Probiotics

What it is

Probiotics are live microorganisms, mostly bacteria such as Lactobacillus and Bifidobacterium, plus a yeast called Saccharomyces boulardii. They are sold as capsules, powders, drinks and yoghurts, and people take them for bloating, irregular bowels, after antibiotics, or as general gut support.

The key point is that the evidence belongs to individual strains, not to the whole category. A strain that helped with one problem in one trial says nothing about the thousands of other products on the shelf. Reviews tend to find modest benefits in some situations and no clear benefit in many others.

Fermented foods such as live yoghurt, kefir and sauerkraut contain live microbes too, but they are a different thing from a dosed, named-strain product.

How it works

Most strains do not permanently colonise the gut. They pass through, and while they are there they may compete with other microbes, produce short-chain fatty acids and other compounds, and interact with the immune cells lining the gut. Effects usually fade soon after you stop.

What the evidence says

• Moderate Reducing antibiotic-associated diarrhoea
  Some strains, notably S. boulardii and certain Lactobacillus strains, show a modest reduction in risk in trial reviews; it is strain- and dose-dependent.
• Limited Irritable bowel symptoms
  Some people report less bloating or pain with particular strains, but results are mixed and effects are generally small.
• Moderate Infectious diarrhoea in children (shortening duration)
  Certain strains have been shown to shorten episodes by a modest amount, alongside rehydration rather than instead of it.
• Weak General immune support
  Some trials suggest slightly fewer minor infections, but the data are inconsistent and the strains vary.
• Weak Mood and mental health
  An interesting research area (the gut-brain axis) but not yet a proven use in healthy people.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Dose is counted in colony-forming units (CFU), but a bigger number is not automatically better. Match the strain and dose to what was studied.
• Take it consistently for at least 4 weeks before deciding whether it is doing anything. If nothing changes, stopping is reasonable.
• During antibiotic courses, bacterial products are often taken a few hours apart from the antibiotic dose; check the product leaflet.
• S. boulardii is a yeast, so antibiotics do not affect it in the same way.

Forms and what to look for

• Look for the full strain name on the label (genus, species and strain code), not just "Lactobacillus blend".
• Capsules stating CFU "at end of shelf life" are better than "at time of manufacture", since counts fall over time.
• Fermented foods are a cheap option but the live count and strains are not standardised.
• Synbiotic products (probiotic plus prebiotic) are common; they are not clearly better than a single well-studied strain.

What to expect, and when

• Gut symptoms: if a strain works for you, you might notice a difference within 1 to 4 weeks.
• Effects generally fade within days to weeks of stopping because most strains do not settle permanently.
• Do not expect a dramatic effect; many people notice nothing.

Side effects

• Gas, bloating and loose stools in the first few days are the most common complaints.
• Some people find symptoms get worse, for example with small intestinal bacterial overgrowth or histamine sensitivity.
• Rarely, mild allergic reactions to ingredients such as dairy or soy.
• Very rarely, the live organisms cause infection in people who are severely unwell or immunosuppressed.

Interactions

• Antibiotics can reduce the survival of bacterial probiotics; spacing doses is common advice.
• Immunosuppressant drugs: take advice first because of the rare risk of infection from live organisms.
• Antifungal drugs may reduce the effect of S. boulardii.
• People with central venous catheters or severe illness should not self-start probiotics.

Who should avoid it

• People who are immunosuppressed, critically ill, or have a central line should only use them under medical advice.
• Premature babies and very sick infants: only under hospital guidance.
• Anyone with severe pancreatitis or a damaged gut lining should check with a clinician first.
• Seek medical advice rather than self-treating persistent diarrhoea, blood in the stool or unexplained weight loss.

Risks and safety

• Supplements are regulated as food in the UK, so quality and live counts vary and are not routinely checked before sale.
• Some tested products have contained fewer live organisms or different strains than the label claims.
• Health claims about probiotics are tightly controlled in the UK and EU, which is why many labels say little beyond "live cultures".

Buying well: quality and testing

• Choose products that name the exact strain and state CFU through the end of shelf life.
• Look for strains with published human trials for the thing you care about.
• Check storage instructions; heat kills the organisms.
• Independent testing is less common for probiotics, so a reputable brand matters more than a long ingredient list.

Bottom line

Probiotics are a reasonable thing to try for a specific problem such as antibiotic-related diarrhoea or IBS-type symptoms, as long as you pick a studied strain and give it a month. They are not a general health upgrade for most healthy people, and a varied diet with plenty of fibre and some fermented foods does similar work more cheaply.