Eighty four studies were included (10,351 participants; range 18 to 756). All trials included in the meta-analysis had a Jadad score of at least 3 except four trials which had a Jadad score of 2.
Probiotics were effective in the treatment and prevention of all gastrointestinal diseases and probiotic species (RR 0.58, 95% CI 0.51 to 0.65; Ι²=61.24%).
Effect by type of diseases: Significant effects of probiotics were observed in the following diseases: Pouchitis, Antibiotic Associated Diarrhoea (27 studies), Infectious diarrhoea (three studies), Irritable Bowel Syndrome (16 studies), Helicobacter pylori (13 studies) and Clostridium difficile Disease (six studies). No significant effects were observed for probiotics for Traveller’s Diarrhoea (six studies) and Necrotizing Enterocolitis (nine studies).
Effect by probiotic species: Across all diseases, significant effects of probiotics were observed in the following species: VSL #3, E. Faecium, C. butyricum, L. acidophilus combined with B. infantis, B. lactis, LGG, L. Casei and S. boulardii. No significant effects were observed for L. acidophilus, L. plantarum and B. infantis.
Effects of age: Across all diseases and probiotic species, significant effects of probiotics was observed for all age groups (infants, children and adults).
Effects of dose: Across all diseases and probiotics species, significant efficacy was observed for three doses: 1- 5x1010 colony-forming units (CFU)/day, 1–5.56, 106, 107, 108 CFU/day and 1–9x109 CFU/day. No significant effect was observed for one dose (1-9x1011, 1012 CFU/day).
Effect of treatment length that probiotic was administered: significant efficacy was observed across all groups (one to two weeks, three to four weeks, five to eight weeks and nine to 240 weeks).
There was no significant difference in efficacy between single and multiple species probiotics. There was evidence of publication bias.