Probiotics are live bacteria and yeasts promoted as having various health benefits, including preventing and treating a range of conditions. They are usually eaten in yoghurts or taken as food supplements, and are often described as "good" or "friendly" bacteria.

Probiotics are thought to help restore the natural balance of your gut bacteria when it has been disrupted. However, there is little evidence to support most health claims made for them.

The strongest of this evidence surrounds the use of probiotics in the prevention of antibiotic-associated diarrhoea (AAD).

There's also evidence, albeit weaker, to suggest they may help in treating irritable bowel syndrome (IBS), lactose intolerance and a complication of ulcerative colitis treatment. It is also thought that probiotics may help protect some premature babies from developing a dangerous gut disease.

However, until more research is carried out, it's uncertain whether the benefits stretch any further than that. Generally, it's hard to see how swallowable bacteria could have an effect on conditions outside of the digestive tract.

It should also be noted that there's likely to be a huge difference between the pharmaceutical-grade probiotics that show promise in clinical trials and the "probiotic" yoghurts and supplements sold in shops, which may not live up to the advertised claims.

The problems with "probiotic" yoghurts and food supplements

In general, probiotics are regulated as foods, and don’t undergo the rigorous testing and approval process that medicines do.

This means we don’t know whether a probiotic yoghurt, supplement or tablet contains what is stated on the label, and whether the amount of "good" bacteria in it is enough to have a beneficial effect. The bacteria in probiotic freeze-dried tablets may not even be "live".

It's also worth noting that the different probiotic strains have completely different effects on the body. Don't assume that the beneficial effects seen with one strain are the same as other similar strains.

However, what we can say is that probiotics appear to be safe. If you wish to try them – and you have a healthy immune system – they shouldn't cause any unpleasant side effects.

Read on to learn when they might help and when we should be sceptical, in the following common uses:

  • antibiotic-associated diarrhoea (AAD)
  • persistent diarrhoea
  • protecting premature babies from gut disease
  • irritable bowel syndrome (IBS)
  • lactose intolerance
  • pouchitis
  • colic
  • "boosting the immune system"
  • vaginal conditions
  • inflammatory bowel disease (IBD)
  • eczema

How they can prevent antibiotic-associated diarrhoea

There's fairly good evidence that giving high doses of Lactobacillus rhamnosus and Saccharomyces boulardii when also taking antibiotics can help prevent children getting AAD.

Without probiotics, the antibiotics tend to wipe out the protective gut bacteria, which results in diarrhoea.

In addition to this, probiotics given with antibiotics may greatly reduce the risk of developing a Clostridium difficile (CD) infection. CD are potentially dangerous bacteria that can multiply and cause a life-threatening infection if the balance of gut bacteria is disturbed by antibiotics.

Probiotics are thought to directly kill or inhibit the growth of harmful bacteria, stopping them producing toxic substances that can make us ill.

How they may treat persistent diarrhoea

There’s some evidence that probiotics can shorten a period of persistent diarrhoea by about a day.

The strain Lactobacillus rhamnosus GG appears to help children with gastroenteritis caused by a rotavirus, and may also help people with traveller's diarrhoea.

However, the evidence isn’t yet strong enough to make any treatment recommendations.

How they can help protect premature babies

Some babies born prematurely are at risk of a serious condition called necrotising enterocolitis (NEC) – this is when tissues in the baby's gut become inflamed and start to die.

There's some evidence that probiotics can reduce the likelihood of premature babies contracting NEC.

How they may help IBS

Probiotics may help reduce abdominal bloating and flatulence in some people with IBS, and they may help relieve pain and provide general relief.

This is supported by a 2010 Cochrane review – however, we don't yet know the extent of the benefits, nor the most effective probiotic species and strain.

Probiotics won't work for everyone with IBS, but if you want to try them, the National Institute for Health and Care Excellence (NICE) recommends taking them for at least four weeks, at a dose recommended by the manufacturer.

How they may help if you're lactose intolerant

Lactose intolerance is a common digestive problem, when the body is unable to digest lactose (a type of sugar found mainly in milk and dairy products).

It's usually the result of a lactase deficiency. Lactase is an enzyme (protein) normally produced in your small intestine. It is needed to break down lactose and to enable you to digest milk.

Lactobacillus acidophilus is thought to help with the digestion and absorption of lactose by producing lactase.

Research on the effects of probiotics on lactose intolerance is in the pipeline. However, in the meantime, people with lactose intolerance may wish to try probiotic preparations (not yoghurts) of Lactobacillus acidophilus to see if they help relieve symptoms. Learn how to manage lactose intolerance.

How they may help pouchitis

Some people with ulcerative colitis need to have part of their bowel removed and a loop of bowel constructed in its place. This loop, or pouch, can become inflamed (possibly due to an imbalance of bacteria), leading to diarrhoea and other problems. This inflammation is known as "pouchitis".

Small studies have shown that taking the probiotic medical food VSL#3 can help to treat pouchitis. However, more research is needed before recommendations for clinical practice can be made.

Disappointing results for colic

In 2013, there were media reports that probiotics could help babies with colic, but the claim was unproven.

The study on which this story was based concluded that Lactobacillus reuteri may help crying infants with colic that are exclusively breastfed. However, generally, it found insufficient evidence that probiotics can help manage colic effectively or prevent infants from crying.

Since then, a small but well-conducted study found that L. reuteri had no effect on infant colic in either breastfed or bottle-fed babies.

Colic is poorly understand, and numerous theories as to its cause have been suggested. There are probably better ways you can manage a colicky baby.

Why we should ignore claims they 'boost the immune system'

Until 2010, the probiotic food industry claimed in adverts that their yoghurts "boost your immune system". However, these claims were ruled unproven by The European Food Safety Authority and are no longer allowed to be made.

Not only is there a lack of evidence for the supposed immune system benefits of probiotics, but research found that in healthy children, probiotic supplements had no effect on antibody levels, days of fever and number of infections.

More research is needed before we can draw any conclusions, and the vague term "boosting immunity" needs clearly defining so we can understand exactly what beneficial effect, if any, probiotic foods and food supplements have on the immune system.

There's also no reason why healthy people should need to "rebalance their gut bacteria", as some marketing material may claim.

Why they can't be recommended for vaginal conditions

There have been suggestions that probiotics can help in the treatment of bacterial vaginosis (BV). However, even when they are taken with antibiotics, there's no evidence of any extra benefits.

Larger, higher quality studies are needed to confirm the benefits of probiotics in the treatment of BV before they can be recommended.

Meanwhile, scientists are testing vaginal pessaries containing probiotic lactobacilli bacteria for the treatment of vaginal thrush.

However, there's currently no evidence that eating yoghurt or inserting it into the vagina will treat thrush or relieve its symptoms.

Why they can't yet be used for inflammatory bowel disease

There's a lack of evidence to make any conclusions about the effectiveness of probiotics for relieving symptoms of Crohn's disease or ulcerative colitis.

Much more research is needed in this area.

Why they're not supported for eczema

There's no evidence to support claims that probiotics can help prevent and treat eczema in children.

A 2008 Cochrane review found that probiotics do not reduce eczema symptoms, such as itching, nor do they change the severity of a person's eczema.

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