CFU (Colony-Forming Units)

What it is

CFU stands for colony-forming units, the standard way to measure the number of live, viable microorganisms in a probiotic. One CFU is a single microbe (or cluster) capable of growing into a colony. A label reading "10 billion CFU" means the product is guaranteed to contain that many live organisms — ideally through the end of shelf life, not just at the time of manufacture.

What it does

• Quantifies the live dose. CFU is the probiotic equivalent of "how much" — it tells you how many viable organisms you're taking.
• Allows comparison to research. Probiotic studies specify CFU doses, so CFU lets you check whether a product matches a studied amount.
• Signals viability guarantees. Good labels state CFU "at expiration," reflecting that live organisms die off over time.

What it does NOT do

• More CFU is not automatically better. The effective dose depends on the specific strain and the intended use — some benefits occur at a few billion CFU, and piling on more doesn't guarantee more benefit.
• CFU says nothing about which strain you're getting or whether that strain is right for your goal. Strain identity matters as much as the number.
• A high CFU at manufacture is meaningless if the organisms are dead by the time you take them — which is why "at time of manufacture" figures can be misleading.

Forms and how to use it

• Look for CFU counts guaranteed through the expiration date, and ideally storage guidance (some need refrigeration).
• Match the CFU and the named strain to what studies used, rather than chasing the biggest number.
• Pair CFU with strain-level labeling (genus, species, and strain designation) for a meaningful choice.

Sources

1. National Institutes of Health, Office of Dietary Supplements. Probiotics fact sheet for health professionals.
2. International Scientific Association for Probiotics and Prebiotics. Probiotics.