What Does CFU Mean on a Probiotic Label?

The CFU number is the most prominent figure on a probiotic label and one of the least understood. It is a count, not a measure of benefit.

What a CFU Count Represents

A colony-forming unit is a unit used in microbiology to estimate how many viable microorganisms are present. One CFU can arise from a single cell or from a clump of cells that grows into one colony, so the count is an estimate of viable cells rather than an exact number.

The count only means something with two other pieces of information: which strains are in the product, and at what point in the product's life the count applies.

"At Time of Manufacture" vs "Through Expiration"

Labels use different conventions:

• At time of manufacture means the count was accurate when the product was made. Viability declines over time, so the number at purchase is lower.
• Through end of shelf life means the label guarantees the count until the expiration date, which requires an overage at manufacture.
• At time of use, less common, is the most consumer-friendly convention.

When two products show the same number, the one guaranteeing it through expiration is providing more.

Why More Is Not Always Better

Probiotic effects are strain-specific. Different strains have different documented roles, and the amount studied varies by strain and by purpose. A product with 50 billion CFU of a strain studied at 1 billion is not necessarily more effective, and it may cost more for no added benefit.

Very high counts can also cause temporary gas or bloating in some people, particularly when a product is new to them.

Reading the Rest of the Label

1. Genus, species and strain. "Lactobacillus acidophilus" is genus and species; a strain designation follows the species name.
2. Number of strains. Multi-strain products list each one, sometimes in a blend with a combined CFU.
3. Serving size. A count per capsule and a count per serving can differ.

Storage instructions. Some strains require refrigeration; others are shelf-stable.

Other ingredients. Look for allergens and for added sugars in chewables and gummies.

Storage and Viability

Heat and moisture reduce viability. Keep the product as the label directs, keep the lid tight and do not store bottles in a hot car or a steamy bathroom. If a product smells or looks different from when it was opened, discard it.

What CFU Cannot Tell You

CFU does not tell you whether a product will help with a specific concern, whether the strains are the ones studied for that concern, or whether the product has been independently verified. It is a quantity, and quantity only matters after the strain and the evidence.

Anyone who is immunocompromised, seriously ill or considering a probiotic for a medical condition should talk with a healthcare professional first.

For related reading, see prebiotics vs probiotics and how to read a supplement facts panel.