An ingredient used in some milk formula here has been found to cause allergic reactions in rare cases, said the authorities yesterday.

Called galacto-oligosaccharide (GOS), the carbohydrate is typically added to help promote good bacteria in the gut.

In a joint statement, the Health Ministry and Agri-Food and Veterinary Authority of Singapore said that GOS is generally deemed safe for human consumption, both in Singapore and abroad.

However, they cautioned those with a history of conditions such as eczema, asthma or allergic rhinitis to be careful when consuming products with this additive for the first time.

Members of the public can find out if a milk product contains GOS by looking at the ingredient label.

Preliminary results from a local research study estimated that 3.5 per cent of people predisposed to certain allergies, aged between five and 60, may be allergic to GOS, said MOH.

Allergy symptoms include sneezing, throat and chest tightness, wheezing, hives and diarrhoea. They usually show up within minutes to hours of the allergen being consumed.

If symptoms are serious, one should go directly to a hospital emergency department to seek immediate medical treatment, said Dr Alison Lee, an associate consultant at the division of paediatric allergy, immunology and rheumatology at the National University Hospital.

"After appropriate medical treatment, the symptoms should resolve quickly within a few hours."

Since 2007, about two cases of GOS allergies on average have been reported each year in Singapore. All of them had a history of allergic conditions and were found to be sensitive to house dust mites.

To date, there have been no reported cases of GOS allergies in children below two years old. Those with no history of allergies are not likely to develop them due to eating GOS.

Dr Chiang Wen Chin, a paediatrician at Mount Elizabeth Hospital, said that this allergy is mostly seen in Asia, although doctors are not yet sure why.