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. MORE