the access for teens. Just the campaigns to legalize marijuana can have the effect of persuading adolescents that marijuana is not dangerous, which can have a devastating impact on their lifelong health and development."

The American Medical Association reaffirmed its opposition to marijuana legalization in 2013, but amended its policy statement to emphasize "public-health-based strategies" instead of punitive approaches like incarceration for reducing use. The organization also reaffirmed its opposition to legalizing marijuana for medical use in 2013, but called for further study of marijuana and cannabinoid compounds as pharmaceuticals.

Some influential physicians do support marijuana legalization. The California Medical Association announced support for the legalization of marijuana in 2011, in part to improve access for physicians who wanted to research the medicinal effects of the drug. (As a Schedule I drug, cannabis is available only on a limited basis for research purposes.) In addition, the group Doctors for Cannabis Regulation formed this year, breaking with official AMA policy. Its founders include former U.S. Surgeon General Dr. Joycelyn Elders and Dr. H. Westley Clark, past director of the Center for Substance Abuse Treatment at the Substance Abuse and Mental Health Services Administration.

What do we really know about marijuana's health effects?

Research on the health effects of marijuana makes clear why proponents and opponents of legalization often seem to be talking about a different drug. Individual studies differ on fundamental results, like whether heavy marijuana use shrinks the teen brain, meaning that it's easy for both sides to cherry-pick results.

For example: A 2014 study in the journal Proceedings of the National Academy of Sciences concluded that the drug reduced volumes of gray matter in the orbitofrontal cortex, but a 2015 study that compared siblings and twins who used pot with those who didn't use the drug and found that the brain effects seemed more linked to pre-existing conditions than to the pot itself. Heavy pot users had smaller volumes than nonusers in the amygdala, a brain region associated with fear and reward, and the right ventral striatum, another reward-related region. But their siblings, who shared their genes and environment, also showed smaller volumes in those regions, researchers reported in the journal JAMA Psychiatry. Other research suggests that marijuana interacts with different brains, well — differently. And it's hard to know who is at risk.

"Because of diversity in genotype and environment, one person's sugar may be another's poison," Dr. David Goldman, a neurogeneticist, wrote in an editorial accompanying the 2015 study.

Very few high-quality studies have been done to examine the effect of marijuana on intelligence. The drug clearly affects memory, learning and attention in the short term, but long-term studies have returned conflicting results. A 2012 study published in the journal Proceedings of the National Academy of Sciences followed 1,037 New Zealanders from birth to age 38 and found that the participants' IQs dropped, on average, about 6 points after they started using marijuana. Another study, published in 2016, followed marijuana users and nonusers into middle age and found that the only effect of marijuana use over time was worsened verbal memory. Meanwhile, another study in 2016 compared twin users and nonusers and found, like the 2015 sibling study, that intelligence declines were explained by shared environment and genes, not pot use.

Regular marijuana smoking damages lung tissue and can cause chronic bronchitis, according to a 2013 review in the Annals of the American Thoracic Society. Heavy use may increase cancer risk or other lung diseases, but the risks appear less serious than those caused by tobacco smoking.

Cannabis seems to suppress the immune system in mice, according to a 2010 study.

More information on marijuana:

Statistics, trends and latest research at the National Institute on Drug Abuse

Marijuana Fact Sheet at the Drug Enforcement Administration.

Medical Marijuana page at the National Center for Complementary and Integrative Health.

Legalities and policy at the Office of National Drug Control Policy

LiveScience's Election 2016 Coverage

Election Day 2016: A Guide to the When, Why, What and How

Democratic Party Platform: We Fact-Checked the Science

Republican Party Platform: We Fact-Checked the Science

Editor's note: This article was updated on Nov. 9 to include Alaska among the states where the recreational use of marijuana is already legal.

Original article on Live Science.