Nobody really knows.  Claims about their efficacy are often based on poorly designed studies.  Efficacy of each strain for specific conditions and with specific dosing is often lacking.  One recent negative study demonstrates that probiotics are often not beneficial (J Pediatr 2012; 161: 40-3).

In this randomized, double-blind placebo controlled study of 106 Polish children (1-48 months of age), Lactobacillus reuteri had no effect in preventing nosocomial diarrhea in patients admitted for non-diarrheal illnesses.  While the authors contemplate that this could be due to the strain of probiotic chosen or the dose, it is clear that evidence that probiotics prevent infectious diarrhea “is still scant.”

This conclusion is backed by a large meta-analysis (JAMA 2012; 307: 1959-69).  While the study concludes that the use of probiotics is associated with a lower risk of antibiotic-associate diarrhea (RR 0.58), it predicted that the number to treat for one person to benefit would be 13.  The study was based on a systematic review of 82 randomized clinical studies.  Yet, overall the quality of the research was considered low; the studies were often had shortcomings:

• 59 studies “lacked adequate information to assess the overall risk of bias”
• 64 did not indicate if treatment randomization was blinded
• 31 did not report an intent-to-treat analysis
• 41 did not include a calculation of the study’s statistical power to detect differences
• 17 trials were industry-sponsored and 52 did not clarify their funding/potential conflicts of interest
• 59 did not report on adverse events specifically related to probiotic use; few trials addressed the risk of fungemia or sepsis
• Trials rarely specified antibiotic agents; thus, it is difficult to know if a particular probiotic would be better with certain types of antibiotic therapy or duration.

Additional references/links:

• Potential and pitfalls of probiotics with necrotizing enterocolitis
• -JPGN 2010; 51:24. VSL#3 helpful for IBS, n=509 (4-18yr olds). 1 per day for <11yr, 2/day in 12-18yr olds
• -Pediatrics 2008; 121:e850. Culturelle, during pregnancy and early infancy, not effective in preventing atopic dermatitis. Did increase wheezing.
• -J Pediatr 2008; 152: 801. Probiotic helped reduce colic sx in 30 preterm infants, Lactobacillus reuteri

• -Pediatrics 2007; 119; e124. Probiotics reduced colic in breastfed babies more than simethicone. n=83, Lactobacillus reuteri, 10-8th power per day. Decreased crying 18 minutes per day at 1 week compared to simethicone & by 94 minutes/day at 4 weeks (95% response vs 7% of simethicone)
• -Neurogastroenterol Motili 2007 (Quigley EM, et al), 19: 166-72. Review of probiotics and IBS.
• -NASPGHAN 2007, author: Brian Dunlap, H. Yu, Y Elitsur. abstract -most commercial yogurts have LOW concentrations of probiotics.
• -JPGN 2006; 43: 550. Review of probiotics for specific conditions.
• -J Pediatr 2006; 149: 367. Probiotics reduce risk of antibiotic assoc diarrhea. If 7 pts (on abx) are treated with probiotics, one fewer will develop AAD.
• -JPGN 2006; 42: 454. Evidenced-based review of probiotics.
• -Pediatrics 2005; 115: 1-4 & 171 editorial.  Probiotics decreased NEC in this study.
• -Gastroenterol 2004; 126: 1620-33.  Review of probiotics, prebiotics and antibiotics in IBD.