Probiotics are live micro-organisms taken to support the balance of bacteria in the gut. They are most commonly used alongside antibiotics, for diarrhoea, for irritable bowel syndrome and for infant colic. The evidence is genuinely mixed, and it depends heavily on the specific strain.
What is Probiotics?
A probiotic is defined as live micro-organisms which, when given in adequate amounts, confer a health benefit. Two words in that definition do most of the work — live, and adequate amounts.
The gut contains an enormous population of bacteria that help digest food, produce certain vitamins, occupy space that would otherwise be available to harmful organisms, and interact with the immune system. Illness, antibiotics and diet can disturb that balance, and probiotics aim to help restore it.
The single most important thing to understand about probiotics is that effects are strain-specific.
Not “probiotics” in general, but a particular strain, at a particular dose, for a particular condition. A strain is identified by three parts:
| Part | Example | Meaning |
|---|---|---|
| Genus | Lactobacillus | The broad group |
| Species | rhamnosus | The species |
| Strain | GG | The specific strain |
A different strain of the same species is not automatically equivalent. Lactobacillus rhamnosus GG has evidence for antibiotic-associated diarrhoea; Saccharomyces boulardii — actually a yeast — has evidence for certain diarrhoeas; Lactobacillus reuteri DSM 17938 has evidence for infant colic.
This is why a general recommendation to “take a probiotic” is weaker than it sounds, and why comparing products means comparing strains and CFU counts rather than brand claims.
Prebiotics are different. They are fibres that feed existing gut bacteria rather than adding new ones. A synbiotic contains both.
Probiotics are not registered as medicines in the same way, so they generally do not appear in a medicine registry. That is normal for the category, and it means your pack’s label is the authoritative source.
What is Probiotics commonly used for?
- Antibiotic-associated diarrhoea — the best-supported use. Antibiotics disturb normal gut bacteria, and certain strains reduce the likelihood of diarrhoea following a course.
- Infectious diarrhoea, where some strains modestly shorten the illness, particularly in children. Rehydration remains the priority.
- Irritable bowel syndrome, where some people get symptom relief, though results vary considerably between individuals and strains.
- Infant colic, where L. reuteri has reasonable evidence in breastfed infants.
- General digestive comfort — bloating, irregularity.
- Vaginal health, using specific strains.
- **Preventing *C. difficile*** in some hospital settings.
- Restoring gut bacteria after illness.
The evidence is strong for some of these and weak for others, and results depend on the strain used. Probiotics are not a treatment for a diagnosed condition unless a doctor has advised it.
“Supporting immunity”, “detox” and weight-loss claims should be treated cautiously. The gut and immune system do interact, but that does not translate into demonstrated benefit from any particular supplement.
Food sources exist too — yoghurt with live cultures, kefir, sauerkraut, kimchi and other fermented foods. These are a reasonable and inexpensive starting point, though the strains and quantities are less defined than in a supplement.
Who commonly looks for Probiotics and why
- People prescribed antibiotics, often on a pharmacist’s suggestion — the most common reason.
- People with irritable bowel syndrome, bloating or irregular bowels.
- Parents of a colicky baby, or a child with diarrhoea.
- People recovering from a stomach bug.
- Travellers, hoping to reduce the chance of traveller’s diarrhoea.
- Women with recurrent thrush or urinary symptoms.
- People comparing brands such as Entiro, Probiflora or Reuterina — where the differences come down to strains, dose and format.
- People generally interested in gut health, often after reading about the microbiome.
What to check on the label before taking Probiotics
- Which strains it contains — genus, species and strain designation. The most important item on the label.
- The number of live organisms (CFU) per dose, usually in billions.
- Whether the CFU count is guaranteed to the expiry date, or only at manufacture. This is a meaningful difference — organisms die off over time, and a count “at time of manufacture” tells you little about what you are taking.
- Storage instructions. Some probiotics need refrigeration; others are shelf-stable. Storing a refrigerated product at room temperature can kill the organisms, leaving a supplement that does nothing.
- The expiry date.
- Whether it contains prebiotics, which can cause wind initially.
- Allergens — dairy, soya, gluten — and whether it suits a vegetarian or vegan diet.
- The intended age, particularly for infant products.
Safety, interactions and when to seek professional advice
For most healthy people, probiotics are well tolerated. Common effects are wind, bloating and mild abdominal discomfort in the first days, usually settling as the gut adjusts.
Some groups should not take probiotics without medical advice, because introducing live organisms carries genuine risk where the immune system or gut barrier is compromised. There are documented cases of probiotic organisms causing bloodstream infections in vulnerable patients.
Speak to a doctor first if you or the person taking them:
- Have a weakened immune system — from HIV with a low CD4 count, chemotherapy, immunosuppressant medicines, or after an organ transplant.
- Are seriously ill or in hospital, particularly in intensive care.
- Have a central venous catheter or other indwelling line.
- Have short bowel syndrome, or significant damage to the gut lining.
- Have had recent gastrointestinal surgery.
- Are a premature or very young infant.
- Have acute pancreatitis.
- Have a heart valve abnormality or prosthetic valve.
Timing around antibiotics: separate the probiotic from the antibiotic by two to three hours, so the antibiotic does not simply kill the probiotic organisms. Continuing for a period after the course is common practice.
See a doctor rather than self-treating if you have:
- Diarrhoea lasting more than two days, or that keeps returning.
- Blood or mucus in the stool.
- Severe abdominal pain, or a swollen, tender abdomen.
- A high fever with diarrhoea.
- Signs of dehydration — dark or scanty urine, dizziness on standing. In a baby: drowsiness, no wet nappies, no tears when crying.
- Unintended weight loss.
- A persistent change in bowel habit, particularly over 45.
- Severe diarrhoea during or after antibiotics, which may indicate a C. difficile infection.
Rehydration matters more than any supplement in diarrhoea. Oral rehydration solution replaces the fluid and salts whose loss actually causes harm.
Related supplement guides
Frequently asked questions
What are probiotics used for? Supporting the balance of gut bacteria — most commonly alongside antibiotics, for diarrhoea, irritable bowel syndrome and infant colic.
Do probiotics actually work? Evidence is strongest for antibiotic-associated diarrhoea and some childhood diarrhoea, and mixed for other uses. Effects depend on the specific strain and dose.
What is probiotics used for in women? Beyond general gut health, specific strains are used for vaginal health and recurrent thrush. Evidence varies by strain.
Should I take them with antibiotics? This is the best-supported use. Separate the probiotic from the antibiotic by two to three hours so the antibiotic does not kill the organisms.
What does CFU mean? Colony-forming units — the number of live organisms per dose. Check whether the count is guaranteed to the expiry date or only at manufacture.
Do they need refrigerating? Some do, some do not. Check the label — storing a refrigerated product warm can kill the bacteria and leave it ineffective.
Can I get them from food instead? Yoghurt with live cultures, kefir, sauerkraut and kimchi are reasonable sources, though strains and quantities are less defined than in a supplement.
Are probiotics safe for everyone? Not quite. People with weakened immune systems, serious illness, indwelling lines or significant gut damage should ask a doctor first.