What is acidophilus? A plain guide

Lactobacillus acidophilus, usually shortened to "acidophilus," is a species of rod-shaped, lactic-acid-producing bacteria that lives naturally in the human gut and vagina and is also cultured into fermented dairy products like acidophilus milk and some yogurts. It's sold as a probiotic dietary supplement, typically as a capsule listing a colony-forming unit (CFU) count rather than a milligram dose.

Where it comes from and how it's used

Acidophilus belongs to a large group of bacteria classified as lactic acid bacteria — organisms that ferment sugars into lactic acid, which is what gives fermented dairy its tang and its acidic, spoilage-resistant environment. It's a normal part of the bacterial community in the human intestine and vagina, not something foreign introduced by a supplement. "Acidophilus milk" is ordinary milk that's been cultured with the bacteria, similar in concept to how yogurt is cultured with different species; it's a food product, not a supplement.

As a supplement, acidophilus is sold on its own or blended with other Lactobacillus and Bifidobacterium species (labels like "acidophilus & bifidus" or "acidophilus probiotic blend" describe a mix of species, not acidophilus alone). Capsule counts on the market range widely, from around 1 billion to over 100 billion CFU, and doses are described in colony-forming units because a live bacterial supplement is measured by how many organisms are viable, not by weight.

Why the exact strain matters

"Lactobacillus acidophilus" names a species, and a species contains many distinct strains — commercial products often add a strain code, like NCFM, DDS-1 or La-14, after the species name. Comparative genome analysis of Lactobacillus strains has found that closely related strains can differ meaningfully in their genes, and a functional trait identified in one strain isn't assumed to carry over to a different strain, even within the same species. That's the reason a careful answer to "what does acidophilus do" has to say which strain a study actually used — a finding for NCFM doesn't tell you what DDS-1 does, and neither tells you what an unnamed "L. acidophilus" on a generic label does. Our acidophilus benefits review covers what's actually been tested, strain by strain.

What it isn't

Acidophilus is one species among many bacteria described as "probiotics" — the term probiotic refers to a broad category of live microorganisms from many different genera and species (Lactobacillus, Bifidobacterium, Saccharomyces and others), so "probiotic" and "acidophilus" aren't interchangeable; acidophilus is one specific member of that category. It's generally treated as a normal, non-pathogenic part of the human microbiota in healthy people. Rare exceptions exist in people with specific health vulnerabilities, which is covered in full in who should not take acidophilus.

Food sources vs. a capsule

Beyond acidophilus milk, the species is used to culture some yogurts (check the label for "Lactobacillus acidophilus" among the live cultures — not every yogurt includes it), kefir, and some fermented vegetables. Food-based sources aren't standardized: a food label isn't required to state a CFU count the way a supplement label is, so there's no reliable way to compare "how much acidophilus" is in a cup of yogurt to a labeled capsule dose.

In the United States, acidophilus is regulated as a dietary supplement, not a drug. That means it hasn't gone through the FDA approval process required of medications, and a supplement label can describe an effect on the body's normal structure or function but can't claim to diagnose, treat, cure or prevent a disease. That framework is why a careful acidophilus label sticks to describing the organism and its CFU count rather than promising a specific health outcome.

Questions people ask

Is acidophilus the same as lactobacillus?

No. Lactobacillus is a large genus containing many species — acidophilus is one species within it. Other common Lactobacillus species sold as supplements include L. rhamnosus, L. reuteri and L. plantarum, each studied separately from L. acidophilus.

What is acidophilus milk?

It's cow's milk that has been fermented with Lactobacillus acidophilus cultures, giving it a mildly tart flavor. It's a food, sold in the dairy case, and is a different product from an acidophilus capsule or tablet.

What is acidophilus bifidus?

It's a supplement that combines Lactobacillus acidophilus with one or more Bifidobacterium species (often labeled "bifidus"). It's a two-or-more-species blend — research on a blend like this doesn't tell you what acidophilus does on its own, and vice versa.

Is lactobacillus acidophilus pathogenic or nonpathogenic?

In healthy people, it's treated as a normal, non-pathogenic resident of the gut and vaginal microbiota — it's the same organism found naturally in the body, not an outside threat. See who should not take acidophilus for the specific health situations where that changes.

What is acidophilus used for?

People buy it mainly for digestive comfort, to take alongside antibiotics, or for vaginal and urinary health. What the actual human trials tested — and where the evidence is thin or missing — is covered in the benefits review, not on this page.

What is the difference between acidophilus and probiotics?

"Probiotic" is the umbrella term for live microorganisms taken for a health purpose; it spans many genera and species. Acidophilus is one specific species within that category, so every acidophilus product is a probiotic, but most probiotics aren't acidophilus.

Related reading: how much acidophilus to take, how to choose a good acidophilus supplement, and acidophilus benefits: what the research does and doesn't show.