There appears to be a shift in the United States in favor of relaxing marijuana laws, a topic that has dipped in and out of the national conversation for decades. Public perceptions about pot have come a long way, from the dire warnings of "Reefer Madness" to growing acceptance of medical marijuana. 
Members of a crowd numbering tens of thousands smoke and listen to live music at the Denver 420 Rally on April 20, 2013. Annual festivals celebrating marijuana are held around the world on April 20, a counterculture holiday. 
A man smokes a joint during the official opening night of Club 64, a marijuana social club in Denver, on New Year's Eve 2012. Voters in Colorado and Washington state passed referendums to legalize recreational marijuana on November 6, 2012. 
People light up near the Space Needle in Seattle after the law legalizing the recreational use of marijuana went into effect in Washington on December 6, 2012. 
Nutrient products are placed on shelves in the weGrow marijuana cultivation supply store during its grand opening on March 30, 2012, in Washington, D.C. The store is a one-stop-shop for supplies and training to grow plants indoors, except for the actual marijuana plants or seeds. Legislation was enacted in 2010 authorizing the establishment of regulated medical marijuana dispensaries in the nation's capital. 
Marijuana activist Steve DeAngelo wears a "Yes on Prop 19" button as he speaks during a news conference in Oakland, California, on October 12, 2010, to bring attention to the state measure to legalize marijuana for recreational purposes in California. Voters rejected the proposal. 
Sonja Gibbins walks through her growing warehouse in Fort Collins, Colorado, on April 19, 2010. Since the state approved medical marijuana in 2000, Colorado has seen a boom in marijuana dispensaries, trade shows and related businesses. So far 20 states and the District of Columbia have made smoking marijuana for medical purposes legal. 
A patient prepares to smoke at home in Portland, Maine, on October 22, 2009, a decade after the state approved a medical marijuana referendum. 
Coffeeshop Blue Sky worker Jon Sarro, left, shows a customer different strains of medical marijuana on July 22, 2009, in Oakland, California. Voters in the city approved a measure during a vote-by-mail special election for a new tax on sales of medicinal marijuana at cannabis dispensaries. 
Medicinal marijuana patient Angel Raich wipes her eyes during a press conference on March 14, 2007, in Oakland, California. The 9th circuit U.S. Court of Appeals in San Francisco ruled that 41-year-old Raich, who used medicinal marijuana to curb pain from a brain tumor as well as other ailments, did not have the legal right to claim medical necessity to avoid the possibility of prosecution under federal drug laws. 
Different varieties of medical marijuana are seen at the Alternative Herbal Health Services cannabis dispensary in San Francisco on April 24, 2006. The Food and Drug Administration issued a controversial statement a week earlier rejecting the use of medical marijuana, declaring that there is no scientific evidence supporting use of the drug for medical treatment. 
People in New York gather for a pro-cannabis rally on May 4, 2002. That same day, almost 200 similar events took place around the world to advocate for marijuana legalization. It was dubbed the "Million Marijuana March." 
Dennis Peron takes notes during a phone interview while Gary Johnson lights up at the Proposition 215 headquarters in San Francisco on October 11, 1996. The ballot measure was approved when voters went to the polls in November, allowing medical marijuana in California. 
A television ad aired in 1996 by Republican presidential candidate Bob Dole's campaign included footage from a 1992 MTV interview of a laughing President Clinton saying he would inhale marijuana if given the chance to relive his college days. 
President George H. Bush holds up a copy of the National Drug Control Strategy during a meeting in the Oval Office on September 5, 1989. In a televised address to the nation, Bush asked Americans to join the war on drugs. 
Robert Randall smokes marijuana that was prescribed to treat his glaucoma in 1988. He became the first legal medical marijuana patient in modern America after winning a landmark case in 1976. 
First lady Nancy Reagan participates in a drug education class at Island Park Elementary School on Mercer Island, Washington, on February 14, 1984. She later recalled, "A little girl raised her hand and said, 'Mrs. Reagan, what do you do if somebody offers you drugs?' And I said, 'Well, you just say no.' And there it was born." She became known for her involvement in the "Just Say No" campaign. 
President Jimmy Carter, with his special assistant for health issues, Dr. Peter Bourne, beside him, talks to reporters at the White House about his drug abuse control message to Congress on August 2, 1977. Among other things, he called for the elimination of all federal criminal penalties for the possession of up to one ounce of marijuana. 
Panel members of the National Commission on Marijuana and Drug Abuse attend a hearing In Denver on January 10, 1972. From left, Dr. J. Thomas Ungerleider, psychiatrist; Michael R. Sonnenreich, commission executive director; Raymond P. Shafer, commission chairman; Mitchell Ware, Chicago attorney; Charles O. Galvin, Dallas law school dean. The commission's findings favored ending marijuana prohibition and adopting other methods to discourage use, but the Nixon administration refused to implement its recommendations. 
Protesters wade in the Reflecting Pool at the National Mall in Washington during the "Honor America Day Smoke-In" thrown by marijuana activists in response to the official "Honor America Day" rally organized by President Nixon supporters at the Lincoln Memorial on July 4, 1970. 
Marijuana reform was the Life magazine cover story in October 1969. The banner read: "At least 12 million Americans have now tried it. Are penalties too severe? Should it be legalized?" 
Police dogs trained to smell out hidden marijuana examine U.S. soldiers' luggage at the airport during the Vietnam War in 1969. Drug use was widespread during the war. 
People share a joint during a 1969 concert in Portland, Oregon. In 1973, Oregon became the first state to decriminalize cannabis. 
Marijuana use became more widespread in the 1960s, reflecting the rising counterculture movement. 
Research scientist Dr. Reese T. Jones, right, adjusts the electrodes monitoring a volunteer's brain response to sound during an experiment in 1969 that used a controlled dosage of marijuana. The tests were conducted at the Langley Porter Institute at the University of California, San Francisco. 
U.S. Customs agents track the nationwide marijuana market during Operation Intercept, an anti-drug measure announced by President Nixon in 1969. The initiative intended to keep Mexican marijuana from entering the United States. 
Members of the Grateful Dead talk with reporters from their home in San Francisco on October 5, 1967. The band was protesting being arrested for marijuana possession. 
A woman buys ready-rolled marijuana cigarettes from a dealer at her door circa 1955. 
Even after Congress cracked down on marijuana in 1937, farmers were encouraged to grow the crop for rope, sails and parachutes during World War II. The "Hemp for Victory" film was released in 1942 by the U.S. Department of Agriculture. 
Marijuana cigarettes are hidden in a book circa 1940. Congress passed the Marijuana Tax Act in 1937, effectively criminalizing the drug. 
A poster advertises the 1936 scare film "Reefer Madness," which described marijuana as a "violent narcotic" that first renders "sudden, violent, uncontrollable laughter" on its users before "dangerous hallucinations" and then "acts of shocking violence ... ending often in incurable insanity." 
Harry Anslinger was named commissioner of the Federal Bureau of Narcotics when it was established in 1930. While arguing for marijuana prohibition, he played on Americans' fear of crime and foreigners. He spun tales of people driven to insanity or murder after ingesting the drug and spoke of the 2 to 3 tons of grass being produced in Mexico. "This, the Mexicans make into cigarettes, which they sell at two for 25 cents, mostly to white high school students," Anslinger told Congress. 
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- David Nathan: Most Americans favor full legalization of medical marijuana
- Nathan: While critics raise important concerns about pot, its dangers are exaggerated
- He says the arguments against pot are not sufficient to warrant its prohibition
- Nathan: I'd rather live in a nation where cannabis and its trade is regulated than not
Editor's note: David L. Nathan, Distinguished Fellow of the American Psychiatric Association, is a clinical associate professor at Robert Wood Johnson Medical School. He teaches and practices general adult psychiatry in Princeton, New Jersey.
(CNN) -- David Frum's recent op-ed on CNN.com exemplifies why Americans are becoming deaf to the critics of pot legalization. Their hyperbolic claims about the dangers of pot -- along with the realistic ones -- are being dismissed by a justifiably cynical public. Anti-marijuana propaganda isn't stopping the march toward national legalization, but the opposition is losing its voice as it screams about the falling sky.
A majority of Americans favor full legalization, and they understand that recreational cannabis is not now and will never be legal for minors. They know better than to believe Frum's prediction that "half the states may soon allow the sale of marijuana to almost anybody determined to buy it" -- an assertion so broad as to imply that even children would be permitted to buy pot legally. This is simply not so.
Americans are also informed enough to realize that medical marijuana is not a "laughable fiction." This week, I saw a longstanding patient with a rare, ultimately fatal neurological disorder that causes chronic, painful muscle spasms. For years, this patient has smoked marijuana to counteract this terrible symptom, and it works.
With medicinal marijuana now legal in New Jersey, this patient is not convinced that she will be safe from federal prosecution if she goes through the state's one operational dispensary, so she continues to purchase illegal, untaxed and potentially adulterated cannabis. While she is technically a criminal, she is neither laughable nor fictional.
Those opposed to marijuana do raise important concerns that are often neglected in the movement toward legalization.
Cannabis is indeed associated with low motivation and poor performance in minors. It may have subtle but long-term negative effects on young users and worsen the condition of some individuals vulnerable to psychosis. Smoking marijuana may be harmful to users' lungs, although vaporizers have long been available as a safer alternative.
Cannabis is habit forming in a small percentage of users. Marijuana intoxication impairs driving, though the risk is similar to that of drivers with a blood alcohol level of 0.05%, which is well below the federally mandated legal limit of 0.08%.
So why can't the opposition discuss these problems realistically?
It's simple: Because the only rational conclusion is that the dangers of pot are not sufficient to warrant its prohibition. Yet those who have an ideological opposition to legalization appear immune to reason.
Interestingly, Frum is a moderate in the opposition camp, in that he favors reducing the penalties for marijuana use through decriminalization. But by maintaining pot's nominal illegality, decriminalization denies our government the power to regulate and tax cannabis while removing obstacles to its consumption.
Free pot handed out in Colorado 
Christie's big marijuana decision Those who would limit the cannabis trade must make a choice between two Americas -- one that crushes the marijuana trade by any means necessary (as nothing less has worked) or one that regulates and taxes a "soft" drug that millions of consenting American adults already consume medicinally as well as recreationally.
Frum warns of the coming of "Big Marijuana," a term he quoted from legalization opponent Kevin Sabet. Frum and Sabet compare this hypothetical Leviathan to the alcohol and tobacco industries, both of which have been accused of marketing to minors and encouraging destructive consumption of their products by adults. I share this concern, which is why several of the 15 steps I proposed for the federal regulation of cannabis are designed to limit the power of the cannabis industry.
Frum predicts that Big Marijuana will target "young people and racial minorities" because "secure and contented people don't tend to be heavy consumers of psychoactive substances." Setting aside the odd assumptions about who are insecure and discontent, minors and minorities could both benefit from legalization.
It is our government, not Big Marijuana, that disproportionately targets racial minorities for prosecution of marijuana crimes, causing much more harm to those communities than cannabis itself ever could. And our prosecution of healthy adults for their relatively benign use of marijuana prevents the justice system from focusing on the diversion of pot to the young people we all want to protect.
But in reality, we already have "Big Marijuana" in the form of violent Mexican drug cartels and an underground economy in the 48 states where pot remains illegal.
We currently live in a nation that does not regulate the ubiquitous, thriving cannabis trade. I want to raise my children in an America that does.
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The opinions expressed in this commentary are solely those of David Nathan.
