Probiotics, prebiotics & synbiotics: the evidence behind the claims

Many health claims are made about probiotics, prebiotics and synbiotics. In some cases, there’s evidence that they may be helpful – but there’s a lack of evidence to support other claims. This blog, by Cochrane UK’s Selena Ryan-Vig, focuses mainly on probiotics and looks at the evidence behind their use for various health conditions, explaining where: 

  • They may have some benefits
  • They may not be worthwhile
  • The evidence is uncertain – so we’re unsure whether or not they’re helpful 

Blog updated 24 October 2023.


What are probiotics, prebiotics and synbiotics?

Probiotics are live bacteria and yeasts which are thought to help restore the natural balance of bacteria in your gut (including your stomach and intestines) when it’s been disrupted by an illness or treatment. Typically, they may be added to yoghurts or taken as food supplements. You might see them marketed as ‘good’ or ‘friendly’ bacteria.

Prebiotics are a form of dietary fibre which encourage the growth of ‘friendly’ bacteria in the gut. They may be found in foods such as bananas and grains or taken as food supplements. 

Synbiotic is the word for a product that contains both a probiotic and a prebiotic.


Health problems where probiotics, prebiotics or synbiotics may have some benefits

This section looks at the evidence on 1) babies and children; 2) children and adults; 3) adults.


Babies and children


Preventing necrotising enterocolitis in very premature or very low birth weight infants 

Necrotising enterocolitis is a severe condition where some lining of the bowel becomes inflamed and dies.  


​​Giving very premature and very low birth weight infants probiotics, compared with placebo or no treatment, may reduce their risk of necrotising enterocolitis, and probably reduces the risk of dying. Probiotics probably have little or no effect on serious infection and may have little or no effect on disability or developmental outcomes. In extremely premature babies (more than 12 weeks early) or extremely low birth weight infants (under 1kg at birth), probiotics may have little to no benefit. Listen to a podcast for more information.


Giving synbiotics to very premature or very low birth weight infants may reduce the risk of necrotising enterocolitis and death, compared with control treatment. Synbiotics may have little or no effect in reducing the risk of serious infection, but the evidence is very uncertain. There is not enough information about the potential effects on disability or developmental outcomes.


However, prebiotics may have little or no benefits in reducing the risk of necrotising enterocolitis, serious infection, or death.


Probiotics to prevent infantile colic 

Infantile colic is when a baby cries a lot with no apparent cause. It typically gets better by itself around 3 months of age. Compared with placebo, taking probiotics may not prevent infantile colic – but it may reduce the time a baby spends crying each day. Probiotics may not increase the risk of side effects. 


Probiotics for preventing children from getting acute middle ear infections

Acute middle ear infection (also known as acute otitis media) is common in children – with symptoms including earache and fever. In children who aren’t prone to getting these infections, probiotics probably reduce the risk of getting an infection, compared to placebo. But probiotics are unlikely to benefit children who are prone to these infections. Taking probiotics may not increase the risk of side effects.

There is not enough reliable information about which strain of probiotic may be best, how often to take them, or for how long.


Probiotics and synbiotics for managing unexplained, long-term stomach pain in children

Lots of children have long-term, frequent stomach pain for which no cause can be found. There is some evidence that probiotics and synbiotics may be better than placebo at improving such pain. However, there is no reliable information about potential side effects. 


​​Probiotics to prevent antibiotic-associated diarrhoea in children

Children taking antibiotics may experience diarrhoea as a result. Taking probiotics probably reduces the risk of getting diarrhoea and may reduce how long someone has diarrhoea (by around one day). Side effects may be infrequent and mild (for example, rash, feeling sick, constipation).


Children and adults


Probiotics for preventing acute upper respiratory tract infections (such as the common cold, ‘flu, or infections of the throat, nose, or sinuses) in children and adults

There is some evidence that, compared with placebo or no treatment, probiotics:

  • may reduce the risk of getting an upper respiratory tract infection 
  • may reduce how long the infection lasts by around 1.2 days
  • may reduce the need for antibiotics in children (but this benefit wasn’t seen in adults)
  • may not increase the risk of side-effects compared with placebo or no treatment. 

Find out more in this blog: Probiotics to prevent colds and flu: what’s the evidence?


Probiotics to prevent Clostridium difficile diarrhoea associated with antibiotic use in adults and children 

Clostridium difficile (C. diff) is a type of bacteria that can cause diarrhoea. People taking antibiotics often get C. diff infection. When probiotics are given alongside antibiotics, the risk of developing diarrhoea is probably reduced and those at high risk of developing diarrhoea from C. diff are particularly likely to benefit from probiotics. The evidence about potential side effects is very uncertain, but probiotics may not increase the risk.


Probiotics for induction of remission in ulcerative colitis 

Ulcerative colitis is a long-term bowel disease, with symptoms such as stomach pain and diarrhoea.  The first aim of treatment is ‘induction of remission’ – where symptoms are reduced. Then the aim is ‘maintenance of remission’ – keeping symptoms under control for a sustained period.

There is some evidence that, compared with placebo, probiotics may improve induction of remission, but there may be little or no difference when probiotics are compared with 5‐ASA (an anti‐inflammatory treatment). The evidence about potential side effects is uncertain. 

It is uncertain whether probiotics can help with maintenance of remission


Probiotics for children and adults with cystic fibrosis

Cystic fibrosis is an inherited condition that causes sticky mucus to build up in the lungs and digestive system, causing lung problems and problems with digesting food.

Overall, probiotics for people with cystic fibrosis “may have some limited health benefits”, when compared with placebo. Pulmonary exacerbations are when a person’s breathing worsens (for example, increased cough, or shortness of breath) and their lung function worsens. Probiotics may reduce these types of events, but this is not uncertain. Probiotics may also reduce a marker of gut inflammation (called calprotectin) – but the benefit of this to the individual is unclear. Probiotics may have little to no other benefits or impact on outcomes such as lung functioning, the need for admission to hospital, or abdominal symptoms. Probiotics may also slightly increase the risk of side effects, such as vomiting, diarrhoea and allergic reactions.




Probiotics for vaginal thrush (vulvovaginal candidiasis) in women who aren’t pregnant

Vaginal thrush (or vulvovaginal candidiasis) is a yeast infection that can cause pain and itchiness around your vagina and vulva. 

In non-pregnant women with vaginal thrush, using probiotics alongside conventional antifungal treatment (compared with just using conventional antifungal drugs) may improve the chance of curing the thrush in the short term (within 5 to 10 days). However, it may not improve the chance of longer-term cure (within one to three months). Adding probiotics to conventional antifungal treatment may not increase the risk of side effects.


Probiotics for people with hepatic encephalopathy 

Hepatic encephalopathy is when a high level of toxins in the blood due to liver damage can lead to disordered brain function. It may be ‘overt’, where symptoms are apparent – such as confusion, agitation, difficulty speaking, or even coma in serious cases. ‘Minimal’ hepatic encephalopathy is a milder form of the same condition, which does not have the same obvious symptoms. 

Compared with placebo or no treatment, probiotics probably improve recovery and may lead to improvements in the development of overt hepatic encephalopathy and quality of life, but probiotics may lead to little or no difference in the risk of dying. The evidence is very uncertain about how probiotics compare with lactulose (a different treatment for the condition). 


When taking probiotics or prebiotics may not be worthwhile 

This section looks at the evidence on probiotics for:

  • treating acute infectious diarrhoea
  • treating eczema
  • preventing gestational diabetes

Plus, the evidence on prebiotics for preventing necrotising enterocolitis (a serious condition affecting the bowel) in premature babies.


Probiotics to treat acute infectious diarrhoea

Acute infectious diarrhoea is short-term and caused by an infection of the gut (for example, from drinking contaminated water). Compared with placebo, no treatment, or standard treatment, probiotics probably make little or no difference to the number of people who have diarrhoea lasting 48 hours or longer, and it is uncertain whether probiotics reduce the length of time someone has diarrhoea. Listen to a podcast for more information.


Probiotics for treating eczema 

Eczema is a common skin condition where people may have red itchy patches of skin. People with eczema sometimes have an inflamed gut and can have different bacteria in their gut compared to people without eczema. It has been suggested that probiotics may help by changing the mix of gut bacteria and/or reducing gut inflammation.

However, compared with not taking probiotics, taking probiotics probably has little or no impact on reducing eczema symptoms, such as itching and sleep loss, and may not improve quality of life. Probiotics may slightly reduce the severity of eczema (scored by patients and their healthcare professionals together) – but it’s uncertain if such a change is meaningful for patients. Probiotics may not increase the risk of side effects. 


Probiotics to prevent gestational diabetes ​​

Gestational diabetes is a condition where a pregnant woman develops high blood sugar levels, typically after 13 weeks of pregnancy. It can bring health risks for both mother and baby. 

There is evidence of possible harm – with little observed benefit – for widespread use of probiotics in pregnancy:

Compared with placebo, probiotics increase the risk of developing pre‐eclampsia – a condition which affects some women in the second half of pregnancy or soon after their baby is born. Signs include high blood pressure and protein in the urine which could lead to symptoms such as swelling, headache or vomiting. Many cases are mild, but it can be serious for mother and baby if it isn’t monitored or treated. 

Many of the other effects of probiotics are uncertain (including if they impact women’s risk of developing gestational diabetes, or babies’ risk of medical problems after birth). Probiotics may make little to no difference to the risk of needing a caesarean section, and probably make no little or no difference to the risk of having a big baby.


Prebiotics to prevent necrotising enterocolitis in premature infants

Necrotising enterocolitis is a severe condition where some lining of the bowel becomes inflamed and dies. In premature infants, prebiotics may have little or no benefits in reducing the risk of necrotising enterocolitis, serious infection, or death. (In contrast, synbiotics and probiotics may have some benefits – see above).


Where the evidence is very uncertain about the potential benefits or harms of probiotics, prebiotics or synbiotics

The evidence is uncertain whether there is any benefit or harm in taking:


Where does this leave you?

Overall, the evidence about probiotics, prebiotics and synbiotics is varied. In some cases, they may have some benefits; in others they may not be worthwhile – or even have harms; and there are some cases where we just can’t be certain. It is wise to question claims about them – benefits may be overstated, and risks may be downplayed. Before taking or buying any supplement for a particular reason, consider speaking with a health professional and asking about the evidence. 

Read more about probiotics on the NHS website.


References and further reading (pdf)


Join in the conversation on Twitter with @CochraneUK or leave a comment on the blog.

Please note, we cannot give specific medical advice and do not publish comments that link to individual pages requesting donations or to commercial sites, or appear to endorse commercial products. We welcome diverse views and encourage discussion but we ask that comments are respectful and reserve the right to not publish any we consider offensive. Cochrane UK does not fact check – or endorse – readers’ comments, including any treatments mentioned.

Selena Ryan-Vig has nothing to disclose.

Probiotics, prebiotics & synbiotics: the evidence behind the claims by Selena Ryan-Vig

is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International

1 Comments on this post

  1. Selena Ryan-Vig provides a nuanced examination of probiotics, prebiotics, and synbiotics, elucidating evidence-backed benefits for specific health conditions, cautioning against unwarranted claims, and emphasizing the importance of informed decision-making. The blog serves as a valuable guide for individuals considering these supplements.

    Bazayan / Reply

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