(Reuters Health) - A widely-used probiotic therapy is ineffective against the diarrhea, vomiting and abdominal pain of gastroenteritis, two large studies in the U.S. and Canada have concluded.

Five days of treatment with a key ingredient in many products, Lactobacillus rhamnosus, was no better at stopping symptoms than placebo among children ages 3 months to 4 years.

In the Canadian study, a second probiotic added to the mix also showed no benefit.

“Taken together, neither of these large, well-done trials provides support for the use of probiotics containing L. rhamnosus to treat moderate-to-severe gastroenteritis in children,” Dr. J. Thomas LaMont of Beth Israel Deaconess Medical Center in Boston writes in an editorial in the New England Journal of Medicine, where the two studies appear.

The human gut is packed with thousands of types of bacteria, many of which are believed to help the intestines do their job. Although doctors are just starting to understand the interplay of those bacteria, probiotics are sold based on the idea that an illness such as gastroenteritis may be cured by introducing more helpful bacteria into the body.

Gastroenteritis is a huge problem for young children, especially in poor countries where it is the second leading cause of death worldwide among children under age 5. In the U.S., the illness generates about 1.7 million emergency room visits each year. There is no effective treatment and doctors mostly try to keep the child hydrated and prevent the disease from spreading until the illness runs its course.

The results of some small, poorly-controlled trials have convinced many hospitals and some medical societies to embrace probiotic therapy, now a $47 billion industry.

“My institution does use probiotics. It’s part of their guidelines on treating diarrhea in kids,” Dr. David Schnadower, coauthor of the U.S. study, told Reuters Health in a telephone interview. “It will be pretty easy to convince them to stop doing that here” based on the new results.

Whether other hospitals will abandon the practice is unclear.

“The problem is, it’s a negative study and it’s tough for doctors to stop doing things they’re accustomed to doing. It’s going to be an uphill battle. But the results here are clear,” said Schnadower, academic director in the Division of Emergency Medicine at Cincinnati Children’s Hospital Medical Center in Ohio.

In the U.S. test, done at 10 pediatric emergency departments and involving 971 children, the duration of diarrhea was 49.7 hours with the probiotic formula and 50.9 hours with placebo.

The illness spread to another member of the household in 11 percent of the cases where the probiotic was used, versus 14 percent when the placebo was given, a difference that wasn’t statistically meaningful.

Similarly, in the Canadian study, the duration of diarrhea and vomiting was the same regardless of treatment for the 886 children, although the number of episodes of vomiting was higher in the probiotic group.

It’s not clear if the results will affect the sale of probiotics.

Using probiotics for gastroenteritis “has been the number one target of probiotic marketing,” Dr. Stephen Freedman, an associate professor of pediatrics at the Cumming School of Medicine at the University of Calgary who led the Canadian study, told Reuters Health by phone.

Both Freedman and Schnadower said the findings don’t mean probiotics are bad - and the new studies uncovered no evidence that they are harmful.

Whether probiotics other than L. rhamnosus might be effective against infectious diarrhea in children remains to be seen, LaMont said.

But the research shows that such claims of benefit need to be rigorously tested before being embraced.

The probiotic used in the U.S. study, made by Culturelle and regarded as a high-quality product, “can cost up to $60 for a five-day treatment,” Schnadower said. But if it doesn’t work, “do you spend $60 on that or $60 on good food for your kids?”

“The bad news is, we don’t have a magic pill that will make the diarrhea and vomiting go away sooner,” Freedman said. “The good news is, hopefully consumers are now informed and healthcare professionals can focus on therapies that have evidence of benefit.”

SOURCES: bit.ly/2DJpBK2, bit.ly/2QQP7jc and bit.ly/2TAy8TY The New England Journal of Medicine, online November 21, 2018.