'Evidence' for probiotics means scientific proof that a particular strain delivers a specific health benefit to a defined group of people.
As with all science, there are nuances to this. Every individual study has its limits, so for evidence to be compelling, we usually need it to come from a range of high-quality studies.
This can all feel a bit complicated when you’re just looking to pick up something that’s good for your health.
To help, we'll dig into what it means when probiotics claim to be 'backed by science', what makes evidence 'good' and what to look out for when choosing a probiotic.
What does it mean when probiotics claim to be ‘backed by science’?
When a probiotic claims to be 'backed by science', it usually means researchers have investigated the strains of bacteria it contains.
The International Scientific Association for Probiotics and Prebiotics defines probiotics as ‘live microorganisms that, when administered in adequate amounts, confer a health benefit on the host’.
What makes things confusing is that the European Food Safety Authority currently states that manufacturers can't make specific health claims on any EU-marketed probiotic products.
Any health claims they do make are tightly regulated. For example:
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Not allowed: ‘Reduces the severity of rotavirus diarrhoea in infants’
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Allowed: ‘Improves gut health’
Probiotic products also don't undergo the same regulatory approval process as pharmaceutical medicines. As a result, there's often more evidence for individual strains than for specific products.
So, the label ‘backed by science’ just means that there’s been some research into the type of bacteria in the product. It’s not surprising that we see this label on probiotics so often.
In order to call something a probiotic, the makers need to know that a certain amount of the bacteria can provide a health benefit.
The label doesn't tell us what this research found, though, or how researchers studied the bacteria. We’ll need to look a bit deeper to find this out.
What do scientists mean when they talk about evidence?
When scientists talk about evidence, they mean all the existing research on how safe and effective specific probiotics are. This can include lab studies, as well as those in humans.
Scientists will look at the most up-to-date, high-quality research available to decide what probiotics they might recommend and when.
This evidence comes from several types of study, including:
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Lab studies, or in vitro studies, where researchers experiment on cells and tissues separately from living subjects (animals or humans).
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Preclinical trials, where researchers test on cells or animals to work out whether something's likely to be safe for humans.
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Clinical trials, where researchers use volunteers to determine if what they're testing is safe and effective for humans.
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Real-world studies, where researchers look at data on how things play out beyond the controlled environments of scientific research.
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Systematic reviews and meta-analyses, where researchers combine data from multiple studies. These are generally seen as the highest level of evidence.
There are pros and cons to each type of research. While real-world studies can show how a probiotic behaves in normal environments, clinical trials give researchers more control over what they're testing, which can make findings more precise.
New studies happen all the time, and researchers usually build on previous findings when designing new experiments.
Our understanding changes as new studies provide fresh information, so scientists will always look at the most current research when they evaluate the evidence for a probiotic.
Does evidence for one probiotic apply to every product that contains it?
Evidence for one probiotic won't necessarily apply to every product that contains it, because effects vary by strain, dose and form.
Strain
Different species of bacteria are a bit like different makes of car, and the strains are like the specific model.
For instance, Lactobacillus salivarius is one species of bacteria with several strains, and there's evidence that these strains may have different effects. While L. salivarius LS03 might help with acne, L. salivarius UCC118 could help prevent food-borne infections.
Some studies look at combinations of strains together, so their evidence only applies to those specific combos.
Dose
Studies usually look at specific doses, measured in colony-forming units (CFUs). If a study looks at the effects of taking 10 billion CFUs of a probiotic twice a day, it won't reflect what might happen if you took 1 billion CFUs once a day.
Form
Finally, the form a probiotic takes, also known as the delivery method, can influence its effects. They most commonly come as capsules, powders and liquids, and research on one delivery method may not apply to a different form of the same probiotic.
What does good probiotic evidence look like?
High-quality evidence usually comes from well-designed human randomised controlled trials, with systematic reviews and meta-analyses helping to bring together findings from multiple studies.
Laboratory and preclinical research also play an important role in understanding how probiotics may work. This depth of research isn't often available, though.
The World Gastroenterology Organisation advises that clinicians should only recommend probiotics shown to help in human studies. It also states that the ideal dose depends on the probiotic’s strain and form.
So, for a particular probiotic to have good evidence behind it, there'd ideally be a human study on that exact product.
Good evidence can also come from human studies on the same strain – at the same dose – as the product. The further the research strays from the exact form of a specific product, the weaker the evidence gets.
What are the limits of looking at a single study when assessing probiotics?
Looking at a single study gives a very narrow view of a probiotic. On its own, a study can only accurately tell us what happened at one point in time among its participants. If a study has limitations, there's no other information to fill the gaps.
On top of this, researchers are constantly conducting new studies. As a result, one study’s findings can go out of date when a newer, better study is published. This makes it difficult to generalise a single study's findings, and it's also why recommendations can shift as our understanding develops.
Reviews of existing research, like systematic reviews or meta-analyses, give a much clearer picture of what the evidence says.
An example
One systematic review looked at 12 studies to assess whether probiotics might help treat ulcerative colitis.
Although some of these studies looked favourable on their own, the reviewers concluded that it was unclear just how effective probiotics could be, due to the limitations of so many of the individual studies.
What should you look out for when choosing a probiotic?
When choosing a probiotic, check whether the manufacturer has done any research to support its use.
A probiotic's effectiveness depends on the specific strains used, the dose and the form it takes. Research on the product itself gives the clearest picture of how it might work for you.
If the manufacturers haven’t done any research, you can look to see what scientists have said about the specific strains the product contains. If you do, check whether the CFU dose used in the study matches the amount in the product.
If you want a probiotic to help with a specific health condition, look for research specifically linking the two. The health conditions with the strongest evidence for specific probiotics are antibiotic-associated diarrhoea and irritable bowel syndrome.
This page from the Office of Dietary Supplements in the U.S. sums up the research on probiotics and specific conditions.
Here’s a quick checklist before you settle on a probiotic:
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Have the manufacturers done any research on their product?
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What specific strains of bacteria does it contain?
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How many CFUs are there? The effective dose depends on the strain and the health outcome studied, so compare the product's dose with the one used in the supporting research.
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What form does the probiotic take? Capsules, powders and liquids may all work differently.
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Have researchers looked at probiotics for the health issue you're interested in?
Summary
The 'backed by science' label means there's some scientific evidence that the strain in the product will benefit a specific group of people.
To get a good idea of what a probiotic might do, it's worth checking the evidence for that specific product. It’s not just the strain that’s important – the dose and form are key, too.
Product-specific research is one of the most useful forms of evidence, because it can show exactly what might happen in real-life situations. Systematic reviews and meta-analyses are also handy because they combine data from multiple studies for a wider view.
Remember that evidence is specific. The closer the research matches the probiotic product, the more useful it is.
Sources
Bacteriocin production as a mechanism for the antiinfective activity of Lactobacillus salivarius UCC118. PNAS. (2007). Link.
Evidence based medicine: What it is and what it isn’t. BMJ. (1996). Link.
New approach in acne therapy: A specific bacteriocin activity and a targeted anti IL-8 property in just 1 probiotic strain, the L. salivarius LS03. Journal of Clinical Gastroenterology. (2018). Link.
Probiotics. (2025). Link.
Probiotics for maintenance of remission in ulcerative colitis. Cochrane Database of Systematic Reviews. (2020). Link.
Probiotics for the management of irritable bowel syndrome: A systematic review and three-level meta-analysis. International Journal of Surgery. (2023). Link.
Probiotics for the prevention of antibiotic-associated diarrhoea: A systematic review and meta-analysis. BMJ Open. (2021). Link.
Strain-specificity and disease-specificity of probiotic efficacy: A systematic review and meta-analysis. Frontiers in Medicine. (2018). Link.
The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology. (2014). Link.
World Gastroenterology Organisation global guidelines: Probiotics and prebiotics. (2023). Link.
