FMT stands for fecal microbiota transplantation, a medical procedure that transfers stool from a healthy, screened donor into the gut of a patient to restore a balanced intestinal microbiome. It is primarily used to treat recurrent Clostridioides difficile (C. diff) infections, where it has success rates above 85%, compared to 40% to 50% with antibiotics alone.
How FMT Works
Your gut contains trillions of microorganisms that help with digestion, immune function, and keeping harmful bacteria in check. When this community gets disrupted, often by repeated courses of antibiotics, dangerous bacteria like C. diff can take over and cause severe, recurring diarrhea and colon inflammation.
FMT works by reintroducing a diverse, functional community of gut bacteria from a healthy person. These transplanted microbes compete with C. diff for space and resources, essentially crowding it out. They also restore the metabolic networks that keep the gut environment hostile to harmful organisms. When those networks break down, the conditions favor exactly the kinds of bacteria you don’t want. Donor bacteria rebuild that defense system.
What FMT Treats
The primary use for FMT is recurrent C. diff infection, meaning cases that keep coming back despite standard antibiotic treatment. C. diff causes colitis (inflammation of the colon) and can be debilitating or life-threatening, particularly in older adults and hospitalized patients. FMT is typically considered after a patient has experienced two or more recurrences that haven’t responded to antibiotics.
There is significant interest in using FMT for other conditions, including inflammatory bowel disease, obesity, and gastrointestinal symptoms in autistic individuals. Clinical trials are underway for some of these uses. A double-blind, placebo-controlled trial is currently recruiting autistic adolescents and adults to test whether oral FMT capsules improve chronic gut symptoms. However, there is not yet enough scientific evidence to confirm FMT is safe or effective for any condition other than C. diff.
How the Procedure Is Delivered
FMT can be administered in two main ways: by colonoscopy or by swallowing oral capsules. Both are effective. A clinical trial comparing the two found that capsules were not inferior to colonoscopy for preventing recurrent C. diff infection over 12 weeks.
Colonoscopy delivery is more invasive and resource-intensive, costing roughly $874 per patient in one study. It does have one advantage: the doctor can visually examine the colon during the procedure and potentially identify other problems. Capsule delivery, on the other hand, can happen in an office setting for around $308 per patient, with shorter wait times. Patients also find capsules more tolerable. In the same trial, 66% of capsule recipients rated the experience as “not at all unpleasant,” compared to 44% of those who received FMT by colonoscopy.
Regardless of the method, preparation looks similar. You stop taking antibiotics and probiotics 24 to 48 hours beforehand, and you do a bowel prep the night before (the same type of colon-cleansing drink used before a colonoscopy).
Donor Screening
Not just anyone can donate stool for FMT. Donors go through an extensive screening process designed to minimize the risk of transmitting infections or other health problems. The process includes a detailed health questionnaire covering HIV status, hepatitis B and C, high-risk sexual behaviors, illicit drug use, recent travel to regions where travelers’ diarrhea is common, and any recent needle-stick injuries.
Donors are also screened for conditions that could affect the quality of their gut bacteria. This includes a history of inflammatory bowel disease, irritable bowel syndrome, chronic diarrhea or constipation, gastrointestinal cancers, obesity, autoimmune diseases, atopic conditions like asthma and eczema, metabolic syndrome, depression, and schizophrenia. Anyone who has taken antibiotics in the past three months is excluded, since antibiotics disrupt the very microbiome diversity the transplant aims to provide.
Blood tests check for hepatitis A, B, C, and E, HIV, and syphilis, among other infections. The FDA also requires stool testing for drug-resistant organisms like MRSA and other antibiotic-resistant bacteria.
Risks and Side Effects
FMT is generally well tolerated, but it carries real risks. The most serious concern is the transmission of dangerous pathogens from donor stool. The FDA has issued safety alerts after patients developed infections with pathogenic strains of E. coli following FMT. In one reported cluster, six patients who received stool from a single bank developed E. coli infections. Four required hospitalization, and two patients died after developing diarrhea following the procedure, though their stool was never tested for the specific pathogen.
These cases underscore why rigorous donor screening matters and why the FDA closely regulates FMT products. Common, less serious side effects can include bloating, cramping, and diarrhea in the days following the procedure.
FDA-Approved Products
For most of its history, FMT existed in a regulatory gray area. The FDA technically classifies it as a biological product requiring an investigational new drug application, but it has exercised enforcement discretion for doctors using FMT to treat C. diff infections that don’t respond to standard therapy. This means individual physicians can perform the procedure without full drug-approval paperwork, as long as they obtain the stool directly rather than from a stool bank.
That landscape shifted in 2023 when the FDA approved Vowst, the first orally administered fecal microbiota product, made by Seres Therapeutics. Vowst is approved specifically for preventing recurrence of C. diff infection in adults 18 and older, taken after a course of antibiotics for recurrent C. diff. As a standardized, manufactured product, it offers more consistency and quality control than traditional FMT prepared from individual donors.