The Food and Drug Administration sent warning letters to four companies on Tuesday, alerting them they cannot market their cannabis products as a treatment for certain diseases.

The agency warned the companies about their unsubstantiated claims that their cannabis-derived products, including oils and capsules made from CBD, could combat tumors and kill cancer cells.

The companies – Greenroads Health, Natural Alchemist, That’s Natural! And Stanley Brothers Social Enterprises – are not allowed to make health claims when a product has never been studied as a treatment for cancer.

The websites feature claims from people that they or their loved ones have been miraculously cured of chronic illnesses.

Stanley Brother, a leading cannabis product company in Colorado, said in an email it takes “regulatory compliance very seriously” and will work with the FDA to better monitor the information on its website.

Tisha Casida, chief executive of That’s Natural, said she would comply with the FDA request, that claimed the company mentioned hemp oil contained an ingredient that “makes cancer cells commit suicide.”

“All free people have a right to experience health and wellness from naturally derived cannabinoids,” she wrote in an email. ‘‘We should not have to only take FDA.-approved synthesized drugs. We should be able to experience natural plant-based medicine in its truest form.’’

Some of the other companies did not respond for comment.

The issue could also have a wider impact on the pharmaceutical industry, that is currently carrying research on CBD as a potential treatment for certain diseases. GW Pharmaceuticals Plc, the global leader in developing cannabinoid-based medicine, is testing CBD to treat certain forms of epilepsy.

The four companies have 15 working days to submit evidence to the FDA that they have corrected the violations.