Choose the strain according to the goal and respect the CFU used in the study. Not all work for everything.
In healthy people, probiotics do NOT significantly increase gut microbiota diversity: neither Shannon, Chao1, Simpson, nor the number of OTUs changes compared with placebo (22 trials, 1,068 people). The benefit, if any, would be in specific populations or specific strains, not as a 'general improvement' of the microbiome in healthy people.
View the study (meta-analysis) →Review: manipulating the microbiota with probiotics could reduce the risk of colorectal cancer and improve the safety and efficacy of treatment, according to animal and human studies. It is preliminary: more clinical research is needed before recommending it.
View the study (study) →⏱ 4 min read
Probiotics, according to the consensus definition by the World Health Organization and the FAO, are "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host"; this definition has three important clinical implications: probiotics must be alive when they reach the target organ (typically the gut), doses must be sufficient to produce detectable effects (typically billions of colony-forming units, or CFU), and clinical effects must be documented, not inferred by extrapolation; these three conditions significantly limit which commercial products are actually effective probiotics, as opposed to products sold as such without meeting the requirements.
The most studied genera and species with significant clinical support include Lactobacillus (several species, especially L. rhamnosus GG, L. acidophilus, L. plantarum, L. reuteri), Bifidobacterium (several species, especially B. lactis, B. longum, B. infantis), Saccharomyces boulardii (a yeast, with its own profile), and, to a lesser extent, Enterococcus, Bacillus, and others; each species and specific strain has its own clinical profile for particular indications, a strain-indication specificity that is one of the most important concepts in modern clinical probiotic therapy: probiotics are not interchangeable, and data for one strain for one indication do not automatically translate to other strains or other indications.
The indication with the strongest clinical support is acute infectious diarrhea, especially in children: several meta-analyses have documented significant reductions in duration and severity with supplementation with Lactobacillus rhamnosus GG, Saccharomyces boulardii (with its own profile), and other specific probiotics, an indication recognized in international pediatric guidelines as an adjunct to oral rehydration.
In the prevention of antibiotic-associated diarrhea (including prevention of Clostridium difficile recurrence), multiple meta-analyses confirm significant reductions with supplementation with Saccharomyces boulardii (the most studied option and the only one that also acts during the antibiotic course, as it is a fungus resistant to antibacterials), Lactobacillus rhamnosus GG, and specific combinations; the effects are quantitatively notable and have led to recommendations for routine prescription of probiotics with antibiotics in high-risk patients.
In irritable bowel syndrome, several specific probiotics (especially Bifidobacterium infantis 35624 and specific combinations such as VSL#3) have shown significant improvements in overall symptoms, abdominal pain, and bloating in randomized trials; the effects are strain-specific, and generic products without declared strains have heterogeneous or null efficacy.
In recurrent bacterial vaginosis and recurrent vulvovaginal candidiasis, several specific probiotics (Lactobacillus rhamnosus GR-1, Lactobacillus reuteri RC-14) given orally or vaginally have shown significant reductions in recurrence, an indication with interesting clinical support for refractory or recurrent situations.
In breastfed infants with colic, Lactobacillus reuteri DSM 17938 at 1×10⁸ CFU/day for 3 weeks has shown significant reductions in daily crying time in several trials; it is an intervention with reasonable clinical support and a low risk profile.
In infant atopic eczema (prevention and treatment), Helicobacter pylori (as an adjunct to eradication), hepatic encephalopathy, and depression-anxiety (through the gut-brain axis, an active line of research with interesting preliminary data), there are variable clinical data with specific strains that require consulting the evidence for each strain-indication combination.
The doses studied are typically 1×10⁹ to 10×10⁹ CFU/day (one billion to ten billion), specific to the strain and indication; the viability of the microorganisms during storage and gastrointestinal transit is critical: refrigerated products are usually more reliable than room-temperature ones, enteric-coated capsules improve survival through the stomach, and sustained regular use is necessary to maintain the effects (most probiotics do not colonize permanently and disappear within days to weeks after supplementation is stopped); cheap products may have much lower viability than declared, and it is advisable to choose products with viability guarantees through the expiration date.
Generally very well tolerated, with an excellent safety profile for most people; the adverse effects described are rare and mild (transient flatulence, abdominal fullness at the start); the most important safety consideration is the sporadic reports of bacteremia or fungemia (systemic infection by the probiotic itself) in patients with severe immunodeficiencies, critically ill patients with central venous catheters, patients with prosthetic heart valves, or those with a severely compromised intestinal barrier; these specific populations should avoid probiotics or use them only under specialized medical supervision; healthy people can use them with confidence; in pregnancy and breastfeeding they are generally considered safe; in pediatrics they are a useful clinical tool with strain-specific profiles; generic probiotic supplements without declared specific strains have questionable clinical value, and it is advisable to choose products with specific strain identification and doses appropriate to the intended indication; a traditional yogurt or kefir with live microorganisms can provide part of the probiotic benefits as part of a normal diet and is an accessible route for people who prefer dietary interventions to specific supplements.
Information, not medical advice. Scientific evidence reflects the quality of the studies.