Should I take probiotics every day?

It depends on what you're using them for. For antibiotic diarrhoea prevention, use during and 1–2 weeks after a course. For IBS, if a specific strain helps, ongoing use may be appropriate. General daily use without a specific indication has no clear evidential basis.

Are fermented foods as good as probiotic supplements?

Fermented foods (yoghurt, kefir, kimchi, miso) contain live microorganisms but in variable amounts and strains. They provide nutritional benefits beyond microorganisms. They are a valuable part of diet but are not a reliable substitute for a clinically studied strain at a specific dose.

Do probiotics need to be refrigerated?

It depends on the strain and formulation. *Lactobacillus* and *Bifidobacterium* species generally require refrigeration; *Saccharomyces boulardii* (a yeast) is shelf-stable. Check manufacturer storage guidelines, and choose products with viability guarantees.

evidence

Probiotics: What the Evidence Actually Supports (and What It Doesn't)

Probiotics are heavily marketed but highly strain-specific. This guide explains which indications have solid evidence, what strains are involved, and why 'probiotic' on a label tells you almost nothing.

3 min read · 583 wordsReviewed May 2026

Close-up of the word 'probiotic' crafted from letter tiles on a wooden surface. - Evidence evidence guide for Probiotics: What the Evidence Actually Supports (and What It Doesn't) — Photo by Alicia Harper on Pexels · Pexels License

Quick Answer

Probiotics have consistent evidence for a narrow set of conditions: antibiotic associated diarrhoea (particularly Lactobacillus rhamnosus GG and Saccharomyces boulardii ) and some types of infectious diarrhoea. Evidence for IBS, immunity, mental health, and general "gut health" is more mixed and highly strain specific.

Key Takeaways

01---
02Most commercial probiotic labels list genus and species but not strain designation. This makes it nearly impossible to match a product to the clinical evidence.
03| Condition | Strains With Evidence |
04|---|---|
05| Antibiotic-associated diarrhoea | *L. rhamnosus* GG, *S. boulardii* |

Quick Answer

Probiotics have consistent evidence for a narrow set of conditions: antibiotic-associated diarrhoea (particularly Lactobacillus rhamnosus GG and Saccharomyces boulardii) and some types of infectious diarrhoea. Evidence for IBS, immunity, mental health, and general "gut health" is more mixed and highly strain-specific. The strain, dose, and formulation on the label determine whether a product matches what was studied.

Why Strain Specificity Is Everything

A probiotic is not like vitamin C where any source provides the same molecule. Each strain (e.g., Lactobacillus rhamnosus GG vs Lactobacillus rhamnosus ATCC 53103) has unique mechanisms, colonisation behaviours, and evidence profiles. A study showing benefits for strain X tells you nothing about strain Y, even from the same genus and species.

Most commercial probiotic labels list genus and species but not strain designation. This makes it nearly impossible to match a product to the clinical evidence.

Conditions With Consistent Evidence

Condition	Strains With Evidence
Antibiotic-associated diarrhoea	L. rhamnosus GG, S. boulardii
Infectious diarrhoea (children)	L. rhamnosus GG, L. reuteri DSM 17938
C. difficile recurrence (adjunct)	S. boulardii
Pouchitis	VSL#3 multi-strain blend
Necrotising enterocolitis prevention (neonates)	Multi-strain; studied in NICU settings

Conditions With Moderate/Preliminary Evidence

Condition	Notes
IBS symptoms	Mixed; some benefit for bloating and transit time but inconsistent across strains
Ulcerative colitis maintenance	VSL#3 shows some benefit; not all products equivalent
Eczema prevention (infants)	Some evidence for L. rhamnosus GG in high-risk infants
Vaginal microbiome	Lactobacillus spp. show plausible effects; RCT evidence limited

What Probiotics Probably Don't Do (Based on Current Evidence)

Boost immunity in healthy adults: General immune claims lack consistent RCT support.
Improve mental health meaningfully: The gut-brain axis is real, but probiotics as antidepressants is a significant overreach of available evidence.
Colonise the gut long-term: Most probiotic strains do not permanently colonise; they transit through and their effects end when supplementation stops.

CFU: What Does That Number Mean?

CFU (Colony Forming Units) is a measure of viable bacterial count. More CFU is not always better — most studied doses are in the range of 1–10 billion CFU/day. Product stability is a bigger concern than raw CFU numbers; many supplements have far fewer viable organisms by the time they reach consumers than what is printed on the label.

Look for products with an expiration date guarantee (not just manufactured date) and appropriate storage conditions.

Safety Notes

Probiotics are generally safe in healthy adults.
Immunocompromised individuals: Risk of bacterial or fungal translocation is a real concern. Several case reports of probiotic sepsis in immunocompromised patients exist. Avoid without medical supervision.
SIBO (Small Intestinal Bacterial Overgrowth): Some evidence suggests probiotics may worsen SIBO symptoms in some patients.
During or after antibiotic courses, take probiotics at least 2 hours apart from antibiotics.

Practical Next Steps

If taking antibiotics: L. rhamnosus GG (e.g., Culturelle) or S. boulardii (e.g., Florastor) are the best-supported choices for preventing diarrhoea.
For general gut health claims, save your money unless you have a specific condition with evidence-backed strain choices.
If you choose a general probiotic: look for a product with a full strain designation and expiration viability guarantee.
For IBS: A dietitian-led low-FODMAP diet has stronger evidence than probiotics for most people.

FAQ

Frequently Asked Questions

Medically reviewed

Last reviewed May 9, 2026 by Migaku Editorial Team