A Norwegian study has shown women who consume a lot of the common pain relievers like acetaminophen and paracetamol during pregnancy are more likely to have children with attention deficit hyperactivity disorder (ADHD) compared to ones who don’t use the drug.

The team used data from a massive 113,000 children and their parents, including 2,246 kids who were already diagnosed with ADHD, said the study which was published in Pediatrics journal. Almost half of the mothers took acetaminophen (also known as paracetamol) at some point during their pregnancy.

“Using the drug during just one trimester was associated with 7 percent higher odds of having a child with ADHD, while the increased risk was 22 percent for women who used acetaminophen in two trimesters and 27 percent with use in all three trimesters,” the study found.

But the study also claimed short-term use didn’t appear to increase the risk of ADHD. If the period of acetaminophen consumption was restricted to less than 8 days, the women were 10 percent less likely to have kids with ADHD than mothers who didn’t use the drug at all during pregnancy, the study found.

The effect of the medicine on the fetus rises exponentially with increasing dosages. The study said when consumed for 22 to 28 days, acetaminophen, which is used for fever and infections, was six times more likely to cause women to have kids with ADHD than mothers who avoided the drug during pregnancy.

“Surprisingly, adjusting for all the medical conditions related to acetaminophen use during pregnancy (e.g., infections and pain) and familial genetic risk for ADHD, children exposed to long-term use of acetaminophen use during pregnancy were more than two times more likely to have ADHD diagnosed by a specialist in a clinic,” said the study’s lead author Eivind Ystrom of the Norwegian Institute of Public Health and the University of Oslo in a report by Reuters.

“This is important, since more than 50 percent of women in western countries use acetaminophen during pregnancy, a large number of children are also exposed to long term use,” Ystrom added.

The study wasn’t a controlled experiment designed to prove whether or how prenatal acetaminophen use might directly cause ADHD. So the researchers studied the percentage of children born with ADHD and juxtaposed it over the data of mothers who consumed acetaminophen.

The report said researchers also “lacked data on the severity of conditions that led women to use the drug.”

The reasons the mothers took the drug was also not taken into account while collecting the data.

Doctors generally advise pregnant women to take acetaminophen in the smallest dose possible for the shortest possible period of time when they have fever.

“I don’t think that a very moderate dose of acetaminophen during only a few days would have any real effect on the developing brain,” Jordi Julvez, a researcher at the Barcelona Institute for Global Health, who wasn’t involved in the study, said in the report .

But such studies shed light on interactions between a fetus and pharmaceutical drugs and over-the-counter medication. The team hopes this will push more studies into analyzing similar impacts.