How long should I take a probiotic before expecting results?

Duration depends on the indication and strain. For IBS, the network meta-analysis in Frontiers in Cellular and Infection Microbiology found that eight-week treatment duration was associated with greater efficacy than shorter courses. The B. longum 35624 observational study (Sabaté and Iglicki, 2022) showed meaningful improvement within 30 days. For antibiotic-associated diarrhoea prevention, the probiotic should be started at the same time as the antibiotic course and continued for at least two weeks after completing it. There is no universal answer — if you see no improvement after eight weeks on a strain with clinical evidence for your specific concern, the strain may not be effective for you.

Do probiotics need to be refrigerated to be effective?

Not necessarily. Refrigeration requirements depend on the strain and the encapsulation technology used by the manufacturer. Some strains are fragile and require cold chain storage; others are stable at room temperature for extended periods. What matters is the viable CFU count at the point of consumption, not the storage method per se. Shelf-stable products with validated stability data are not inferior to refrigerated products. Check the manufacturer's stability data where available, and avoid products that have been stored improperly (e.g. left in direct sunlight).

What is the difference between probiotics and prebiotics?

Probiotics are live micro-organisms that, when consumed in adequate amounts, confer a health benefit. Prebiotics are non-digestible food components — typically fermentable fibres such as inulin, fructooligosaccharides (FOS), or galactooligosaccharides (GOS) — that selectively feed beneficial bacteria already present in the colon. In other words, probiotics introduce bacteria; prebiotics feed them. The two are complementary, and some products combine both (called synbiotics). Prebiotic fibre from dietary sources (onions, garlic, leeks, chicory, Jerusalem artichoke) is likely sufficient for most people without supplementation.

Can probiotics be harmful?

For the vast majority of healthy adults, probiotics present no serious safety concerns. A 2023 expert consensus paper in Gut Microbes (Merenstein et al.) concluded that established practices adequately address traditional probiotic safety for healthy populations. However, caution is warranted in specific groups: immunocompromised individuals (including those on chemotherapy or immunosuppressants), critically ill patients, premature infants, and elderly individuals with multiple comorbidities. Rare but serious cases of bacteraemia (bacteria entering the bloodstream) and fungaemia have been reported in these populations. Healthy individuals may experience transient bloating or gas when starting a new probiotic; this typically resolves within a week or two.

Are probiotics safe and effective for children?

The ESPGHAN 2023 position paper provides the most comprehensive strain-specific guidance for paediatric use. L. rhamnosus GG and S. boulardii received the strongest recommendations for acute gastroenteritis and antibiotic-associated diarrhoea in children. Other commonly sold paediatric strains received no recommendation due to insufficient strain-specific evidence. Probiotics for necrotising enterocolitis prevention in preterm infants show promise in Cochrane review data but are not yet recommended as routine clinical practice. Parents should use only products with named strain designations and consult a paediatrician before use in infants or unwell children.

Do probiotics work for inflammatory bowel disease (IBD)?

The evidence for IBD is more limited than for IBS. A 2024 meta-analysis in the United European Gastroenterology Journal (Estevinho et al., overview of 22 systematic reviews and 45 RCTs) found probiotics beneficial for ulcerative colitis remission maintenance and pouchitis prevention, with multi-strain formulations appearing superior. No significant benefit was demonstrated for Crohn's disease. For active ulcerative colitis, evidence was inconclusive. IBD is a serious condition requiring medical supervision; probiotics should be considered an adjunctive intervention only and discussed with a gastroenterologist.

What CFU count should I look for in a probiotic?

CFU (colony-forming units) is a measure of the number of viable micro-organisms in a dose. While it seems intuitive that more is better, the clinical evidence does not fully support this. The 2022 network meta-analysis in Frontiers in Cellular and Infection Microbiology found that probiotic dose did not significantly influence IBS outcomes. Conversely, the BMJ Open 2021 meta-analysis on antibiotic-associated diarrhoea found higher doses were associated with greater protective effects. As a general guide, most clinically studied products use formulations in the range of 1–50 billion CFU per dose. A product with 100 billion CFU but no named strain designation provides less meaningful information than one with 5 billion CFU of a well-characterised, evidence-backed strain.

Can probiotics help with mental health or anxiety?

This is an emerging area of research involving what is termed the gut-brain axis. A 2025 meta-analysis in Nutrition Reviews (Asad et al.) found that probiotics demonstrated a significant reduction in depression symptoms in clinically diagnosed populations, and produced moderate improvements in anxiety scores. The authors recommended probiotics as an adjunctive intervention alongside standard psychiatric treatment. The evidence is early-stage: most trials are small, strain-specific data are limited, and effect sizes are modest. Probiotics should not be used as a replacement for evidence-based mental health treatment. However, the gut-brain axis is a scientifically credible mechanism, and research is ongoing.

Probiotics: Do They Work? What the Evidence Says for IBS, Antibiotics & More

Nutripedia Research Team14 April 2026

Probiotics are among the most purchased supplements in the UK, yet most products on shelves have no clinical evidence for their specific strains. Here is what the research actually shows — and why the strain on the label matters more than the CFU count.

Not medical advice

Nutripedia summarises published peer-reviewed research. This content is for informational purposes only and is not a substitute for professional medical advice. Product mentions are not endorsements.

What Are Probiotics — and Why Do Strains Matter?

The World Health Organisation defines probiotics as 'live micro-organisms that, when administered in adequate amounts, confer a health benefit on the host.' That definition contains two critical qualifiers: adequate amounts, and a health benefit. Both are strain-dependent. A probiotic strain is identified by three parts: genus, species, and a strain designation — for example, Lactobacillus rhamnosus GG. The 'GG' is the strain. Clinical evidence generated for L. rhamnosus GG does not transfer to L. rhamnosus Lcr35, let alone to a generic 'Lactobacillus blend.' This is not a technicality — it is the central problem with the probiotic supplement market. Most commercially sold probiotics in the UK are multi-strain blends formulated without reference to a specific indication or clinical endpoint. The World Gastroenterology Organisation (WGO) 2024 global guidelines are explicit: recommendations require at least one adequately powered, double-blind randomised controlled trial per strain-indication pair. The vast majority of strains in commercial products have no such trial. This article covers three areas where strain-specific evidence is strongest: irritable bowel syndrome (IBS), antibiotic-associated diarrhoea (AAD), and paediatric gut health. It also covers what to look for when purchasing a probiotic in the UK.

Our research is based on 139 peer-reviewed studies. View the full evidence database

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Nutripedia is an educational resource. Content is sourced from peer-reviewed studies and does not constitute medical advice. Product mentions are not endorsements. Consult a healthcare professional before starting any supplement.

Reviewed by

Archie Roberts

Founder, Nutripedia — ALDR Ltd

This page summarises published research from PubMed, NHS, EFSA, and SACN. It does not constitute medical advice; consult a qualified healthcare professional before changing any supplement regimen.

Last reviewed: 14 Apr 2026Methodology