Jeff Anderson says it's been 20 years since he smoked pot. But he's out of a job anyway, after failing a drug test.

Anderson, 62, drove a bus for the Beaverton School District for 18 years. Last November, he started taking a daily dose of a hemp tincture to treat the pain from his psoriatic arthritis—a chronic inflammation of the skin and joints.

He says he was pain-free for months, thanks to the tincture's active ingredient: cannabidiol, better known as CBD. That's the cannabis compound that eases pain without the psychoactive "high" of THC.

Employers aren't looking for CBD on drug tests. And Anderson says the Beaverton marijuana shop he visited assured him the level of THC in the tincture couldn't be detected by tests.

But when the school district urine-tested Anderson in January, his THC levels were massive: five times the testing threshold. The district forced him into an early retirement.

"I was emphatic when I walked into the dispensary that I can't test positive for THC," Anderson says. "My job was on the line, and I don't know about this stuff. I'm not a biochemist. But I lost my job, I lost health insurance for me and my daughter, I'm living on peanuts. And I don't feel like I did anything wrong other than trust in a company."

Experts say Anderson's story checks out. Daily use of a CBD tincture with a small amount of THC—like the 20 milligrams of Ra Hemp CBD Indica Tincture that Anderson took three times a day for three months—could build up in his body's fat cells and show up in a urinalysis.

"It's entirely plausible," says Dr. Michelle Sexton, a cannabis researcher who helped the Washington State Liquor and Cannabis Board set up its lab-certification process for recreational marijuana. "If he was taking it three times a day for months, we can only imagine what kind of accumulation might have been going on."

Anderson's story is a dramatic example of a widespread misconception.

In a state where weed is as common as cough drops, many employers still use urine tests to keep pot users off their payrolls. Cannabis retailers assure customers that CBD products won't show up on drug tests. But in fact, those products often contain small amounts of THC—and the drug tests don't distinguish between somebody who smoked a joint yesterday or who had THC build up in his system over several months.

Even the tincture's manufacturer, Sun God Medicinals in Eugene, warns against using its product in advance of a drug test. "We would never recommend to anybody that they use our product if they are subject to a drug test," says Mark Weir, the company's operations manager.

Yet that's what Green Mart, the Beaverton store where Anderson bought his tincture, told him. Jami Arvon, manager of Green Mart, confirms that she recommended the product, knowing Anderson was subject to random drug tests.

"I genuinely believe Sun God's product is not the reason why he failed," she says. "If it is, I'm not a scientist, but I have anecdotal-only evidence that this product will not make you fail a drug test."

She's not alone. WW called a half-dozen cannabis stores in Portland, and all recommended various CBD products for pain relief, claiming they wouldn't show up on a drug test.

In Colorado and Washington, where recreational cannabis has been legal longer, the same inquiry received a different response. A random selection of budtenders in Denver and Seattle declined to recommend a CBD product for pain relief that would avoid detection by urinalysis.

Anderson's story now has his labor union sending out a warning.

Kim Bonner represented Anderson in his dealings with the district for the Oregon School Employees Association. She said the situation jolted her into issuing a warning in its February newsletter: "Buyer beware," it read.

"I think he was misled," Bonner tells WW. "People don't know that if you consume any kind of cannabis product, you're going to have some level of THC involved, even if it says zero. Dispensaries should not be in the position of telling people you're OK to go drive a school bus when legally you are not."