Probiotics are not effective in preventing diarrhea associated with Clostridium difficile infection, according to a large study that calls into question previous findings.

The study included about 17,400 patients at risk for C. diff-related diarrhea, who were divided into two groups. One group received a probiotic preparation of Lactobacillus acidophilus and bifidobacterium. Probiotics are small organisms that help maintain the natural balance of organisms in the intestines that are disturbed by C. diff infection. The other group received a placebo. They were tracked for development of antibiotic-associated diarrhea (ADD) within eight weeks and C. diff diarrhea (CDD) within 12 weeks.

There was no statistical difference in outcomes between the two groups, according to the researchers, who are affiliated with various British hospitals and research institutions. The probiotics group had 159 cases of ADD (including CDD), while the placebo group had 153 cases.

The results contradict previous findings, which suggested probiotics are effective in combating CDD. Earlier this year, researchers with the Cochrane Collaboration analyzed 23 studies and found that probiotics could reduce CDD risk by 64%.

The findings of the U.K. study are especially significant because its size “dwarfs” previous studies, wrote Nick Daneman, M.D., FRCPC, of the University of Toronto. The results coming out of the U.K. may “tip the balance of probiotic evidence,” Daneman wrote in his editorial accompanying the findings.

However, Daneman did note some possible shortcomings of the British research, namely that the total incidence of Clostridium difficile among participants was low, and only the two types of probiotics were tested.

Full study results were published in The Lancet.