Evidence Level
Probiotics have an extensive evidence base, though the field is complicated by strain specificity. Ford et al. (2018) published a landmark meta-analysis of 53 RCTs in IBS, demonstrating significant symptom improvement with probiotics, particularly multi-strain combinations. Hao et al. (2015) conducted a Cochrane systematic review showing probiotics reduce the risk of acute upper respiratory tract infections and support immune function. The gut-brain axis has emerged as a frontier, with Wallace & Milev (2017) reviewing the evidence for "psychobiotics" — specific probiotic strains that influence mood and cognition through neural, immune, and endocrine pathways. Suez et al. (2018) provided important nuance in a Nature study showing that post-antibiotic probiotic use can delay, rather than accelerate, native microbiome recovery in some individuals, highlighting the importance of strain selection.