(CNN) The US Food and Drug Administration sent a warning to 15 companies that have illegally sold cannabidiol products, better known as CBD, by adding it to food or marketing it as a dietary supplement.

The agency also released a consumer update about CBD , the non-psychoactive ingredient in marijuana increasingly used to treat pain and anxiety. Until it learns more about the effectiveness and safety of CBD, the FDA said it cannot generally recognize the ingredient as safe or approve products that contain it.

"We remain concerned that some people wrongly think that the myriad of CBD products on the market, many of which are illegal, have been evaluated by the FDA and determined to be safe, or that trying CBD 'can't hurt,'" FDA Principal Deputy Commissioner Amy Abernethy said.

The FDA sent letters to the following 15 companies for selling CBD products that violate the Food, Drug and Cosmetic Act, for marketing the products to treat illnesses or for therapeutic use, claiming it's a dietary supplement or adding it to food for humans and animals.

-Red Pill Medical Inc. of Phoenix, Arizona

Read More