The Food and Drug Administration recently issued warnings to two businesses that are illegally marketing and making "unproven" claims that their products, which contain a drug called kratom, can treat or cure opioid addiction and withdrawal.

Kratom, which is an herbal extract that naturally grows outside of the U.S., is not approved by the FDA. The drug acts as a stimulant at low doses, according to the Mayo Clinic, but can reduce pain and potentially cause euphoria when taken in higher quantities.

Though some advocates say kratom can be used to quell the symptoms of opioid withdrawal and be used as an alternative to pain medications, the FDA says the drug affects the same brain receptors as morphine and can expose users to addiction and abuse.

The FDA has issued a number of safety warnings associated with the use of kratom, including a multi-state salmonella outbreak that was linked to kratom products. Negative side-effects of the drug can include dizziness, drowsiness, breathing suppression, coma and even death.

On Tuesday, the FDA also advised consumers not to use products made by one of the companies, KratomNC, because of various microorganisms found in the products the FDA said could potentially cause illness. The agency said on June 14 it recommended the company recall all of its products, but KratomNC has yet to do so.

"Despite our warnings, companies continue to sell this dangerous product and make deceptive medical claims that are not backed by science or any reliable scientific evidence," acting FDA Commissioner Dr. Ned Sharpless said in a statement Tuesday.

By marketing kratom as a potential treatment for opioid abuse, the FDA says the two companies — California-based Cali Botanicals and North Carolina-based KratomNC — could prevent people who are addicted to opioids from seeking proper care.

"As we work to combat the opioid crisis, we cannot allow unscrupulous vendors to take advantage of consumers by selling products with unsubstantiated claims that they can treat opioid addiction or alleviate other medical conditions," Sharpless said.

Reliance on products that don't work put those who abuse opioids at "greater risk of addiction," the FDA said.

The agency's warnings come as lawmakers across the country look to tackle the devastating opioid epidemic, which kills more than 130 people a day, according to the Centers for Disease Control and Prevention.

The FDA said the two companies also asserted their kratom-containing products could treat other conditions like anxiety, depression and cancer.

KratomNC claims on its website that the drug is used for energy, to increase attention, relaxation and to treat pain and addiction. "Here is just some of what our customers have used kratom to treat," the company's website says. "Chronic Pain, Migraines, Opiate Addiction, ADHD/ADD, Anxiety, Depression, Arthritis, Insomnia and much more!"

The description is still available on the company's website.

Cali Botanicals on its website claimed kratom can be used to treat chronic pain, depression and anxiety and "has essential uses in combating opiate addiction and the harrowing withdrawal that comes with trying to kick the habit," according to the FDA.

Cali Botanicals has since removed it's webpages that said kratom can treat opioid addiction.

The FDA said the companies have up to 15 working days of receiving their warning letters to inform the agency of the steps they have taken to correct their violations. KratomNC and Cali Botanicals were warned on May 16 and June 11, respectively.

The FDA said both cases are currently open and deferred additional comment on responses to the warnings to the companies.

Cali Botanicals and KratomNC did not immediately respond to CNBC's request for comment.