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.

“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,” Pennington recalled recently.

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.

Pennington 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.

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.

Even in states that have legalized marijuana, it remains legal for employers, child protective services, public housing authorities and other entities to test for THC.

Amanda Chicago Lewis is a New York Times writer.