Evidence explainerProbiotics
Probiotic CFU Explained: Why the Strain Name Matters More Than the Billions
Probiotic labels shout big CFU numbers. ODS says the strain designation and the count at the end of shelf life matter more, and that more CFU is not necessarily better.
Key takeaways
- ODS says labels must list the total weight of microorganisms, which includes dead cells; CFU may be listed voluntarily [1].
- A full strain name has a genus, species, subspecies and an alphanumeric strain designation [1].
- Look for the CFU count at the end of shelf life, not at manufacture; more CFU is not necessarily more effective [1].
- Two strains of the same species can test differently: LGG lowered antibiotic-associated diarrhea, while an R0011 combination did no better than placebo in children with acute stomach illness [2][3].
Safety at a glance
From our Probiotics monograph. Published interactions and cautions only, each with its source.
Who should be careful
- Preterm infants. FDA warns that preterm infants given probiotic products in hospitals are at risk of invasive, potentially fatal disease or infection from the bacteria or yeast they contain, including one infant death in 2023 and more than two dozen other adverse events since 2018.[10]
- Severely ill or immunocompromised people. Bloodstream bacterial and fungal infections linked to probiotics have occurred mostly in severely ill or immunocompromised people; the WGO advises restricting use in these groups to strains and indications with proven efficacy (ODS). In one ICU study, bloodstream Lactobacillus matched the probiotic given.[9][7]
- People with central lines or in intensive care. Many reported Saccharomyces fungemia cases involved ICU patients, central venous catheters, tube or IV feeding, or broad-spectrum antimicrobials (ODS).[9]
- Before you start. Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[9]
The evidence, graded
Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.
- Grade BWorsenedDiarrhea risk during antibiotic treatmentAntibiotic-associated diarrhea fell from 22.4% to 12.3% (RR 0.49; low quality of evidence). Separately, the reduction was significant in children (5 RCTs, n=445) but not in adults (6 RCTs, n=863), except in a subset on H. pylori eradication therapy.
- No effectNo effectModerate-to-severe illness and diarrhea duration in children with acute stomach illnessModerate-to-severe gastroenteritis within 14 days: 26.1% with the probiotic vs 24.7% with placebo (odds ratio 1.06). No differences in diarrhea or vomiting duration, unscheduled visits or adverse events.
- No effectNo effectModerate-to-severe illness and diarrhea duration in children with acute stomach illnessModerate-to-severe gastroenteritis within 14 days: 11.8% with LGG vs 12.6% with placebo (RR 0.96). No differences in diarrhea duration (median 49.7 vs 50.9 hours), vomiting, day-care absence or household spread.
What CFU means on the label
CFU stands for colony-forming units, a count of live microbes. ODS says supplement labels must list the total weight of microorganisms, which includes dead cells, and CFU can be added voluntarily [1].
ODS advises looking for the CFU count at the end of the product's shelf life rather than at manufacture, and notes that a higher CFU count is not necessarily more effective [1].
Read the strain designation
Probiotics are identified by genus, species, subspecies and an alphanumeric strain designation, and some effects are strain-specific [1].
Two Lactobacillus rhamnosus strains show why. LGG lowered antibiotic-associated diarrhea in pooled trials [2]. A combination of L. rhamnosus R0011 and L. helveticus R0052 did no better than placebo in 827 children with acute stomach illness (26.1% vs 24.7% moderate-to-severe illness) [3].
How many CFU?
There is no single number. In the LGG antibiotic trials, ODS describes amounts from 4 x 10^8 to 12 x 10^10 CFU [2][1]. A large US trial gave children LGG at 1 x 10^10 CFU twice daily for 5 days and found no benefit for acute stomach illness [4].
Match the strain and amount to the trials for the specific use. See probiotics with antibiotics.
Quick answers
Is 10 billion CFU enough?
The right amount depends on the strain and use; LGG antibiotic trials ranged from 4 x 10^8 to 12 x 10^10 CFU [2], and ODS says more CFU is not necessarily better [1].
Does a probiotic need refrigeration?
Not always. ODS reports World Gastroenterology Organisation advice to check the label for storage conditions, since some products need refrigeration and others can be kept at room temperature [1].
Are probiotics safe for everyone?
ODS says side effects are usually minor in healthy people, but serious infections have occurred mostly in severely ill or immunocompromised people [1].
Keep reading
Full monographs
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- Probiotics With Antibiotics: Two Strains Have the Data, and Strain Is Everything
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References
- [1]Probiotics: Fact Sheet for Health Professionals (Updated March 25, 2025). NIH Office of Dietary Supplements, 2025.
- [2]Systematic review with meta-analysis: Lactobacillus rhamnosus GG in the prevention of antibiotic-associated diarrhoea in children and adults. Aliment Pharmacol Ther, 2015.
- [3]Multicenter Trial of a Combination Probiotic for Children with Gastroenteritis. N Engl J Med, 2018.
- [4]Lactobacillus rhamnosus GG versus Placebo for Acute Gastroenteritis in Children. N Engl J Med, 2018.