Why CFU Isn't the Whole Story (How to Read a Women's Probiotic Label)

Every women's probiotic on the UK market sells on the same number: CFU count. "50 billion live cultures." "10 billion CFU." "25 billion guaranteed." It's the headline figure on the front of the box, and it's almost entirely the wrong thing to compare.

CFU (Colony-Forming Units) is a count of how many live bacterial cells a capsule or sachet contains. It tells you the supplement has a lot of bacteria. It doesn't tell you which bacteria, whether the count still holds at the end of shelf life, or how that total is split between the strains in the formula. A big number spread across the wrong species is still the wrong product.

This article is about what to look at instead. It's the same checklist we used for our own Femme Flora label: every strain named, a strain code and a count for each one, and every other amount printed.

What does CFU actually mean (and why it's not enough)?

CFU stands for Colony-Forming Units. It's a microbiology measurement: the number of live bacterial cells in a sample capable of forming a visible colony when cultured under standard lab conditions. On a supplement label, CFU is the count of viable bacteria the manufacturer guarantees at a stated point in time, usually either at manufacture or at the best-before date.

The CFU number has two real problems as a buying signal.

The first is the manufacture-vs-best-before question. Many probiotic labels give CFU "at time of manufacture", which means the viable count when you swallow the capsule is lower, sometimes much lower. Bacteria die during storage. Heat, humidity and time all reduce the viable count. A more transparent label states CFU "at best-before" or "guaranteed until expiry", meaning the number on the box is the minimum count up to the best-before date.

The second is the composition question. CFU counts total live cells, not which species they are or how they're split. 50 billion CFU of a generic gut blend is not the same supplement as 50 billion CFU of a different formula. The total looks identical. The composition is not.

The same trap we covered in our piece on what "clinical dose" actually means applies here. The headline number on the label borrows credibility from research, but the formulation underneath determines whether the research applies.

The four things that matter more than CFU count

If CFU isn't the right comparison, what is? Four things, in this order.

1. Are the species named individually? A label that lists "Probiotic Blend 10 billion CFU: Lactobacillus species" tells you nothing about what's actually in the capsule. A label that lists each species separately (Lactobacillus rhamnosus, Lactobacillus reuteri, Lactobacillus crispatus and so on) is at least transparent. Proprietary blends in probiotics do the same job as proprietary blends in any other supplement: they stop you seeing how much of each component is in the capsule.

2. Does each strain have a code, and its own count? A species name such as Lactobacillus crispatus covers many different strains. The letters and numbers after it (the strain code) identify the exact strain in the bottle, and published research is usually about a specific strain, not a whole species. A count for each strain tells you how the headline total is split. Without it, one strain could make up almost all of the number on the front.

3. Is a prebiotic fibre included, and is the amount given? Probiotics are live bacteria. Prebiotics are fibres, such as XOS or inulin, that some formulas include alongside them. If one is included, the label should name it and give the amount.

4. Is the count stated at best-before? A count "at manufacture" describes the capsule on the day it was made. A count at best-before is the minimum you're getting for the life of the bottle.

Four checks. None of them visible from the front of the box. All of them visible from the ingredient list on the back.

Why many "women's probiotics" are actually gut probiotics

This is the part of the women's probiotic category that the marketing doesn't say out loud: many products positioned as "women's probiotics" contain the same species as the general-purpose gut probiotics on the next shelf. The packaging is different. The composition often isn't.

The vaginal microbiome is genuinely different from the gut microbiome. Most of the published research on taking live cultures by mouth has used a small number of specific strains. L. rhamnosus GR-1 and L. reuteri RC-14 are the best known, studied since a 2003 trial by Reid and colleagues. That research is strain-specific: it describes those strains, not live cultures in general.

The practical point is that a species list tells you whose microbiome a formula resembles. Lactobacillus crispatus is the species most often dominant in vaginal microbiome samples. A "women's probiotic" that contains only Lactobacillus acidophilus and Bifidobacterium species (both typical of the gut) has a gut-style composition, whatever the packaging says.

The useful question to ask of any women's probiotic is what the formulation contains, strain by strain. The answer is on the ingredient list.

How UK rules treat probiotic claims

Here's a fact that almost no UK probiotic marketing makes visible: no specific health claim has been authorised for any probiotic. Not one. The GB Nutrition and Health Claims Register, the UK's post-Brexit successor to the EU framework, contains no authorised health claims for any probiotic species or strain.

This has a practical consequence. Under UK advertising rules, "probiotic" is itself treated as a general health claim: a brand can only use it alongside a specific authorised health claim. That's why you'll often see UK labels describe the ingredient as "live cultures" instead. The cultures are a permitted ingredient; there is simply no authorised claim for what they do.

Several UK authorities are involved. The Food Standards Agency regulates probiotics as foods, not medicines. The Advertising Standards Authority polices what brands can claim about them. The Medicines and Healthcare products Regulatory Agency draws the line between food supplement and medicine. For more on how this works across the supplement category, see our Compliance policy and the Health Claims References page, where Evori publishes the authorised wording behind every claim we make.

What this means for buyers: no live-culture product in the UK can point to an authorised claim for the cultures themselves. Composition (which strains, how many of each, and guaranteed until when) is the part of the label you can check.

Four questions to ask before buying a women's probiotic

If the framework feels like a lot to remember, distil it to four questions:

1. Are the species individually named on the label? If the answer is "Lactobacillus and Bifidobacterium blend", the brand isn't telling you what's in the bottle.
2. Does every strain have a code and its own count? If only the total is given, you can't see how it's split.
3. Is a prebiotic fibre included, and is the amount given? If it's listed without an amount, you can't tell how much is there.
4. Is the count guaranteed to best-before? If it says "at manufacture", the number on the front is a starting point, not what you get.

One last thought

The CFU number on a probiotic label answers a question most buyers aren't actually asking. The questions that matter (what's in the formulation, strain by strain, and how much of each) are answered in the small print, not the big.

No health claim has been authorised for live cultures in the UK, and there's no minimum standard for what goes into a women's probiotic. That's why the back of the label is worth reading closely.

Frequently Asked Questions

What does CFU mean on a probiotic label?

CFU stands for Colony-Forming Units: the count of live bacterial cells in a serving capable of forming a visible colony under standard lab conditions. It's a measure of total live bacteria, not which bacteria or how the total is split between strains. Composition matters more than count.

What is a strain code on a probiotic label?

The letters and numbers after a species name, such as the code after Lactobacillus crispatus, identify the exact strain. Many different strains share one species name, and research is usually about a specific strain. A label that gives a code and a count for every strain tells you exactly what's in the bottle.

What's the difference between a gut probiotic and a vaginal probiotic?

The composition. Gut probiotics typically contain species found in the digestive tract, such as Lactobacillus acidophilus and Bifidobacterium species. Some formulas marketed to women also include species found in the vaginal microbiome, particularly Lactobacillus crispatus, Lactobacillus gasseri and Lactobacillus jensenii. The ingredient list tells you which kind you're looking at.

How do probiotics taken orally reach the vagina?

Research on this is strain-specific. A small number of strains, such as L. rhamnosus GR-1 and L. reuteri RC-14, have been studied for whether they can be detected in the vaginal tract after being taken orally. Findings for one strain don't transfer to another, so they can't be read as a general statement about live cultures.

Is L. crispatus included in most women's probiotics?

Not usually. Many UK women's probiotics contain Lactobacillus rhamnosus, Lactobacillus reuteri or Lactobacillus acidophilus, but not L. crispatus, the species most often dominant in vaginal microbiome samples. Some products now include it, so check the ingredient list, and check for a strain code after the name.

Does the UK regulate probiotic health claims?

Yes. No specific health claim has been authorised for any probiotic species or strain on the GB Nutrition and Health Claims Register. Under UK advertising rules, "probiotic" is treated as a general health claim, which can only be used alongside a specific authorised claim. That's why many labels say "live cultures" instead.

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