Paracetamol, a common pain reliever considered safe for pregnant women, has been linked for the first time to an increased risk of attention deficit hyperactivity disorder (ADHD) in children, according to a new study.

More studies are needed to confirm the findings, but experts said the research points to a new potential cause for the worldwide rise in cases of ADHD, a neuro-behavioural condition which has no known cause and affects as many as 5% of US children.

Women who took paracetamol, also known as acetaminophen, while pregnant had a 37% higher risk of having a child who would be later given a hospital diagnosis of hyperkinetic disorder, a particularly severe form of ADHD, said the study in the 24 February edition of the Journal of the American Medical Association (JAMA) Pediatrics.

Compared with women who did not take paracetamol while pregnant, women who did also had a 29% higher chance of having children who were later prescribed medications for ADHD, and a 13% higher chance of exhibiting ADHD-like behaviors by age seven.

Previous research has suggested paracetamol can interfere with normal hormone function and may affect the developing foetal brain. The painkiller has also been linked to a slightly increased risk in boys of cryptorchidism, a condition in which the testicles do not descend.

The latest research was based on survey data on more than 64,000 Danish women from 1996 to 2002.

More than half said they took paracetamol at least once during pregnancy.

Outside experts cautioned that the observational findings do not prove that taking such pain relievers causes ADHD, only that a preliminary link between the two has appeared and would need to be confirmed by further research.

“Findings from this study should be interpreted cautiously and should not change practice,” said an accompanying editorial in JAMA Pediatrics by Miriam Cooper and colleagues at the Cardiff University school of medicine. “However, they underline the importance of not taking a drug’s safety during pregnancy for granted.”

The reasons the women took the painkillers could have also had a confounding effect on the outcome, they added.

The study was led by Zeyan Liew, of the University of California, Los Angeles, and was co-written by Jorn Olsen of the University of Aarhus in Denmark.