People without any specific illnesses who claim probiotic products give them health benefits may just be experiencing the placebo effect, a specialist claims.
The so-called "friendly" bacteria, which are similar to those found in the human digestive system, are now included in everything from drinks to snack bars, and even baby formula.
They come with vague health claims of "regulating your digestive health" or "strengthening your body's defences".
Experts believe probiotics are generally safe, and in some cases might be helpful, but more research is needed.
Millions of "good" bacteria live in the intestinal tract and help keep "bad" bacteria at bay, and scientists increasingly believe that people can become ill when this balance is affected.
Dr Sri Komanduri, a gastrointestinal specialist at Chicago's Rush University Medical Center, prescribes powerful probiotic pills containing 450 billion live lactic acid bacteria for inflammatory bowel disease.
But he does not recommend them for patients without specific complaints, and does not recommend probiotic foods because "there is no evidence that they are as effective".
He is now warning people not to be influenced by marketing campaigns and seek medical advice if they believe they have a gastrointestinal problem.
"Not hype. The Lancet has twice published data demonstrating that a supplement of the probiotic Lactobacillus rhamnosus GG almost halved the incidence of atopic dermatitis (eczema) in infants followed up in the first instance until two years and in the second publication until they were four years of age. The infants were exposed to the probiotic through their mothers taking a supplement containing the L rhamnosus probiotic in the last 4 weeks of pregnancy and then throughout 6 months of breast feeding. If the infant was not breastfed the infant received the same probiotic by mouth for 6 months. The authors concluded that the results suggest that the preventative effect of Lactobacillus rhamnosus GG on atopic eczema extends beyond infancy. References: Kalliomaki M et al. Lancet 2001, Kalliomaki M et al, Lancet 2003." - Name and address supplied
"Can probiotics help people with IBS?" - Name and address supplied