Washington becomes second US state to legalise sale of marijuana for recreational use.

People have queued for hours after the US state of Washington became the second in the country to allow the legal sale of marijuana for recreational use.

America's biggest social experiment since Prohibition began six months ago when Colorado became the first US state to allow marijuana to be sold to anyone over the age of 21.

Both states approved the cannabis sales in November 2012 referendums and the first Colorado pot shops opened their doors on January 1.

And just as Colorado experienced on its first day, there were long queues in Washington and not enough product to meet demand, especially in Seattle, where only one store has been licensed for the entire city.

Store employees clapped and cheered at Top Shelf Cannabis in the northern city of Bellingham as its first buds were sold to a 29-year-old from Kansas, Cale Holdsworth.

"I'm happy to be a part of history," said Mr Holdsworth, who was on vacation visiting family and took his place at the front of the line about 4:00am.

After his ID was checked to ensure he was 21 or older, Mr Holdsworth picked out two grams of OG Pearl Kush for $US26.50, which included about $US6 in taxes.

His marijuana was placed in black plastic packaging labelled with its name and strength.

"This is exactly what we need: proper education and information," he said. "It's a great step forward."

Brian Smith, a spokesman for the Washington State Liquor Control Board, said the regulator issued 25 retail licences to "dedicated stores".

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"They can sell smoking, liquid marijuana and all forms of paraphernalia - that is bongs and pipes," he said.

Out of a total 7,000 licence applications, authorities have issued 115 since March to both marijuana growers and processors, according to Mr Smith.

He said he expects other US states to keep a close eye on their trailblazing counterparts to see if legalising recreational cannabis sales might be something for them too.

"I think most states are looking at the experiences in Washington and Colorado and looking forward to how it would work in their state," he said.

Customers are legally allowed up to 28 grams of marijuana before buying more. They also can buy up to 454 grams of marijuana-infused product in solid form or up to two kilograms of marijuana-infused product in liquid form.

Shortages may mean prices of $US25 to $US30 a gram, roughly twice the price at the state's weekly regulated medical marijuana dispensaries.

However, some retailers said limiting early shoppers to buying a few grams would help deal with shortages, due in part to limited harvests by licensed growers, regulatory hurdles and an applicant backlog.

And popular edibles, such as hash brownies, are not expected to be available as no processor has been cleared to operate a cannabis kitchen.

The start of retail sales in Colorado marked not only a legal turning point in how marijuana can be purchased, but also in terms of how it will be consumed and the market it will create.

Opponents of legalised cannabis warn it can lead to higher rates of marijuana use and addiction, even among young people who are not sanctioned to use the drug.

But supporters see it as historic and a possible sign of things to come elsewhere.

Almost half the states in the US already allow medical marijuana to be sold.