Washington has become the second state in the US to permit cannabis to be sold for recreational use.
In November 2012, voters backed a change in the law, which came into effect on Tuesday.
In Colorado, the first state to legalise the sale of cannabis earlier this year, concerns have already been raised about “unforeseen problems” caused by the change in the law.
Washington now allows those aged 21 and over to buy up to an ounce of dried cannabis.
The system in the state means suppliers who want to grow cannabis to provide to shops have to apply for licences.
Around 2,600 suppliers applied, and fewer than 100 were granted permits.
In Colorado, Charlie Parker, a councillor who led the committee that implemented the legalisation cannabis in Denver, said: “It’s an industry that hires a lot of people and is bringing a lot of tax revenues, but one has to ask at what cost, especially to young people.
“And that’s going to take years to determine. My fear is that we could lose a generation of young people if we’re not careful.”
He told the Daily Mail’s US correspondent Tom Leonard that there had been a “lot of unforeseen problems”.
Lieutenant Mark Comte, who works in the Colorado Springs police department, said that illegal drug dealing had increased as users attempted to get around taxed cannabis.
He said: “If you can get it tax-free on the corner, you’re going to get it on the corner.”
In March, 19-year-old Congolese student Levy Pongi jumped to his death from a Holiday Inn balcony after consuming a highly potent cannabis cookie in Colorado.
And in April, Richard Kirk, 47, shot dead his wife in their Denver home after he began hallucinating – police reports suggest he ate a form of edible cannabis before the attack.