Jayne O'Donnell, Ken Alltucker and Shari Rudavsky | USA TODAY

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In less than 25 years, marijuana has gone from illegal everywhere in the United States to legal for at least some uses in all but four states.

Advocates say the drug can help patients who are suffering from chronic pain, multiple sclerosis-triggered muscle spasms and the grueling side effects of chemotherapy. Some states are exploring whether cannabis could help wean people from addiction to opioids.

Beyond the medical claims, 10 states and the District of Columbia have legalized marijuana for recreational use, and more are considering it. The advocates' long-repeated argument: It's safer than alcohol or tobacco.

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But as cultural acceptance of cannabis grows, opponents are warning of potential downsides.

These critics – doctors, police and auto safety officials, parents – point to stories and studies that link the drug to suicide, schizophrenia and car crashes.

Marijuana might be safer than alcohol or tobacco, they say. But that doesn't make marijuana safe.

An increase in impaired driving by people under the influence of drugs including marijuana, for example, is threatening the huge progress made in recent decades to reduce drunk driving crashes.

Car crashes rose 6 percent from 2012 to 2017 in four states that legalized marijuana during that period – Nevada, Colorado, Washington and Oregon – a greater rate than in four comparable states that didn't, the Highway Loss Data Institute found.

Matt Moore is senior vice president at the institute, which is funded by the insurance industry.

"It makes me very nervous about highway safety as many more are considering legalizing it for recreational use," he says.

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Some in medicine warn of possible links between marijuana and psychosis. They say more study is needed.

Bucknell University neuroscientist Judy Grisel, author of "Never Enough," a new book on addiction, warns that the laws have outpaced the science.

"It’s astounding how short our memory is," she says. "We always think the next thing is the answer."

But others say worries about marijuana are mostly overblown.

Sue Sisley, a Phoenix, Arizona, doctor who has studied cannabis, says the reason that states are loosening marijuana laws is that lawmakers and the public realize the plant is largely safe.

Sisley recently studied veterans who smoked marijuana to treat post-traumatic stress disorder under a $2.1 million grant from the State of Colorado. She called the drug "relatively benign overall."

"The cannabis plant is far safer than prescriptions I write for patients every day here in my clinic," Sisley says.

Jolene Forman is a senior staff attorney at the Drug Policy Alliance. The group advocates for the legalization and regulation of marijuana.

Forman says says there's no convincing evidence that marijuana legalization worsens mental health outcomes or increase vehicle crashes.

A better measure of traffic safety, she says, is whether more people are arrested for driving under the influence of marijuana.

"We are seeing DUI rates stay steady or even decline in states that have legalized," Forman said. "Moreover, marijuana use is not new. It was widely accessible and socially acceptable in these states before marijuana was legalized there."

California became the first state to authorize the medical use of marijuana in 1996. States have been easing their own laws ever since.

Now all but four – Idaho, Kansas, Nebraska and South Dakota – allow marijuana for medical or recreational use, or permit public use of low-THC cannabidiol formulations, according to the National Conference of State Legislatures.

Several Democratic presidential candidates have said they would seek legalization.

Sen. Cory Booker, D-New Jersey, last week introduced the Marijuana Justice Act, which would legalize marijuana nationwide and expunge convictions of people who have been convicted of marijuana offenses. Sens. Kamala Harris, D- California, Elizabeth Warren, D-Massachusetts, Kirsten Gillibrand, D-New York and Bernie Sanders, I-Vermont, are co-sponsors.

Emily Paine, Bucknell University

Addiction psychiatrist Dr. Andrew Saxon opposes legalizing marijuana for medical use because he says evidence that it works is scant.

Saxon, a professor in the University of Washington department of psychiatry and behavioral sciences, chairs the American Psychiatric Association's Council on Addiction Psychiatry.

"I just don’t think it’s a good idea," he says. "It's not like any other medication I might prescribe that I can tell you exactly how much to take, how to take it and decrease the dosage and increase.

Saxon does support legalization for recreational use, but "not because I think it’s a good thing."

People are going to use it whether it's legal or not, he says. So it's better to have it regulated and for "some of the profits to go back to the states."

Some states have turned to marijuana in hopes of finding an answer to the opioid crisis. New York, New Jersey, Pennsylvania and Illinois allow marijuana as a substitute for addictive painkillers such as OxyContin, or as a way for people to wean themselves from opioid-use-disorder.

But some believe that's another example of state laws outpacing science.

"The evidence is just not there yet," says Ziva Cooper, research director of the UCLA Cannabis Research Initiative. "What we have are anecdotal reports."

Scientists and doctors are also concerned about the potency of modern-day cannabis products.

In the 1990s, Cooper says, marijuana plants typically contained about 3 percent THC, the psychoactive component that makes users feel "high."

Today, Cooper says, marijuana sold at medical marijuana dispensaries can be as much as 25 percent THC. Other products, such as wax, vaping pens and dabs of concentrated marijuana, can be packed with still higher levels.

These stronger formulations could get more young people hooked.

"The concentrations of THC are certainly increasing in the varieties of cannabis that are available," Cooper said. "It's hypothesized that the higher strength will engender greater risk for developing cannabis-use disorder."

Cannabis links to schizophrenia

The Colorado state committee that oversees legalization reported in February that for the first time, more adults of all ages are using marijuana, using it more often, and ingesting it in more ways, including edibles.

Eating food that contains marijuana gives users a far stronger dose of THC.

The National Academies of Sciences, Engineering, and Medicine concluded in 2017 that schizophrenia and other psychoses are correlated with, but not necessarily caused by, heavy marijuana use.

Cooper, who was part of the National Academy research team, said the comprehensive review of existing studies concluded frequent users face more risk. But she said it's "premature" to say heavy use causes schizophrenia and psychosis.

"We don’t know what’s coming first," Cooper said. "Is it that people who are heavy users are more likely to develop schizophrenia and psychotic disorders? Or is it that heavy users might be quote-unquote self medicating?"

Further underscoring the complicated relationship between cannabis use and mental health, Cooper says, is evidence that cannabis users with a history of psychotic disorders had better cognitive performance.

"There is an association there that shows cannabis might be protective to some degree," she said.

Craig Geller

Alex Berenson, author of the new book "Tell Your Children: The Truth About Marijuana, Mental Illness and Violence," believes there's plenty of evidence that today's high-strength cannabis is causing psychosis.

"People are not well aware of the strength of the science," he says.

And he believes the evidence has only gotten stronger since he finished his book.

"The stories are now so much worse than that kids failed out of school or went on to use other drugs," says Berenson. "A lot of suburban families who never thought this would be a problem" are starting to share their stories.

"That’s what turned the opioid crisis," Berenson said.

Berenson met last month with Florida state legislators considering legislation. He says interest groups and legislators in about a dozen other states have asked him to speak.

Maria McFarland Sánchez-Moreno, executive director of the Drug Policy Alliance, said Berenson's claim that marijuana “frequently causes” psychosis is "over the top." Most scientists, she says avoid saying marijuana causes psychosis.

"The fact is that the overwhelming majority of people who use marijuana do not develop a mental disorder as the simple result of using it," she said in a recent online discussion hosted by the Marshall Project.

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Lori Robinson is also speaking out. She started the group Moms Strong in 2016. Her son, Shane, died by suicide during marijuana withdrawal in 2012.

Shane Robinson was 25. He had been hospitalized twice with psychosis.

Lori Robinson says doctors denied his mental illness could be linked to his heavy cannabis use.

"This is all about money," she says. "And unfortunately, our kids are the ones who have been sold down the river." Some swear by the benefits of marijuana in alleviating pain. Studies confirm that it can reduce chronic pain, muscle spasms caused by multiple sclerosis and the nausea and vomiting from chemotherapy.

'Parents should have that opportunity to make the decision here'

Just six months ago, Heidi and Dave Curtis would never have advocated for the legalization of medical marijuana.

But after seeing how it helped their youngest daughter, the Indiana couple says, they want their voices added to the chorus calling for legalization of medical marijuana in their state.

That’s not likely to happen this year. The Indiana General Assembly has not advanced any of the handful of bills on marijuana proposed this session.

And even if a bill were to pass, Charly would not benefit. The six-year-old, who had a rare genetic condition that causes severe autism and seizures, died in her sleep last month.

As the seizures grew more frequent and more severe, her parents began to give her small doses of THC.

The day Charly first took a peanut-sized piece of a brownie with THC, they say, she had no seizure. For the first time in her life, they say, she went to her room and sat quietly flipping through books.

View | 20 Photos

Medical marijuana finds supporters in parents who lost their daughter

The Curtises continued to give Charly THC drops twice a day. But the decision was not easy. Heidi had always been against drugs for her other children, and she feared the consequences of doing what she knew was illegal.

Now that Charly is gone, her family wonders what might have happened if they given her THC earlier. In the short time she was on the drug, they say, she had fewer seizures and her behavior improved dramatically.

All Heidi Curtis wants now, she says, is the chance for other parents in similar situations to be able to use medical marijuana to help their children.

“This is not a be-all and end-all miracle drug," she says. "We as parents should have that opportunity to make the decision here.”

Medical experts do not endorse the use of THC to control seizures for patients of any age.

Dr. Anup Patel is section chief of neurology at Nationwide Children’s Hospital in Columbus, Ohio, and medical cannabis editor for the Epilepsy Foundation of America.

Based on limited animal data and case reports, he says, THC is thought to have a neutral effect on seizures or, if anything, promotes the episodes.

CBD, which is also derived from cannabis, has been thoroughly researched. CBD does not produce the same high as THC because it binds to a different part of the brain.

The Food and Drug Administration last year approved Epidiolex, the first drug with purified CDB oil, to treat seizures.

Scientists performed rigorous studies on the drug before its approval, says Patel, who helped conduct the trials. They found that about 40 percent of patients saw at least a 50 percent reduction in seizures.

'I couldn't wait to smoke'

Some teacher and parent groups say the risks of legalizing marijuana for recreational use are too high.

Marijuana use by high school students has remained flat for several years. Fewer students use marijuana than drink alcohol or vape nicotine.

But there have been significant increases in methods that deliver greater doses of THC, and considerable research that it leads to cannabis use disorder for young people.

Adolescent marijuana use can also affect intelligence, according to at least one study.

Researchers at Duke University gave subjects IQ tests at age 13, before any of them had smoked marijuana, and again at age 38. They reported in 2012 that those who started using cannabis in adolescence and continued for years afterward showed an average decline in IQ of 8 points.

Quitting cannabis apparently didn't reverse the loss.

The researchers who conducted the more comprehensive review by the National Academy found that learning, memory and attention are impaired immediately after cannabis use. But they concluded that the evidence that use impairs academic achievement is limited.

They also found that brain and cognitive development is critical during adolescence. It is also during those crucial years that people are more likely to experiment using cannabis and other drugs.

After David Childs turned 18, a child psychiatrist warned him that if he continued smoking marijuana, he would develop irreversible schizophrenia.

Childs told his father the psychiatrist was "an idiot," and continued smoking regularly.

David Childs' pot smoking increased to help "handle the pain" after he got kicked out of private school in 9th grade, his father says. Once he started working, James Childs says, he had more money to buy more cannabis – which led to notable behavior changes.

James Childs asked USA TODAY not to publish the name of their hometown.

The day after Thanksgiving last year, he says, his son stabbed his right palm with his pocket knife. His family had him admitted to a mental health facility, which discharged him into an outpatient program with the antidepressant Prozac, the father says.

Four days later, James Childs says, his son smoked some weed and died by suicide.

Khadejah Cox, Urban Health Media Project

Ruth Pointer, the last original member of the Pointer Sisters, was celebrating 35 years "clean and sober" in 2017 when she decided to try medical marijuana for her arthritis.

"There was a lot of talk about the benefits," she says. "I quickly grabbed onto that. It was legal!"

The singer, whose hits included "I'm so Excited," says she was overly excited on weed: "It made me anxious and crazy and paranoid." She began drinking wine again, which she used to do with cocaine, which was her primary addiction.

She stopped smoking and drinking in November.

"Now, I like the way I feel," she says.

Whatever people want a drug to do, Grisel says, the brain often produces the opposite effect. Teenagers who became psychotic after smoking marijuana, for example, tell their parents that the drug was the only thing that would make them feel better.

That kind of thinking is familiar to Pointer.

"I used the excuse that it was for my arthritis, but it didn't do a whole lot for that," she says. "I know where it took me and it took my mind back into addictive behavior.

"I couldn't wait to smoke."