A probiotic is useful for specific, bounded situations, above all diarrhoea: antibiotic-associated and acute infectious. Beyond that, the evidence is patchy or non-existent. If you are healthy and have no defined digestive problem, taking them “to look after your microbiota” has no scientific backing.
This does not mean they’re smoke. It means the benefit depends on the exact strain, the dose and the reason you’re taking them, and almost no pack on the shelf gives you that information in full.
What a probiotic actually is
The reference definition comes from an expert consultation convened by the FAO and the WHO in 2001: live microorganisms which, administered in adequate amounts, confer a health benefit on the host. All three parts of that sentence matter: live, in sufficient quantity, and with a demonstrated benefit.
Here comes the first surprise. In European Union food law there is no legal definition of a probiotic, no list of authorised probiotics and no specific requirements. The Spanish Agency for Food Safety and Nutrition confirms this itself: the use of these microorganisms is governed by general food safety rules, and that’s that.
A probiotic is identified at three levels: genus, species and strain. In Lacticaseibacillus rhamnosus GG, “Lacticaseibacillus” is the genus, “rhamnosus” the species and “GG” the strain. That last item is the one that governs, and it’s the one many packs omit.
Where there is evidence and where there isn’t
The US National Institutes of Health periodically reviews what research has shown. Here is the summary, and it’s worth reading in full before buying anything:
| Situation | What the evidence shows |
|---|---|
| Antibiotic-associated diarrhoea | Some strains may reduce the risk in children and adults, especially if started within the first 48 hours |
| Acute infectious diarrhoea in children | Some studies: shortens the episode by around a day. Others find no effect |
| Irritable bowel syndrome | May help with symptoms, with results varying by strain, duration and the specific symptom |
| Ulcerative colitis | Alongside medical treatment, might slightly reduce symptom intensity |
| Crohn’s disease | Does not appear to provide benefit |
| High cholesterol | Contradictory results: some studies see small falls, others none |
| Body weight | Contradictory. Some studies see a minimal reduction, others none or the opposite effect |
| Atopic dermatitis | Might reduce the risk in infants, with effects that depend heavily on strain and timing |
| A healthy person with no symptoms | There are no official recommendations for use |
The last row is the most important and the least often mentioned. There are no official recommendations on the use of probiotics in healthy people. In addition, EFSA’s nutrition panel has assessed the health claim applications submitted for multiple bacterial species and has issued unfavourable opinions in practically every case, either for lack of trials to support them or for being too generic.
The only authorised health claim in the EU
Of all the applications assessed, only one has passed the filter, and it says nothing about immunity or defences:
“Live cultures in yoghurt or fermented milk improve lactose digestion of the product in individuals who have difficulty digesting lactose.”
It applies to Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, the yoghurt ferments, provided the product contains a minimum of 10⁸ colony-forming units per gram.
This explains a labelling oddity. The AESAN accepts the word “probiotic” appearing on the label, but that term cannot be accompanied by any health claim that is not expressly authorised. That is why packs say “probiotic” and little else: legally they cannot promise you anything.
Take care with fermented foods too. Sauerkraut, kimchi, kefir and kombucha are made with microorganisms, but that does not mean they contain probiotics with demonstrated benefits. They are different things.
How to read the label
Three pieces of data, in this order:
- The complete strain. Genus, species and strain. If the pack only says “Lactobacillus”, you don’t know what you’re buying, because the benefit, where it exists, is specific to each strain.
- The CFUs and when they’re measured. Colony-forming units reflect how many live microorganisms there are, better than total weight. Some products declare CFUs at the time of manufacture and others through to the expiry date. The figure falls over time, so the second is the useful one.
- More CFUs is not better. A high figure on its own does not imply more benefit. It’s determined by the specific strain and the quantity, together.
And check the storage: some need refrigeration and others don’t. It’s on the pack.
The ones that are medicines, not supplements
There’s a practical distinction almost nobody knows about. Some microorganisms are authorised in Spain as medicines, with a technical data sheet assessed by the AEMPS, approved indications and contraindications. That is the case of Saccharomyces boulardii CNCM I-745, a yeast classified as an antidiarrhoeal microorganism.
Its authorised indication is specific: symptomatic treatment of acute non-specific diarrhoea, and prevention and symptomatic treatment of diarrhoea caused by antibiotics, in adults and those over 12. The usual regimen is 250 to 500 mg a day split into two doses, preferably before meals.
Several precautions come out of its data sheet that apply to any product with live microorganisms:
- Nothing above 50 °C. Don’t mix it with very hot drinks or food, with ice cream, or with anything containing alcohol, because these are living cells.
- Not alongside antifungals. It should not be given with oral or systemic antifungal treatments, because of the yeast’s fungal nature.
- It does not colonise. It passes through the digestive tract without settling and disappears from the stools 2 to 5 days after stopping. You are not permanently “repopulating” anything.
- Store in the original packaging, to protect it from moisture.
Who should not take them
In healthy people probiotics are generally safe and at most produce wind. The problem lies at the extremes.
The Saccharomyces boulardii data sheet contraindicates them in patients with a central venous catheter, in critical condition or immunosuppressed, because of the risk of fungaemia. Very rare cases of fungaemia and sepsis have been described in these groups, with fatal outcomes in some critically ill patients. The precaution goes to a point that conveys the scale of the risk: the capsules must not be opened in these patients’ rooms because of the risk of airborne contamination.
In the same vein, probiotics can cause infections in seriously ill people or those with a weakened immune system, and risks have been flagged in premature newborns. This is not material for self-medication in those cases.
A minor but real detail: some presentations contain lactose as an excipient, relevant if you have hereditary galactose intolerance or total lactase deficiency.
When to seek advice
Stop self-treating and seek medical assessment if any of these appear:
- Diarrhoea lasting more than 2 days without improving.
- Blood in the stools, or black stools.
- Fever accompanying the digestive illness.
- Signs of dehydration: very dry mouth, passing little or very dark urine, dizziness on standing, confusion.
- Severe abdominal pain or pain that progressively worsens.
- Vomiting that prevents you keeping fluids down.
- If you are immunosuppressed, on cancer treatment, have a central venous catheter or a serious illness: ask before taking any product with live microorganisms.
- If the diarrhoea appears weeks after finishing an antibiotic.
- Diarrhoea in an infant or small child, where dehydration sets in far faster.
This article is general information and does not replace assessment by a healthcare professional. Sources: AESAN — Probiotics in food, AEMPS/CIMA — Technical data sheet for Ultra-Levura 250 mg hard capsules (Saccharomyces boulardii CNCM I-745), NIH Office of Dietary Supplements — Probiotics and FAO/WHO — Probiotics in food: health and nutritional properties and guidelines for evaluation.