US officials have sent warning letters to 15 companies for selling CBD products on unproven claims that they can treat diseases, the Food and Drug Administration (FDA) announced Tuesday.

The agency also declared that CBD is not considered 'generally recognized as safe' (GRAS) to use in foods meant for animals or humans - a selling point for some of the 15 companies it sent letters to.

Last year, the market for CBD - an abbreviation for cannabidiol, a non-psychoactive derivative of cannabis - was worth an estimated $1.34 billion, and the trendy products have been advertised for everything from pain relief to cancer treatment.

The FDA has struggled to rein in health claims made about CBD, most of which it continually labelled 'unproven,' amid the industry's boom.

Tuesday, officials warned consumers that the products may have the potential to harm them, and should not be considered broadly 'safe.'

FDA officials sent warning letters to 15 CBD companies, including Daddy Burt Hemp Co, which are marketing their products for unproven health claims

'We remain concerned that some people wrongly think that the myriad of CBD products on the market, many of which are illegal, have been evaluated by the FDA and determined to be safe, or that trying CBD "can't hurt,"' said Principal Deputy Commissioner of the FDA, Dr Amy Abernathy, said.

The safety agency has approved exactly one CBD product: a drug derived from CBD that's used to treat epilepsy.

Outside of that single drug, CBD's medicinal uses are not yet understood or proven, and its legal status is convoluted and hazy.

Since the 2018 legalization of hemp containing less than 0.3 percent THC - the psychoactive component of cannabis - and CBD derived from it, everyone has been eager to get in on the CBD game.

CBD oil is now sold in dispensaries as well as delis, herb shops, supplements stores and many coffee shops in places like Los Angeles and New York City - despite attempts by local officials to crack down on the illegal drops upsold for adding to lattes and teas.

It's commonly advertised for treating pain, anxiety and nausea.

But there are a broad range of more far-fetched benefits pitched by those hawking the hemp oil, including protecting against diabetes and fighting cancer.

Some studies have suggested CBD might indeed have some of these benefits, but they're far from conclusive, and the compound has not been through he regulatory approval process.

The FDA has not reviewed and approved any CBD products.

And despite the vast ongoing research on CBD, scientists' and experts' opinions of the substance have not risen to the level of making it 'GRAS', or broadly agreed upon as harmless.

In fact, the FDA warned, it's received many reports that suggest CBD might well have the potential to cause harm.

Now, the agency has revised its consumer notice on CBD, noting that the compound may cause liver damage, drowsiness, diarrhea, mood changes and may alter sexual behavior and fertility in male animals.

Yet at least the 15 companies the FDA warned Tuesday continue to sell products - often online and across borders - that suggest CBD is a safe supplement or food additive.

'We want to be clear that a number of questions remain regarding CBD’s safety – including reports of products containing contaminants, such as pesticides and heavy metals – and there are real risks that need to be considered,' said Dr Abernathy.

'We recognize the significant public interest in CBD and we must work together with stakeholders and industry to fill in the knowledge gaps about the science, safety and quality of many of these products.'