Key messages

• Probiotics (live bacteria that provide health benefits to the digestive tract) given to newborns as a supplement or added to infant formula in the first six months of life may slightly reduce the development of eczema (dry, itchy and inflamed skin) in infants by the age of two years, but there are not enough good-quality studies to be certain about the result.

• Probiotics may have little to no effect on the development of allergic diseases such as asthma (a respiratory condition with symptoms like cough, wheezing, chest tightness and breathlessness) and allergic rhinitis (often called hay fever), and dietary allergies (to food and cow's milk) during infancy.

• The effects of synbiotics (a mixture of probiotics and prebiotics (non-digestible fibres that help bacteria grow)) are uncertain.

What are allergies?

Allergies to certain foods, such as cow's milk, peanuts and eggs, as well as non-food allergies, are common. Some infants become sensitive to foods, including cow's milk, through their digestive tract. This may be affected by the bacteria in the digestive tract. Other infants may become sensitive through the skin. Sensitisation to allergens (the component that causes the allergic reaction) tends to follow a pattern, with allergies to food occurring in the first two to three years of life, followed by indoor allergens (for example, house dust mites and pets) and subsequently outdoor allergens (for example, rye and Timothy grass) that present as asthma, eczema or allergic rhinitis.

How can they be prevented?

Probiotics are helpful to the live gut bacteria that live in the digestive tract, and there is interest in whether probiotics might prevent sensitisation to allergens that lead to food allergies and non-food allergies.

What did we do?

We wanted to find out the impact of giving probiotics or synbiotics (probiotics with added prebiotics (non-digestible fibres that help bacteria grow)) to infants in the first six months of life compared to giving a placebo (pretend treatment) or no treatment, on preventing the development of allergic diseases, including asthma, eczema, allergic rhinitis, food allergy and cow's milk allergy.

We searched for studies that gave infants probiotics or synbiotics as an intervention compared with no probiotics or synbiotics during the first six months of life. We compared and summarised the results of these studies and rated our confidence in the evidence, based on factors such as study methods and sizes.

What did we find?

We included 24 studies that involved 7077 mother–infant pairs.

Probiotics may result in little to no difference in asthma, allergic rhinitis and cow's milk allergy by two years of age. Probiotics may slightly reduce eczema by two years of age, but there were not enough good-quality studies for us to be certain about the result. Probiotics may have little to no effect on the occurrence of food allergy by two years of age, but the evidence is very uncertain.

The evidence is very uncertain about the effect of synbiotics on eczema by two years of age. Synbiotics may result in little to no difference in food allergy by two years of age. We found no data for the effect of synbiotics on asthma, allergic rhinitis and cow's milk allergy by two years of age.

Probiotic or synbiotic supplementation may result in little to no difference in potential unwanted effects, including infection with probiotic bacteria by two years of age. There were no serious unwanted effects related to the use of probiotics or synbiotics reported.

What are the limitations of the evidence?

We had some concerns about how some of the studies were conducted. Not all the studies provided data about everything that we were interested in. Most studies looked at probiotics and reported on eczema. There were not enough studies for us to be certain about the effects of probiotics and synbiotics on other allergic diseases and dietary allergies.

How up to date is this evidence?

The evidence is up to date to December 2023.