Researchers from UCLA and the University of Aarhus in Denmark have released a study that associates acetaminophen use by pregnant women with higher rates of ADHD and associated behavior in their children later in life.

Acetaminophen (paracetamol) is a highly common pain reliever found in over-the-counter products like Tylenol. It is generally recommended as safe for use during pregnancy, although doctors suggest it be used in low doses, as they do with most medications.

The researchers say they need further research to verify the findings, but the results of the study show that the children of women who take acetaminophen during pregnancy have a 13 percent to 37 percent higher risk of later receiving a hospital diagnosis of hyperkinetic disorder, being treated with ADHD medications or having ADHD-like behaviors at age 7.

The researchers studied 64,322 mothers and children enrolled in the Danish National Birth Cohort from 1996 to 2002. The cohort is a nationwide study of the long-term health effects of “exposures in the period from conception to early childhood.”

The researchers interviewed women three times during pregnancy and followed up with them when their child turned 7 years old. Using data from the cohort and via interviews they determined the prevalence of ADHD, its more severe form hyperkinetic disorder and other ADHD-like behavior.

More than half of the women reported using acetaminophen during pregnancy. The researchers say the results show the longer into the pregnancy a mother took acetaminophen, the higher risk of later disorders.

UCLA says “the risks for hyperkinetic disorder/ADHD in children were elevated 50 percent or more when the mothers had used the common painkiller for more than 20 weeks in pregnancy.”

Dr. Beate Ritz of UCLA, an author of the paper, said Acetaminophen can cross the placental barrier, meaning it could affect the way the brain develops in a fetus. “It’s known from animal data that acetaminophen is a hormone disruptor,” says Ritz, “and abnormal hormonal exposures in pregnancy may influence fetal brain development.”

The research was undertaken as a look into avoidable causes of ADHD and related behavior. Acetaminophen was chosen because of it is the most commonly used painkiller by pregnant women.

Dr. Jørn Olsen, who took part in the study, says “we need further research to verify these findings, but if these results reflect causal associations, then acetaminophen should no longer be considered a 'safe' drug for use in pregnancy."