The sale of marijuana 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 7 grams of dried flowers at roughly 200 existing medical-use marijuana dispensaries as a new law took effect that backers hope will help curb a flourishing black market.

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