Liz Szabo, USA Today

Doctors say they're increasingly fielding questions about the safety of marijuana, as use of the drug rises and more communities consider legalizing it. Colorado and Washington state have legalized recreational marijuana, and medical use is allowed in 21 states and Washington, D.C.

USA TODAY's Liz Szabo talked to experts about what scientists know and don't know about marijuana's risks and benefits.

Q. How common is marijuana use?
A. About 12% of people of Americans over age 12 have used it in the past year, according to the Substance Abuse and Mental Health Services Administration.

Use of marijuana among high school students has been increasing since the 1990s. If current trends continue, marijuana use among high school seniors could soon become more common than cigarette smoking, says Nora Volkow, director of the National Institute on Drug Abuse.

Q. Is marijuana addictive?
A. Sometimes. About 9% of those who experiment with marijuana overall — and nearly 17% of those who use it as teenagers — will become addicted, according to the definition of addiction used in the Diagnostic and Statistical Manual of Mental Disorders, Volkow says. Up to half of people who smoke marijuana daily become addicted.

According to the 2012 National Survey on Drug Use and Health, 2.7 million people over age 12 meet criteria for addiction to marijuana.

Q. What are the potential medical uses of marijuana?
A. Smoking marijuana may stimulate appetite, especially in patients with AIDS, according to an Institute of Medicine report.
Smoking pot also may alleviate the nausea and vomiting caused by chemotherapy treatments for cancer. It also may relieve severe pain and spasticity, when muscles become overly tight and rigid. Marijuana also could potentially help treat glaucoma, by decreasing pressure in the eye.

But marijuana also may make the cognitive problems associated with HIV even worse, according to a 2004 study.
"If an individual comes in with severe pain and I haven't been able to manage it with any other means, I am willing to consider it, but with a lot of precaution," says Steven Wright, a pain and addiction medicine specialist in Denver. "But unless you are really severely affected by pain, it's not in your favor in the long term."

Q. Is medical marijuana approved for use by the Food and Drug Administration?
A. No. But the FDA has approved two drugs made with cannabinoids – active ingredients in marijuana – called dronabinol and nabilone. Both are approved to treat chemotherapy-related nausea and vomiting in patients who aren't helped by other therapies.

The FDA is reviewing an unapproved drug, called Sativex, to treat multiple sclerosis. It's already approved in Canada, the United Kingdom and other countries. And researchers are testing an experimental drug called Epidiolex to treat childhood epilepsy.

Q. How have the risks of marijuana changed in recent years?
A. Marijuana products today are far stronger than in the past, which may explain why more people are overdosing or getting into car accidents, Volkow says.

The concentration of the main active ingredient in marijuana – tetrahydrocannabinol, or THC – has quadrupled since the 1980s, increasing to about 12% today, according to a June article by Volkow in The New England Journal of Medicine.
Some of the new, edible marijuana products – from cookies to chocolates – can be 10 times stronger than traditional joints, Wright says.

Because smoked marijuana affects people so quickly — reaching the brain in only 7 seconds – users often feel satisfied after a few drags, so that they don't actually smoke very much, Wright says.

But because a cookie's full effects may not set in for an hour or more, people may end up consuming far more THC than they intended to, Wright says.

Q. What are the risks to children?
A. Emergency room doctors are treating more small children for accidental overdoses of marijuana, says Kathryn Wells, president of the Colorado chapter of the American Academy of Pediatrics. Doctors at Children's Hospital Colorado in Aurora have treated 12 children for marijuana overdoses just since January, when recreational use became legal in Colorado. Doctors treated eight children in all of 2013.

Of those treated this year, seven needed intensive care, says hospital spokeswoman Elizabeth Whitehead.
Children also may be exposed when their mothers use pot during pregnancy or breastfeeding, Wells says. She says a number of women now tell her that they're trying marijuana for morning sickness or other uses while pregnant. Other parents bring their children to the doctor, reeking of marijuana smoke. Wells says parents tell her, "it's legal, so there's nothing wrong with it."

Q. Why are young people more vulnerable to the risks of smoking marijuana?
A. Their brains are still developing, Volkow says.

The brain isn't fully mature until about age 25, Wright says.

That may help explain why people who use pot frequently, beginning as teens, often have significant declines in their IQ as adults, Volkow wrote in a June article in The New England Journal of Medicine. Marijuana impairs critical thinking skills for days after people sober up. That means that teens who use marijuana on weekends may not be able to learn properly when they return to school. Like alcohol, marijuana impairs judgment, so that teens may take more risks – from speeding to sex – that can endanger their lives.

Q. Is marijuana a "gateway drug" that leads to use of more dangerous substances?
A. It's unclear. Studies show that people who use marijuana are more likely to later abuse other drugs. But scientists aren't sure of the reason. It's possible that marijuana, like alcohol and nicotine, "prime the brain for a heightened response to other drugs," Volkow wrote. But it's also possible that people who are susceptible to drug abuse simply start with marijuana because it's more accessible.

Q. How is marijuana related to mental illness?
A. Marijuana increases the risk of psychosis, in which people lose touch with reality and may experience delusions, hallucinations and paranoia, Volkow says. Marijuana is also associated with chronic psychotic disorders, such as schizophrenia, in people who are genetically susceptible. Heavy marijuana use can lead these people to experience a psychotic episode two to six years earlier than otherwise.

Colorado police have reported two deaths this year related to psychosis-like episodes in pot users. In March, a 19-year-old African exchange student jumped off a hotel balcony after eating a marijuana cookie. The next month, a Denver man who had purchased marijuana, including an edible form called "Karma Kandy," began hallucinating and fatally shot his wife.

Q. How is marijuana related to car accidents?
A. Marijuana doubles the risk of a car accident when people try to drive soon after using it. Marijuana causes more car accidents than any other illicit drug, a 2013 study shows. In comparison, being legally drunk — with a blood alcohol level of 0.08% — increases the risk of an accident by five times.

In a study released earlier this year, Columbia University researchers found that marijuana contributed to 12% of traffic deaths in 2010, triple the rate from a decade earlier.

Q. Does marijuana cause cancer?
A. Although some studies have linked heavy marijuana smoking to lung cancer, the link isn't totally clear. But marijuana is associated with a variety of lung problems, including inflammation of the airways, symptoms of chronic bronchitis and an increased risk of pneumonia and respiratory infections, according to the New England Journal review.

Q. Does marijuana cause heart disease?
A. People who smoke marijuana have changes in their blood vessels that increase the risk of heart attack and stroke, the New England Journal review shows.

Q. What don't researchers know about marijuana?
A. Most research is based on heavy use of marijuana and daily smoking, Volkow says. Scientists know much less about the effects of using marijuana once a week or once a month.

Older studies also may not be as relevant to the more potent marijuana products sold today, Volkow says. Doctors at the National Institute of Health are prioritizing research in young people, and hope to better understand the effects of marijuana in real-world situations, in which teens often combine it with alcohol. It's possible that some of the side effects of marijuana relate to impurities found in illegal products, says Jahan Marcu, a senior scientist at Americans for Safe Access, a medical marijuana advocacy group. As marijuana becomes better regulated, it also may become safer.