Taking acetaminophen during pregnancy is linked to a slightly higher risk of children being diagnosed with ADHD and behaviour problems, but it is still the pain reliever of choice when expecting, doctors say.

Acetaminophen, found in Tylenol and some cold medications, is the most commonly used medication for pain and fever during pregnancy. While it has no known links to birth defects, studies in humans and animals suggest it could affect hormones that are key to brain development.

To learn more, researchers in Denmark, the U.S., Spain and Taiwan studied 64,322 children and mothers in the Danish National Birth Cohort who were followed with questionnaires and checks of hospital and prescription records. The focus was on the children's risk of developing attention-deficit hyperactivity disorder-like behaviour problems or a severe form of ADHD called hyperkinetic disorder or HKD.

"In this large pregnancy cohort with prospective data, children born to mothers who used acetaminophen during pregnancy were at higher risk for receiving a hospital diagnosis of HKD, ADHD medications, or having ADHD-like behaviours during followup," Dr. Jorn Olsen of the Institute of Public Health at the University of Aarhus and his co-authors concluded in Monday’s online issue of the journal JAMA Pediatrics.

While the Danish database was large, by age seven only 551 children were diagnosed with HDK among the pain reliever group compared with 283 in those who never used it.

"At this stage, I do not believe that a study of this type of quality can cause us to change what we tell women," said Dr. Gideon Koren, director of the Motherrisk program at the Hospital for Sick Children in Toronto, which provides advice to women and health professionals on the safety of drugs during pregnancy and breastfeeding.

"Use of acetaminophen in low dose over a short period of time is safe. If women need a medication for longer period of time, she should see her physician. There may be an issue there."

The database findings run counter to acetaminophen’s long track record of use worldwide, Koren notes."Here comes a database that correlates with very small risk."

In the study, the associations were stronger when the drug was used for more than one trimester.

ADHD can also run in families, a variable that wasn't fully accounted for in the study.

"Findings from this study should be interpreted cautiously and should not change practice," concludes an accompanying editorial by psychiatric researcher Miriam Cooper and colleagues at Cardiff University in the United Kingdom.  "However, they underline the importance of not taking a drug's safety during pregnancy for granted."

Other pain relievers, such as Aspirin and ibuprofen, can increase the risk of bleeding, Koren said. 

The researchers considered why women were taking the drugs, such as fever, infections and inflammatory conditions, but there could be other reasons, Cooper said.

For pain in pregnancy, non-medical options include massage, heat, and physiotherapy.

The study was funded by the Danish Medical Research Council.

With files from CBC's Kas Roussy