Officials in Washington announced on Monday that the state's nearly 500 licensed marijuana retailers could begin hosting vaccine clinics and offering a single, free pre-rolled marijuana cigarette to any 21-plus adult who received a shot there.

It's one of many vaccine incentives being offered in Washington, including free pints of beer, sports tickets and prize money to lure those who have been hesitant or just lazy.

Washington and Colorado in 2012 were the first states to legalise the adult use of marijuana, with regulated sales beginning in 2014. The industry brought Washington close to $US474 million in taxes in the last fiscal year.

But few things are simple in an industry that's illegal under federal law, and the hurdles to offering the free joints are substantial enough that few of the state's legal pot shops are saying they will participate, even if they would like to do so.

Retailers told the state Liquor and Cannabis Board during a meeting onn Wednesday that many don't have the space to host a vaccine clinic.

Some health care providers are queasy about setting up a clinic on the site of a marijuana business because they don't want to jeopardise federal funding by being involved in the distribution of an illegal drug.

And the program is set to expire July 12 - too soon for them to offer a second shot to customers who might show up for a first shot in mid- to late June.

The retailers also ruefully noted that the Liquor and Cannabis Board allowed breweries, wineries and bars to offer a free drink to customers who merely showed proof of vaccination - no onsite clinic required.

"We're hearing from retailers that they want to be a part of this," said Aaron Pickus, a spokesman for the Washington CannaBusiness Association, an industry group.

"Why can't we do this like the wineries and breweries did it?"

Board chair David Postman told them it was a member of the business group who late last month suggested the program, which he and Governor Jay Inslee's office thought was a great idea.

While a marijuana retailer in Arizona offered free vaccines along with complimentary joints or edibles early this month, no other state appears to have a similar "joints for jabs" program. "We are out ahead on this," Postman said.

The requirement that the pot shops hold onsite vaccination clinics was partly motivated by the fact that people aren't allowed to consume cannabis at licensed retailers, unlike alcohol at a brewery or winery, he said - suggesting that people who showed their vaccine cards at multiple pot shops could wind up with a lot of free joints that might be given to youngsters.

He also said that the board gave the marijuana stores an advantage it didn't give to alcohol purveyors: a tax break on the product they give away.

"I guess I just beg for a little understanding," Postman said.

"If it's not the right thing, then that'll be too bad, because I think we all need to get as many people vaccinated as possible."

Australian Associated Press