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In June of 2018, Mark Pennington received troubling news from his ex-girlfriend, with whom he shared custody of their 2-year-old son.

She had taken a hair follicle from the boy, she said, and had it analyzed at a lab. A drug test had returned positive for THC, the intoxicating compound in marijuana; evidently their son had been exposed to it, presumably in Pennington’s presence.

He was told that, from then on, he would be permitted to see the child only once a week, and under supervision.

“I was mortified,” Pennington recalled. “My jaw hit the floor. I just knew from the bottom of my heart I hadn’t gotten any THC in my son’s system.”

However, Pennington had been providing his son with honey infused with cannabidiol, or CBD, a nonintoxicating compound that, like THC, is found in varying amounts in the plant known as cannabis. THC is federally illegal and until recently so was all cannabis.

But in December, the Farm Bill legalized hemp — cannabis that contains less than 0.3% THC. With that, CBD became legal. It can now be found at stores across the country, in everything from tinctures and massage oils to coffee and makeup.

Pennington, who lives in Colorado, where growing hemp for CBD has been legal since 2014, worked for Colorado Hemp Honey, a company that sells CBD-infused raw honey across the country.

He was despondent about possibly losing custody of his child, until he spoke with Frank Conrad, the chief technology officer and lab director at Colorado Green Lab, a scientific consultant to the cannabis industry. Conrad directed him to a little-known study published in 2012 in the Journal of Analytical Toxicology that showed that a common forensic drug testing method could easily mistake the presence of CBD for THC.

In short, the drug testing lab may have erred; it was entirely possible that the CBD Pennington had given his child had caused the drug test to produce a false positive for THC.

Two chemists with Cascade Chemistry, a private chemical-research company in Eugene, Ore., independently reviewed the study and confirmed the validity of the potential drug testing problem.

With Conrad as an expert witness, Pennington won equal custody. Now, on behalf of his son, he plans to sue the lab that did the drug test, to raise awareness of the problematic testing method, which could have broad implications for average Americans as CBD becomes mainstream.

In every court case in which Conrad has explained the problem with this specific drug testing method, prosecutors have dropped the charges.

“Anyone who’s on probation getting a random urine test — if this happens to them and they’re taking CBD oil, they’re going back to jail,” Conrad said.

Bruce Houlihan, director of the Orange County, Calif., crime lab and chair of the emerging drugs and opioids committee for the American Society of Crime Lab Directors, expressed concern.

“If any labs are using this method, they’ll have to be careful,” Houlihan said.

He added that there was no way to estimate how many drug testing labs might be accidentally mistaking CBD for THC, because forensic labs generally determine their methodology in house.