Correction: An earlier version of the article described Sean ­Azzariti, the former Marine who became the first person to buy the drug under the state’s new law, as a veteran of the wars in Iraq and Afghanistan. Azzariti served in Iraq but not Afghanistan. This version has been corrected.

At 8 a.m. New Year’s Day, in an industrial area a few miles from downtown Denver, a former Marine named Sean Azzariti walked into a giant store and bought a bag of weed. Legally. To smoke just for fun, if he’s so inclined.

Azzariti’s transaction — 3.5 grams of Bubba Kush for $40 and some pot-laced chocolate truffles for an additional $9.28 — was the first in the state’s grand experiment in legalizing marijuana for recreational use.

The first-in-the-nation law was greeted with long lines at retailers and a lot of “Rocky Mountain High” jokes. But beyond the buzz, the measure represented the institution of a major new public policy in America — one that opponents fear will turn the state into a dangerous land of debauchery and that backers hope sets a nationwide precedent.

If Colorado is able to successfully legalize marijuana without causing a social backlash, the tourism, tax and other considerations are likely to compel several other states to quickly follow suit.

Backers say enough signatures have been collected to put legalization before voters this year in Alaska. Oregon would probably come next, and by 2016, they hope to see measures on the ballot in six other states: Arizona, California, Maine, Massachusetts, Montana and Nevada. Supporters are also hopeful that lawmakers will push for legalization in Delaware, Hawaii, Maryland, New Hampshire, Rhode Island and Vermont.

Washington state has legalized pot, but sales there won’t begin for at least a few months.

If problems arise in Colorado — whether that means residents get sick of stoner tourism or there are a rash of marijuana-
related accidents or crimes — it could set back a decades-old movement that has gained substantial momentum in recent years.

Experts say there really is no way to know which way it will go. “Nobody on Earth has ever done this before,” said Jonathan Caulkins, a drug policy expert and professor at Carnegie Mellon University.

Before Wednesday, the pro-marijuana movement’s biggest breakthrough came in 1996, when California became the first state in modern history to allow marijuana use of any kind when it greenlighted medicinal use. Now 20 states and the District allow it.

Colorado has approved 136 licenses for retail sales, three-quarters of them in the city and county of Denver and all at sites that have been legally selling marijuana for medical purposes. Eighteen city stores had completed the full process in time to open Wednesday. State officials expect dozens more to open across the state, and some have estimated that pot sales could add more than $200 million to Colorado’s economy.

Colorado residents 21 and older are allowed to buy up to an ounce of marijuana per transaction, and out-of-state customers are allowed to purchase up to a quarter-ounce.

Azzariti’s involvement was not by chance. He was active in the campaign to legalize recreational sales, and, although he can use the pot however he pleases, the veteran of the Iraq war said he needs it to deal with post-traumatic stress disorder.

His purchase was the culmination of a large media event hosted by advocates and the industry. Dozens of reporters filled the 3D Cannabis Center for a 7:30 a.m. news conference as customers waited in line outside, where a light snow was falling. News releases were distributed, and advocates were on hand for interviews. Other stores welcomed press, too, with media handlers in tow.

Toni Fox, the owner of the outlet, said she expects her average monthly revenue of $30,000 to grow more than eightfold, to $250,000, once improvements are made.

Doors also opened at 8 a.m. at Medicine Man, which boasts an even larger, 20,000-square-foot production space that the owners expect to double.

At Medicine Man, two non­residents who bought the legal limit of a quarter-ounce of marijuana said it cost roughly $130. Prices are expected to remain high in the short term, with only a few retailers and a lot of demand. But over the long term, experts expect prices to fall with competition.

At Medicine Man, where the line Wednesday morning stretched up to 75 deep by 10 a.m., a security guard checking identification at the door estimated that well over half of the customers were from out of state. One customer, Kevin Schatz of Nebraska, said his 90-minute wait and the taxes paid were “well worth it.”

Not everyone was celebrating Colorado’s new marijuana reality Wednesday.

“Today, we’re witnessing the dawn of Big Marijuana, in a similar way that we had Big Tobacco for over 80 years,” said Kevin Sabet, who co-founded Project SAM (Smart Approaches to Marijuana) with former congressman Patrick J. Kennedy (D-R.I.). “We’re opening the doors to allowing a new, powerful industry to downplay the effects of a substance they will be profiting off of and to downplay the effects of addiction.”

Sabet, who worked in the White House’s Office of National Drug Control Policy during President Obama’s first term and serves as director of the Drug Policy Institute at the University of Florida, said the negative consequences of marijuana legalization include advertising aimed at kids, an increase in drugged-driving incidents and a spillover of marijuana from Colorado into surrounding states, where the drug remains illegal.

Possession of marijuana remains a federal crime, but, for now, the federal government is taking a wait-and-see approach to legalization. Last summer, the Obama administration said it would not challenge laws legalizing marijuana in Colorado and Washington as long as those states maintain strict rules involving the sale and distribution of the drug.

Local police are taking a similar approach, stressing that compliance and education are the primary goals.

“For the police department, the concern is safety,” said Cmdr. Les C. Perry, whose district includes the Medicine Man business.

Opponents of legalization face an uphill battle. In October, Gallup reported that a clear majority of Americans favor legalization — the first time it found such results since tracking began in 1969.

Barry Sacharow said his relationship with the drug dates back even further than Gallup’s polls. The 58-year-old from Hollywood, Fla., said he began smoking in 1968 in, of all places, Colorado.

“To go from Colorado 1968 and New York 1968 through the ’80s and the ’90s and all of the years to be here today,” he said, “is an amazing thing.”

Brady Dennis in Washington contributed to this report.