Probiotics

Strong strain-specific evidence for antibiotic-associated diarrhoea, infectious diarrhoea, and some IBS symptoms. Little support for general wellness use in healthy adults, and effects do not transfer between strains.

The single most important fact about probiotics is that "probiotic" is not a thing. It is a category, like "drug". Asking whether probiotics work is like asking whether pills work — the answer depends entirely on which organism, at what dose, for what condition.

Where specific strains have been tested against specific conditions, the results can be good. *Saccharomyces boulardii* and certain lactobacilli meaningfully reduce antibiotic-associated diarrhoea. Several strains shorten infectious diarrhoea in children. Particular formulations improve IBS symptoms and reduce necrotising enterocolitis risk in preterm infants — the last one being a genuinely life-saving application. These are real clinical results with real strain names attached.

What does not follow is the shelf full of general-purpose products. A strain proven for antibiotic-associated diarrhoea tells you nothing about whether it improves mood, immunity, or "gut health" in a healthy adult. Effects are not a property of the genus, or even the species — they are properties of the individual strain, which is why labels list three-part designations like *Lactobacillus rhamnosus* GG.

Two findings complicate the picture further. Colonisation is highly individual: the same product takes root in some people's guts and passes straight through others, apparently determined by the microbiome already present. And in one striking study, probiotics taken after antibiotics actually *delayed* the return of the native microbiome compared with letting it recover on its own — a useful reminder that the intuitive story is not always the true one.

The practical position is narrow but usable. If you have a specific indication — a course of antibiotics, IBS, travellers' diarrhoea — look up which strain was studied for it and buy that strain at that dose. If you are a healthy adult buying a daily probiotic for general wellness, the evidence does not support it, and the money is better spent on the fermented foods and dietary fibre that feed whatever is already living in you. Quality control in the category is also poor: viable counts at expiry frequently fall short of the label.

How it works

Probiotics act by competing with pathogens for adhesion sites and nutrients, producing antimicrobial compounds and short-chain fatty acids, reinforcing the gut barrier, and signalling to gut-associated immune tissue. Most do not permanently colonise — they transit, act while present, and disappear within weeks of stopping.