(CNN) The US Food and Drug Administration is cracking down on companies it says are making unsubstantiated claims that certain products made from marijuana can cure cancer.
This week, the agency responsible for policing the American food and drug market issued warning letters to four companies that are "illegally selling products online that claim to prevent, diagnose, treat, or cure cancer without evidence to support these outcomes." It said in a statement, "The illegally sold products allegedly contain cannabidiol (CBD), a component of the marijuana plant that is not FDA approved in any drug product for any indication."
FDA Commissioner Scott Gottlieb said in the statement, "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."
The companies that received the warning letters are required by law to respond within 15 working days, indicating what steps they have -- or will -- take to address the FDA's concerns.
"Failure to promptly correct these violations may result in legal action without further notice, including, without limitation, seizure and injunction," each of the four letters warned.
The FDA said the 25-plus products that are part of this crackdown include oil drops, capsules, syrups, teas, topical lotions and creams.
The four companies that received warning letters on Wednesday are Greenroads Health, Natural Alchemist, That's Natural! Marketing & Consulting and Stanley Brothers Social Enterprises LLC.
Greenroads Health did not respond to a request for comment.
Natural Alchemist declined a request for comment.
In a statement, That's Natural! said, "We are excited about the opportunity to address the FDA's Warning letter by complying to their warning letter and taking down the requested links and information, including that to peer-reviewed journal articles, on our site and social media."
Stanley Brothers said in a statement, "We take regulatory compliance very seriously. Our customers love to share their very personal stories about how our products helped improve their lives or those of their loved ones. ... We will work with the FDA to ensure that we better monitor how we share third-party testimonials."
CNN featured the Stanley brothers in a 2013 documentary, "Weed," about a young girl who struggled with severe seizures.
"There are a growing number of effective therapies for many cancers," Gottlieb said. "When people are allowed to illegally market agents that deliver no established benefit they may steer patients away from products that have proven, anti-tumor effects that could extend lives."
When issuing warning letters about other "illegal" cancer treatments in April, the FDA said consumers should not use them "because they may be unsafe and could prevent a person from seeking an appropriate and potentially life-saving cancer diagnosis or treatment," according to Douglas W. Stearn, director of the FDA's Office of Enforcement and Import Operations."We encourage people to remain vigilant whether online or in a store, and avoid purchasing products marketed to treat cancer without any proof they will work. Patients should consult a health care professional about proper prevention, diagnosis and treatment of cancer."
"I think the biggest red flag would be that any product that hasn't undergone FDA review is making a claim that it can treat or cure cancer," Jason Humbert, a regulatory operations officer in the FDA's Office of Regulatory Affairs, told CNN in April. "Only products that have been evaluated -- approved FDA drugs -- can make those claims. So if a consumer happens upon a website or a social media site and they see that this product is marketed as a natural cure for cancer or a natural treatment for cancer, they should be very skeptical, because unless that product has been evaluated by FDA, there's no reason to believe it's safe or effective for that use."
Although claims vary from product to product, the FDA says fraudulent cancer products "often use a particular vocabulary." The agency identified these phrases as the most common red flags:
"The overarching point is that these products are untested, and some of the ingredients may present direct risk to the consumer's health or interact with any medications they might be taking," Humbert said in April. "They're not a substitute for appropriate treatment, and using these products can not only endanger consumers' health but waste their money and waste their time, as well."
In this week's statement, Gottleib said, "We have an obligation to provide caregivers and patients with the confidence that drugs making cancer treatment claims have been carefully evaluated for safety, efficacy, and quality, and are monitored by the FDA once they're on the market.
"We recognize that there's interest in developing therapies from marijuana and its components, but the safest way for this to occur is through the drug approval process -- not through unsubstantiated claims made on a website. We support sound, scientifically-based research using components derived from marijuana, and we'll continue to work with product developers who are interested in bringing safe, effective, and quality products to market."