A new study has found a link between prenatal exposure to paracetamol and symptoms of autism and hyperactivity.

According to Spanish researchers, this is the first study of its kind to find an independent association between the use of paracetamol during pregnancy and autism symptoms in children.

They followed the progress of over 2,600 mother-child pairs, assessing them during pregnancy, when the child was 12 months old and again when the child was five years old.

The mothers' use of paracetamol during pregnancy was classified as never, sporadic or persistent.

The study found that around four in 10 of the children had been exposed to paracetamol while still in the womb. When assessed at the age of five, those who had been exposed to paracetamol had an increased risk of displaying impulsivity or hyperactivity symptoms.

In particular, children who had been persistently exposed to the drug tended to perform poorer in computer tests measuring impulsivity, inattention and visual speed processing.

Overall, boys exposed to paracetamol displayed more symptoms of autism.

"Although we measured symptoms and not diagnoses, an increase in the number of symptoms that a child has can affect him or her, even if they are not severe enough to warrant a clinical diagnosis of a neurodevelopmental disorder," the researchers explained.

They suggested that paracetamol could be harmful to a child's neurodevelopment for a number of reasons.

"First of all, it relieves pain by acting on cannabinoid receptors in the brain. Since these receptors normally help determine how neurons mature and connect with one another, paracetamol could alter these important processes.

"It can also affect the development of the immune system, or be directly toxic to some foetuses that may not have the same capacity as an adult to metabolise this drug, or by creating oxidative stress," they said.

They also suggested that boys may be more likely to display autism symptoms because ‘the male brain may be more vulnerable to harmful influences during early life'.

The researchers concluded that the widespread exposure of children to paracetamol while still in the womb could increase the number of children with symptoms of autism or ADHD (attention deficit hyperactivity disorder). However, they called for more studies into the exact amounts of paracetamol that appear to have an impact.

Details of these findings are published in the International Journal of Epidemiology.