All dog owners who have had to clean up after their beloved pooches know that people aren't the only ones who experience gastrointestinal disorders. Many animals, including man's best friend, also suffer from both short-lived and chronic digestive woes. As a result, some veterinarians have begun prescribing supplements containing friendly bacteria, or "probiotics," to ease related symptoms such as diarrhea, vomiting, gas and bloating. But some animal experts say there is little solid evidence that the supplements are effective.

Gastrointestinal (GI) issues are the second-most common health problem in dogs after skin diseases, according to Richard Hill, a small-animal internist and clinical nutritionist at the University of Florida College of Veterinary Medicine in Gainesville. He notes that common causes include parasites such as hookworm, whipworm and giardia as well as "dietary indiscretion," which could mean anything from gorging on table scraps to drinking from the toilet to raiding the garbage.

Canines may also suffer from chronic diarrhea, stemming from a bacterial imbalance or infection such as salmonella, campylobacter or Escherichia coli, says Susan Wynn, a nutritionist at the University of Tennessee's College of Veterinary Medicine in Knoxville.

To resolve both acute and chronic GI conditions, some vets are now turning to supplemental probiotics, the gut-dwelling "good bacteria" that may assist in digestion and help fend of gastrointestinal maladies in animals as well as people. Wynn says there is evidence that probiotics help stave off infections in chickens, pigs and other farm animals but no "hard clinical evidence" that they do so in dogs. She notes, however, that preliminary research indicates that they may benefit pups, as well.

Cincinnati-based Procter & Gamble makes a soft, chewable probiotic supplement called Prostora Max containing a special strain of Bifidobacterium animalis that kills the disease-causing bugs salmonella and Clostridium difficile in laboratory studies, according to Liam O'Mahony, an immunologist at the Alimentary Pharmabiotic Center at University College Cork in Ireland. O'Mahony, who led the team that discovered and isolated the strain, says that results from a yet-to-be-published clinical study suggest the bacterium helps clear up diarrhea in various canine breeds.

The recent study included 31 dogs—10 German shepherds, 11 Labrador retrievers, two golden retrievers, and eight Labrador–golden retriever mixes—suffering from the trots. The 13 dogs given probiotics recovered about 40 percent more quickly—in four instead of seven days—than did the other animals. The reason? Mahoney speculates that the good bacteria outcompete those responsible for causing the GI upset, or perhaps the probiotics fortify the gut barrier in some way.

Meanwhile, Purina has developed a product called Fortiflora containing Enterococcus faecium that it claims may boost doggies' immune responses. Some research suggests that taking E. faecium leads to higher blood concentrations of certain antibodies—proteins that fight pathogens.

"There is accumulating evidence that probiotics do something," Hill says. But at this juncture "there are still as many questions as there are answers."