Antibiotics administered to children younger than two are associated with several ongoing illnesses or conditions, ranging from allergies to obesity, warn researchers.

For the study, published in the journal Mayo Clinic Proceedings, the research team from Mayo Clinic in the US analysed data from over 14,500 children.

About 70 per cent of the children had received at least one treatment with antibiotics for illness before age 2.

The findings showed that children receiving multiple antibiotic treatments were more likely to have multiple illnesses or conditions later in childhood.

Types and frequency of illness varied depending on age, type of medication, dose and number of doses. There also were some differences between boys and girls.

Conditions associated with early use of antibiotics included asthma, allergic rhinitis, weight issues and obesity, food allergies, attention deficit hyperactivity disorder, celiac disease, and atopic dermatitis.

The authors speculate that even though antibiotics may only transiently affect the microbiome, the collection of microbes in the body, this may have long-term health consequences.

"These findings offer the opportunity to target future research to determine more reliable and safer approaches to timing, dosing and types of antibiotics for children in this age group," said study author Nathan LeBrasseur from the Mayo Clinic.

While recent data show an increase in some of the childhood conditions involved in the study, experts are not sure why.

Other than the issue of multi-drug resistance, antibiotics have been presumed safe by most paediatricians.

Researchers also noted the ultimate goal is to provide practical guidelines for physicians on the safest way to use antibiotics early in life.