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Marijuana doesn’t cure cancer. It has not been shown to somehow explode tumor cells or promote the growth of new brain cells, the Food and Drug Administration said Wednesday.

The FDA warned four companies to stop making these unproven claims and to stop selling cannabis-based hemp and marijuana products that claim to treat cancer or any other medical condition.

“Substances that contain components of marijuana will be treated like any other products that make unproven claims to shrink cancer tumors,” FDA Commissioner Dr. Scott Gottlieb said in a statement.

The FDA says it is not trying to shut down the sales -- just to stop sellers from making unproven medical claims.

“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.”

Medical marijuana is legal in 29 states, as well as in Guam and Puerto Rico. Studies have shown that it can have some benefits in reducing anxiety, counteracting some types of nausea, and that it may relieve some symptoms of glaucoma and multiple sclerosis.

A cannabis derivative called cannabidiol or CBD for short has been shown to help kids with a severe form of epilepsy called Dravet syndrome and perhaps it can help treat Parkinson’s disease.

Related: Pediatricians Urge Legalizing Medical Marijuana

The federal government still regulates marijuana as a controlled substance, but the Obama administration had a policy deferring to the states to regulate it.

With loosening enforcement of laws, sites and clinics have escalated offers of various marijuana and hemp-based treatments, including many selling CBD oils or creams.

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"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."

The FDA warning comes down on four of them: CW (Charlotte’s Web) Botanicals; Natural Alchemist; Greenroads Health; and That’s Natural Marketing and Consulting.

They all have made unproven claims that their products treat cancer, the FDA said. Not only are the claims not proven, but they could discourage patients from using treatments that will help them.

“There are a growing number of effective therapies for many cancers. 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,” Gottlieb said.

One website uses the phrase “anti-tumor” over and over again, the FDA says in its warning letters to the four companies. Another claims that CBD oil can help asthma, Alzheimer’s disease, arthritis, autism, bipolar disorder, and various types of cancer.

CW Hemp, which makes a cannabinoid product called Charlotte's Web, has received extensive media coverage, including from NBC News, for some of its products, but the FDA says it’s gone too far with some of its claims for its dietary supplement products.

Related: Cannabis-based Drug Reduces Seizures

“Your products are not generally recognized as safe and effective for the above referenced uses and, therefore, these products are ‘new drugs’,” it says in its warning letter to Joel Stanley, CEO fo the Colorado Springs-based company.

There’s a law that exempts dietary supplements from FDA approval, but the FDA strongly regulates anything sold as a new drug.

Most of the companies contacted by NBC News did not immediately respond. But CW Hemp blamed its customer testimonials for going too far.

“Our customers love to share their very personal stories about how our products helped improve their lives or those of their loved ones,” the company said in a statement.

“We will work with the FDA to ensure that we better monitor how we share third-party testimonials on the CW Hemp website and social media channels.”

The CEO of That's Natural, Tisha Casida, pushed back against the FDA.

"That’s Natural believes it is individual’s natural and constitutional right to consume non-psychoactive cannabinoids (like CBD) from the hemp plant, or any plant for that matter," she said in an email.

" Anything that comes naturally from a plant should never be able to be taken away from the people. People have a right to grow and consume plant-based medicines without the approval of any government agency."

But she said her websites would comply and remove the language FDA pointed out.