Earlier this year, the US Food and Drug Administration issued a warning regarding the safety of so-called fecal microbiota transplants (FMTs). The bulletin came after two patients were infected with a drug-resistant form of E. coli after receiving poop transplants from the same donor. One of the patients recovered from their illness, but the other, a 73-year-old with blood cancer, ultimately died.

Poop transplants are a relatively new form of treatment for patients who have imbalances in the bacteria present in their digestive system. The tiny organisms that live in the gut are important to a person’s health, and not having the right mix can cause a whole host of problems. FMTs are one way to fix it, but there are plenty of risks.

“The Food and Drug Administration (FDA) is informing health care providers and patients of the potential risk of serious or life-threatening infections with the use of fecal microbiota for transplantation (FMT),” the agency’s bulletin in June explained. “The agency is now aware of bacterial infections caused by multi-drug resistant organisms (MDROs) that have occurred due to transmission of a MDRO from use of investigational FMT.”

A report on the incident was just published in the New England Journal of Medicine and it reveals that some disturbingly lax screening guidelines may have played a role in the infections.

After some of the potential risks of infection warranted a more strict screening process for poop donors, the medical center made changes to the way they verify each sample. Unfortunately, the hospital didn’t apply these new screening guidelines retroactively, and older samples the facility had in storage weren’t given the same treatment.

Additionally, the doctors believe that a round of antibiotics that were given to both patients ahead of their poop transplants may have actually made it easier for the E. coli to fester, decreasing the ability of the patients to fight off the infections.

It’s clear that a lot of work needs to be done to ensure that these kinds of procedures are safe for everyone involved. When it works, it can be a major improvement in a patient’s life, but when it doesn’t, well, we’ve already seen the troubling results.