By Courtney Sherwood

PORTLAND, Ore. (Reuters) - Marijuana sales for recreational use began in Oregon on Thursday as it joined Washington state and Colorado in allowing the sale of a drug that remains illegal under U.S. federal law.

Oregon residents 21 years and older can buy up to a quarter-ounce (seven grams) of dried pot at roughly 200 existing medical-use marijuana dispensaries as a new law took effect.

Backers hope the law will help curb a flourishing black market, but opponents say it heightens drug use and access by children.

About 40 people lined up outside the medical pot dispensary Shango in a strip mall near Portland International Airport for the chance to buy recreational pot one minute after midnight, when the changes went into effect.

"We came to be part of the experiment," said Juliano Hamana, 24, in line with his girlfriend.

"I'm worried about the 25 percent tax coming in January," he said, "but for a $10 gram that's only a bit over $2 more. I think it might be worth it for the quality."

Voters in Oregon and Alaska last year approved marijuana use and possession in state-regulated frameworks. Alaska's law took effect in February, but regulations for stores are still in the works. Pot shops created specifically for the recreational market, like those operating in Washington state and Colorado, are expected to start in 2016 in both Oregon and Alaska.

The District of Columbia has also legalized marijuana possession.

While marijuana use remains illegal for any reason under federal law, 23 states allow cannabis for medical purposes. Legalization measures will be on the ballot in Ohio in November and in other states in 2016.

In Oregon, possessing and growing pot became legal in July. Through 2015, sales of pot for recreational use will be untaxed, though that will likely change next year.

"You can get all the best strains from Oregon, which can make this into a top tourist spot," said Sue Vorenberg, editor of the Cannabis Daily Record.

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Roughly 30 municipalities in Oregon have enacted bans to the sale of recreational pot, while others have sharply limited the nascent industry.

In Portland, the state's largest city, lawmakers on Wednesday approved limits on the recreational marijuana stores that will open there, such as a requirement they be no closer than 1,000 feet (305 meters) from a school.

Portland commissioners said they expected to take further steps to refine their rules for the industry.

(Writing by Suzannah Gonzales; Additional reporting by Alex Dobuzinskis; Editing by Susan Heavey and Stephen R. Trousdale)