“Substances that contain components of marijuana will be treated like any other products that make unproven claims to shrink cancer tumors. We don’t let companies market products that deliberately prey on sick people with baseless claims that their substance can shrink or cure cancer and we’re not going to look the other way on enforcing these principles when it comes to marijuana-containing products,” said FDA Commissioner Dr. Scott Gottlieb in a statement.

The FDA sent warning letters to four companies—Greenroads Health, Natural Alchemist, That’s Natural! Marketing and Consulting, and Stanley Brothers Social Enterprises LLC—for claiming various products they sell online containing cannabidiol (CBD), a non-psychoactive cannabis component (i.e., the part that doesn’t get you high), can thwart the biological processes that cause cancerous cells to spread.

As medical and fully legal recreational marijuana become increasingly common in America, claims about cannabis’ health effects have finally been getting serious scrutiny. The medical literature to date paints a mixed picture of marijuana’s effect on health. For instance, while there’s no real evidence it can treat cancer, it is fairly clear that marijuana can treat nausea in cancer patients undergoing chemotherapy. Cannabis can also be used to soothe chronic pain, mollify muscle issues in people with multiple sclerosis, and improve sleep for people with certain disorders in the short term, according to a review of more than 10,000 individual studies published by the National Academies of Sciences, Engineering, and Medicine earlier this year.

FDA chief Gottlieb notes that there aren’t any marijuana-derived products approved the FDA. But there are several going through later-stage clinical trials, including an experimental epilepsy treatment from GW Pharmaceuticals and Zynerba Pharmaceuticals’ cannabis-based gel for a rare autism spectrum disorder called Fragile X syndrome.