Probiotic vs Prebiotic FAQ: 25 Expert Answers (2026)
Every answer cites primary evidence. Evidence-tier badge on claims where applicable.
Basics
What is the difference between probiotics and prebiotics?+
Probiotics are live beneficial microorganisms (bacteria or yeast) that, when consumed in adequate amounts, confer a health benefit. Prebiotics are non-digestible fibres that selectively feed beneficial gut bacteria already present. Probiotics add bacteria; prebiotics feed them. The ISAPP consensus definitions (2014 for probiotics, 2017 for prebiotics) are the authoritative references.
Do I need both probiotics and prebiotics?+
For general gut health, a varied diet with adequate fibre covers both. Targeted probiotic supplementation is most useful for specific conditions (post-antibiotic recovery, IBS-specific strains). A combined synbiotic approach may provide additive benefit. Our decision tool on the homepage gives a personalised evidence-based starting recommendation.
What is a synbiotic?+
A mixture comprising live microorganisms and substrate selectively utilised by host microorganisms that confers a health benefit (ISAPP 2020). Seed DS-01 and Ritual Synbiotic+ are consumer synbiotics. Evidence for synbiotics outperforming standalone probiotics is emerging and condition-specific.
What is a postbiotic?+
A preparation of inanimate microorganisms and/or their components that confers a health benefit (ISAPP 2021). Butyrate, heat-killed bacteria preparations, and bacterial cell wall fragments are examples. Unlike live probiotics, postbiotics carry no infection risk, which is their key safety advantage in vulnerable populations.
Foods
What foods are highest in probiotics?+
Kefir (10-20 billion CFU/cup, 30+ strains), yogurt with live cultures (1-10 billion CFU/serving), kimchi (L. kimchii, L. plantarum), unpasteurised sauerkraut, miso, natto, and tempeh. Check labels - pasteurised versions have no live cultures. Refrigerated, unpasteurised products only.
What foods are highest in prebiotics?+
Chicory root (~41g inulin per 100g), Jerusalem artichoke (16-20g inulin), dandelion greens (12-15g), garlic (9g FOS per 100g), oats (4g beta-glucan per 40g serving), cooked and cooled potato (3.5-5g resistant starch). Full guide with grams per serving at our prebiotic foods page.
Conditions
What is the best probiotic for IBS?+
Strain-specific. B. infantis 35624 (Align) has the strongest single-strain RCT evidence for IBS across subtypes. L. plantarum 299v is best for IBS-D. Multi-strain products VSL#3/Vivomixx and Symprove have RCT support. Low-FODMAP diet first; probiotics are an evidence-based adjunct.
What is the best probiotic after antibiotics?+
S. boulardii CNCM I-745 (Florastor) - start on Day 1, take simultaneously with antibiotics (it is a yeast, unaffected), 250-500 mg twice daily, continue for 2-4 weeks post-course. Add L. rhamnosus GG (Culturelle) 10 billion CFU/day, separated 2-3 hours from antibiotic doses.
What are prebiotics good for?+
Strongest evidence: increasing Bifidobacterium counts (inulin, FOS - Strong Evidence), improving stool regularity (psyllium - Strong Evidence), and lowering LDL cholesterol (beta-glucan, psyllium - Strong Evidence, EFSA-recognised). Emerging: improving immune function, supporting metabolic health, reducing colorectal cancer risk markers.
Can probiotics help with weight loss?+
The evidence is very weak. Probiotics for weight loss earns a Limited Evidence grade. L. gasseri SBT2055 shows some emerging signal in small studies. No probiotic supplement has strong RCT evidence for clinically significant weight loss in healthy adults. Diet and exercise remain the only evidence-based interventions.
What is the best probiotic for constipation?+
B. lactis BB-12 has the most consistent RCT evidence for adult constipation. L. reuteri DSM 17938 (BioGaia) is the top choice for children. Psyllium husk (not a probiotic - a prebiotic fibre) has NICE-endorsed evidence for constipation and is often more effective than any probiotic alone.
Safety
Are probiotics safe?+
Yes, for healthy adults and children. The NIH Office of Dietary Supplements notes rare but serious adverse events (bacteraemia, fungaemia) in immunocompromised individuals, those with central venous catheters, and very premature infants. For healthy people, the most common effects are transient gas and bloating for 1-2 weeks. Always consult a clinician if you are immunocompromised, have a CVC, or have recently had major surgery.
Can I take probiotics with antibiotics?+
Yes - and it is recommended. S. boulardii CNCM I-745 (Florastor) is a yeast unaffected by antibiotics; take it simultaneously from Day 1. Bacterial probiotics (LGG, L. acidophilus) should be separated by 2-3 hours from antibiotic doses. Continue for 2-4 weeks after the course ends.
Are probiotics safe during pregnancy?+
Generally yes. L. rhamnosus GG and B. lactis BB-12 have been studied in pregnancy with no adverse signals. The evidence for preventing eczema in high-risk infants (L. rhamnosus GG during late pregnancy) is emerging. Avoid high-CFU experimental strains without safety data in pregnancy. Always discuss with your obstetrician before starting any supplement.
Can children take probiotics?+
Yes. L. rhamnosus GG (Culturelle Kids) has Cochrane-level evidence for reducing acute diarrhoea duration in children and is ESPGHAN-recommended. L. reuteri DSM 17938 (BioGaia) has strong evidence for infant colic. Both are available in age-appropriate forms. Always check age-appropriate dosing - most paediatric products use 5-10 billion CFU.
Are prebiotics safe if I have IBS?+
Many prebiotic foods (garlic, onion, asparagus) are high-FODMAP and can trigger IBS symptoms. Low-FODMAP prebiotic alternatives: psyllium, acacia fibre, partially hydrolysed guar gum, cooked and cooled potato/rice, kiwifruit. Always follow the Monash University FODMAP diet protocol during the elimination phase.
Can probiotics cause side effects?+
Common: transient gas and bloating for 1-2 weeks as the microbiome adjusts. Rare but documented: in immunocompromised patients, bacteraemia (from Lactobacillus/Bifidobacterium) or fungaemia (from S. boulardii) can occur. S. boulardii fungaemia risk is highest in patients with central venous catheters. In healthy individuals, serious side effects are extremely rare.
Practical
How long does it take for probiotics to work?+
For acute diarrhoea, S. boulardii shows effect within 24-48 hours. For IBS and general gut symptoms, clinical trials use 4-8 weeks as the minimum assessment period. If no benefit after 6 weeks of consistent use at an appropriate dose, discontinue and consider alternative strains.
Do I need to refrigerate probiotics?+
Depends on the strain. Lactobacillus and Bifidobacterium strains require refrigeration; spore-forming Bacillus species (Bacillus coagulans, Bacillus clausii) and S. boulardii are naturally shelf-stable. Labels should state CFU at expiry, not at manufacture. If only 'at manufacture' is stated, actual counts at consumption may be much lower.
What is CFU and how much should I take?+
CFU = colony-forming units, the standard measure of viable microorganism count in a probiotic. Most clinical trials use 1-10 billion CFU per day. B. infantis 35624's pivotal IBS trial used exactly 1x10^8 (100 million) CFU - showing more is not always better. Match the dose to the clinical trial dose for the specific strain and condition.
Do prebiotics cause gas?+
Yes, especially when introduced rapidly. High-FODMAP prebiotics (inulin, FOS, garlic, onion) are fermented rapidly by colonic bacteria, producing hydrogen and CO2 gas. Increase prebiotic intake by 1-2 g per day per week to minimise gas. Low-FODMAP prebiotics (psyllium, acacia) are gentler and produce less gas.
When is the best time to take probiotics?+
Most evidence suggests taking probiotics with or just before a meal. Food buffers stomach acid, improving probiotic survival. Some strains (Bacillus species, S. boulardii) are sufficiently acid-resistant to be effective without food. The most important factor is consistency - daily use at any time beats sporadic use at the 'optimal' time.
Strains
What is the difference between L. acidophilus and B. lactis?+
L. acidophilus is a Lactobacillus species (colonises the small intestine) with evidence for lactose digestion and modest IBS bloating reduction. B. lactis is a Bifidobacterium species (colonises the large intestine) with stronger evidence for constipation and immune support. They have different habitats, mechanisms, and evidence profiles. Both are commonly found together in multi-strain supplements.
What is Lactobacillus rhamnosus GG?+
LGG (ATCC 53103) is the world's most-studied probiotic strain (800+ clinical papers). Isolated in 1983 by Gorbach and Goldin at Tufts. Cochrane-level evidence for reducing acute diarrhoea duration in children. Strong evidence for antibiotic-associated diarrhoea prevention. Available as Culturelle. Transient coloniser - effect stops when dosing stops.
What is Saccharomyces boulardii and why is it different?+
S. boulardii CNCM I-745 (Florastor) is the only probiotic yeast - not a bacterium. Key advantage: unaffected by antibiotics. Mechanism: secretes proteases that cleave C. difficile toxins A and B. Strong Evidence for antibiotic-associated diarrhoea prevention (54% risk reduction, Cochrane 2015), C. diff prevention, and traveller's diarrhoea. Can be taken simultaneously with antibiotics.