Probiotic
Also called: live bacteria supplement, good bacteria
Probiotics are live bacteria taken in supplements or fermented foods that may benefit gut health when taken in adequate amounts. The evidence is strain-specific, not species-specific, which means a label that just says Lactobacillus is not enough. Most healthy people do not need a probiotic, but specific strains have good evidence for antibiotic-related diarrhoea, IBS, and infant colic.
What counts as a probiotic
The official definition from the World Health Organization is live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. The key parts are live, adequate amount, and health benefit. Many products on shelves do not meet all three.
Strains with the strongest evidence
- Lactobacillus rhamnosus GG: reduces antibiotic-associated diarrhoea and shortens infectious diarrhoea in children.
- Saccharomyces boulardii (a yeast, not a bacterium): same antibiotic and infection picture, also studied in C. difficile prevention.
- Bifidobacterium infantis 35624: studied in IBS for bloating and gas, with modest improvements over placebo.
- Visbiome / Vivomixx (the multi-strain blend originally sold as VSL#3 before the 2016 reformulation, when the original strains moved to these brands): studied in pouchitis and ulcerative colitis under specialist care.
- Lactobacillus reuteri DSM 17938: reduces crying time in colicky breastfed babies.
What probiotics do not do
They do not colonise. Most probiotic strains pass through the gut and are gone within a week of stopping. They do not work for everyone. Response is highly individual, with up to a third of users seeing no effect even on well-evidenced strains. They do not replace fibre. Feeding the bacteria you already have is more important than adding new ones.
How to pick one
- Look for the strain code on the label, not just the species. Lactobacillus rhamnosus GG is different from any other Lactobacillus rhamnosus.
- Check the colony-forming unit (CFU) count at end of shelf life, not at manufacture. Many products lose viability over time.
- Match the strain to the reason. Antibiotic recovery, IBS bloating, and infant colic each have their own evidence base.
- Give it 4 to 6 weeks. If nothing has changed, it is unlikely to. Stop and try a different strain or stop altogether.
Fermented foods are not the same
Yoghurt, kefir, kimchi, sauerkraut and kombucha contain live bacteria but rarely meet the dose, strain identification, or stability requirements of clinical probiotics. They are useful for general gut diversity, not for treating specific conditions. They are also cheaper and more pleasant than capsules.
Common questions
- Should I take a probiotic every day?
- Probably not without a specific reason. The evidence is for short courses targeting a particular issue (antibiotic recovery, IBS flare, traveller diarrhoea). Daily long-term use in healthy adults has not been shown to improve any outcome that matters.
- What is the difference between a probiotic and a prebiotic?
- A probiotic is live bacteria. A prebiotic is food (usually fibre) that feeds the bacteria already living in your colon. Most people get more benefit from prebiotics, because they support the bacteria that already work for you.
- Can probiotics cause side effects?
- Mild bloating and wind in the first week are common as your gut adjusts. These usually settle. Probiotics are generally safe in healthy adults but can be risky for people with severe immune suppression or central lines, where translocation has been reported.
- Do probiotics survive stomach acid?
- Some strains do, some do not. Saccharomyces boulardii is naturally acid-resistant. Many Lactobacillus and Bifidobacterium strains rely on enteric coatings or buffer formulations to survive. The label should mention this. If it does not, assume most of the dose is lost.