The United States Food and Drug Administration has issued a warning letter to a company that appears to sell products containing human fecal matter without approval from the agency.

In March, the FDA wrote to Human Microbes, a company advertising itself as "the world's largest, highest quality stool donor bank" for fecal microbiota therapy transplants, after reviewing its website a month prior.

This type of therapy should only be used in the context of an authorized clinical trial, or to treat patients with recurrent Clostridium difficile, also known as C. difficile, a bacteria that causes diarrhea and intestinal conditions, like colon inflammation, the agency said.

Per Human Microbes' website, the therapy yields "promising results in clinical trials" to treat other conditions, such as irritable bowel syndrome, Parkinson's disease, multiple sclerosis and mental health disorders like depression and anxiety, among others.