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 Mr. 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,” Mr. Pennington recalled recently. “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, Mr. 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 last December, the Farm Bill legalized hemp — cannabis that contains less than 0.3 percent 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. Mr. 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.