U.S. officials sent a warning to the marijuana industry, alerting online sellers they cannot market their products as a treatment for cancer.

The Food and Drug Administration sent letters to four companies on Tuesday, warning them about unsubstantiated claims that their marijuana-derived products can combat tumors and kill cancer cells. The firms sell products including oils and capsules made from cannabidiol, also known as CBD, a component of the marijuana plant that doesn’t cause the mind-altering effects of the other main component, tetrahydrocannabinol, or THC.

The agency told the companies they cannot make claims to treat or cure a disease when a product has never been studied as a treatment. Curbing the sale of CBD products with health claims could put a damper on the medical-marijuana market. Producers that are required to nix references to medical ailments may move toward the recreational side of the legal cannabis industry.

Eight states and Washington, D.C., have legalized pot for recreational use. Twenty-one additional states have legalized for medical purposes.

“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,” FDA Commissioner Scott Gottlieb said in a statement.

The crackdown could also have a wider impact on the pharmaceutical industry. CBD is being researched in labs as potential treatment for certain diseases. Biotech company GW Pharmaceuticals Plc, for instance, is testing the component to treat certain forms of epilepsy.

Gottlieb hinted almost a month ago at a congressional hearing that the FDA may get tough on unproven marijuana claims. The companies that received warning letters are: Greenroads Health, Natural Alchemist, That’s Natural! and Stanley Brothers Social Enterprises. The companies have 15 working days to tell the FDA what corrective steps they will take.

The companies didn’t immediately return calls seeking comments.