In a bulletin that will make many people do a double-take, the Food and Drug Administration just issued a very serious warning for individuals considering a controversial treatment where one individual’s poop is transplanted into another.

Yes, poop transplants are a real thing, and they’ve actually proven to be incredibly beneficial for certain people suffering from a variety of ailments related to their intestines. However, the FDA warning emphasizes caution due to a recent case where two individuals receiving the fecal transplants became seriously ill, with one of the recipients actually dying as a result.

“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 reads. “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.”

The experimental procedure has shown promise for patients whose digestive systems have been thrown into disorder by the use of antibiotics. In some, a dramatic loss of certain intestinal bacteria causes a serious imbalance that is difficult to regain. Fecal transplants from individuals with healthy guts can actually restore the delicate balance and promote wellbeing, but it’s not without its risks.

In this case, the donor poop that caused problems came from a single individual and, when that poo was transplanted into two patients, it brought with it a nasty strain of E.coli which ravaged their bodies.

In its bulletin, the FDA emphasizes the need for detailed screening of poop donors in order to filter out any potential problems. The agency also suggests speaking openly with primary care doctors so that individuals considering the controversial treatment option can better understand the potentially deadly risks of undergoing the transplant.