The Food and Drug Administration (FDA) agency in the US has warned companies advertising marijuana-based treatments for cancer that what they're offering hasn't been proven as either safe or effective.

Much of the FDA's focus is on the cannabidiol (CBD) component of the marijuana plant, which hasn't been approved for any kind of treatment, but still shows up in various oil drops, capsules, syrups, teas, and creams.

That's the part of cannabis that doesn't get you high or cause mind-altering experiences.

Saying that CBD-based products can kill off cancer cells or combat tumours in any way is simply unfounded, according to the FDA, and companies should stop advertising them as anything close to being cures for cancer.

"Substances that contain components of marijuana will be treated like any other products that make unproven claims to shrink cancer tumours," says FDA Commissioner Scott Gottlieb.

"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."

These CBD drops, creams, and other treatments haven't been tested to see how effective they are, what side effects they could cause, how they might react to other drugs, or what the proper dosage levels should be.

Not only might they be dangerous to a person's health, suggests the FDA, they could also stop that person from looking for treatments that actually could make a difference.

"There are a growing number of effective therapies for many cancers," says Gottlieb. "When people are allowed to illegally market agents that deliver no established benefit they may steer patients away from products that have proven, anti-tumour effects that could extend lives."

The companies getting a rap across the knuckles for making unsubstantiated claims are Greenroads Health, Natural Alchemist, That's Natural! Marketing and Consulting, and Stanley Brothers Social Enterprises LLC.

Medical marijuana can help with certain conditions, and research is ongoing in to what kind of positive effects it can have on various diseases, including cancer. It has been recognised as one way of dealing with the nausea that comes with chemotherapy.

And some research – like a 2014 lab study on using cannabinoids and radiotherapy to tackle aggressive brain cancer – has shown promising results. We're still at the "inconclusive evidence" stage though.

For now, emphasises the FDA, company's need to make sure they're not talking up the benefits of this weed ingredient way beyond what science has found so far. Sales can still be made as long as these products are properly advertised.

Part of the problem is there's still a lot we don't know about marijuana, but with recent moves to legalise the drug (at least in the US), scientists are busy trying to gather detailed evidence about how it can affect the body.

Until more of that evidence comes in, let's all just chill with the over the top claims.