Children born to women who take acetaminophen during pregnancy may be at increased risk of attention deficit hyperactivity disorder (ADHD) and similar behavioural problems, new findings suggest.
The study does not show that prenatal exposure to the medication causes ADHD, and the increase in risk is small, Dr. Jorn Olsen, one of the study's authors, told Reuters Health.
Nevertheless, "it's reasonable to say that there's no reason to use these drugs during pregnancy unless there is a clear medical indication," said Olsen, a professor of epidemiology at UCLA and at Aarhus University in Denmark.
Acetaminophen, or paracetamol, has been available over the counter since the 1950s, Olsen and his colleagues note in their report in JAMA Pediatrics.
While the medication is widely considered to be safe to use during pregnancy, they add, recent studies have shown it can disrupt hormone function in pregnant rats and mice.
Given that hormones play a key role in guiding fetal development, the researchers decided to investigate whether acetaminophen exposure might be related to ADHD risk. They looked at 64,322 children and mothers enrolled in the Danish National Birth Cohort between 1996 and 2002.
More than half - 56 per cent - of mothers reported using acetaminophen during pregnancy.
Children born to these women were 37 per cent more likely to be diagnosed with hyperkinetic disorder (HKD), a severe form of ADHD. They were 29 per cent more likely to be prescribed ADHD medications, and 13 per cent more likely to exhibit ADHD-like behaviours at age 7.
The acetaminophen-ADHD link was stronger for women who used the medication during more than one trimester of pregnancy, and increased with the frequency of exposure.
Five to six per cent of babies born today will develop ADHD symptoms during their lifetimes, Dr. Olsen noted; based on the current findings, that risk would increase to about 7 per cent for children exposed to acetaminophen prenatally.
"It's still a modest increase," he said. "For the women that are taking these drugs there are no special reasons for concern....for women who are pregnant and who have not taken these drugs, I think that the take-home message would be a lot of the use of these particular drugs during pregnancy is not really necessary."
An editorial accompanying the study pointed out that Olsen's team adjusted for things that might have also influenced fetal development, such as the mothers' inflammation or fever, while possibly explaining why the women took acetaminophen. But the researchers could not account for every reason the women took acetaminophen, so more study is needed.
Pregnant women should consult their physician about whether or not they should be taking acetaminophen during pregnancy, one of the editorial's coauthors, Dr. Kate Langley, told Reuters Health.
Even if acetaminophen exposure does turn out to have a causal relationship with ADHD, she added, it is only one of many environmental and genetic risk factors involved in the disease, said Langley, a professor of psychology at Cardiff University and the MRC Center for Neuropsychiatric Genetics and Genomics.
"In terms of research, it's definitely something that should be followed up, but it's the first step on a long road to try to determine how we should interpret these findings," she said.