The popular botanical drug kratom is already under fire from U.S. health officials as an addictive opioid, and now new reports are linking its use with salmonella poisoning.

In a news release issued Tuesday, the U.S. Centers for Disease Control and Prevention said that, along with several states and the U.S. Food and Drug Administration, it is "investigating a multistate outbreak of 28 salmonella infections in 20 states" linked to kratom use.

Kratom grows naturally in the Southeast Asian countries of Thailand, Malaysia, Indonesia and Papua New Guinea. It has been sold as a dietary supplement -- typically to help manage pain and boost energy.

Get Breaking News Delivered to Your Inbox

But the CDC said that, so far, 11 people have been hospitalized with salmonella illness linked to their use of the leafy herb, although no deaths have yet been reported.

"Investigation findings link the outbreak to kratom products," the CDC said. "Out of 11 people interviewed, eight (73 percent) reported consuming kratom. Ill people in this outbreak report consuming kratom in pills, powder or tea. No common brands or suppliers of kratom products have been identified."

For now, the CDC is urging Americans to avoid kratom due to the salmonella threat. The agency noted that their investigation into the outbreak is ongoing.

This isn't the first bad news for users of kratom, which is growing in popularity in the United States.

On Feb. 6, the FDA issued a statement that declared the botanical to be an opioid.

Computer analysis of the herb found that nearly all of kratom's major compounds bind to opioid receptors on human brain cells, and two of the top five most prevalent compounds activate those receptors, FDA Commissioner Dr. Scott Gottlieb said in the statement.

In addition, there have been 44 reported deaths associated with the use of kratom, often in combination with other substances, Gottlieb said.

"Kratom should not be used to treat medical conditions, nor should it be used as an alternative to prescription opioids," Gottlieb said at the time. "There is no evidence to indicate that kratom is safe or effective for any medical use."

Claims that kratom is harmless because it's just a plant are "shortsighted and dangerous," Gottlieb continued, noting that heroin also is derived from poppy plants.

Gottlieb urged people to seek help from a health care provider if they are using kratom to self-medicate for pain or to treat opioid withdrawal symptoms.

"There are safe and effective, FDA-approved medical therapies available for the treatment of opioid addiction," Gottlieb said. "Combined with psychosocial support, these treatments are effective."

Concerns over kratom have been growing in recent years. For example, calls to poison centers regarding kratom increased tenfold between 2010 and 2015, rising from 26 to 263, according to the CDC.

More than one-third of the poison center calls reported use of kratom in combination with other substances, such as illicit drugs, prescription opioids or over-the-counter medications, the CDC said.

"Cases of mixing kratom, other opioids and other types of medication is extremely troubling because the activity of kratom at opioid receptors indicates there may be similar risks of combining kratom with certain drugs, just as there are with FDA-approved opioids," Gottlieb said.

In a statement, the American Kratom Association said the FDA analysis is "an unprecedented abuse of science to create a new computer program that is clearly garbage in/garbage out, avoiding the rules of the Controlled Substances Act and making unproven claims that have been proven to be untrue."

The FDA's claims also have been questioned by kratom researcher Scott Hemby, chair of the department of basic pharmaceutical sciences at High Point University in North Carolina.

Hemby has found that kratom's principal chemicals do bond to opioid receptors and cause opioid-like effects such as pain relief and a euphoric rush from a release of the neurotransmitter dopamine. At least one of the chemicals may also have some addictive properties.

However, Hemby told CNN that kratom acts much less effectively than prescription opioids or heroin, and that the total amount of these compounds in the plant as a whole is so low that it's unlikely to lead to abuse or addiction.

"Just because it binds, it doesn't mean it has the same efficacy" as an opioid, Hemby said.