Written by Ryan Jaslow, CBS News
(CBS News)-- Women who take acetaminophen during pregnancy may face an increased risk their child develops behavior problems commonly associated with attention-deficit/hyperactivity disorder (ADHD).
"If these results reflect causal associations, acetaminophen should no longer be considered a safe drug for use in pregnancy," wrote lead study author Zeyan Liew, an epidemiologist working on her PhD at UCLA's Fielding School of Public Health in Los Angeles.
Acetaminophen is an active ingredient in the pain and fever reliever Tylenol, and is also found in over-the-counter cold medications and in combination with prescription pain medications like hydrocodone (Vicodin).
Acetaminophen is the most popular medication for pain and fever during pregnancy and is taken by more than 50 percent of pregnant U.S. women, according to the authors of the study, which was published Feb. 24 in JAMA Pediatrics.
Recent studies in animals and people have found the drug might disrupt hormone levels, which in turn may affect brain development and cause behavioral problems.
Researchers looked into the link by studying more than 64,000 children and moms who gave birth in Denmark between 1996 and 2002.
More than half of mothers reported using acetaminophen while pregnant. Their children faced an increased risk for ADHD-like behaviors by age 7, the researchers found.
Those children were also more likely to be using ADHD medications, and faced an increased risk for hyperkinetic disorders (HKD), a particularly severe form of ADHD.
ADHD risk increased if acetaminophen was taken during more than one trimester, especially in later pregnancy. Risk increased the more frequently the drug was taken.
Women who took acetaminophen for 20 or more weeks during pregnancy almost doubled the risk the child would have HKD, and increased the chance a child would take ADHD medications by more than 50 percent, compared to moms who did not take the drug.
The study was observational, meaning it does not imply a cause-and-effect relationship between taking acetaminophen during pregnancy and these problems.
"Medication that we thought was fairly safe to use during pregnancy for pain relief actually might increase the chance of a child to later develop ADHD," study co-author Dr. Beate Ritz, vice chair of the epidemiology department at UCLA, told CBS News' Teri Okita.
About 6.4 million U.S. kids -- 11 percent of all children -- have ADHD, which is characterized by behavioral problems like impulsivity, trouble paying attention, being overly active and having difficult getting along with others, according to theCenters for Disease Control and Prevention (CDC).
In an editorial published in the same journal issue, researchers at Cardiff University in Wales said the study should not change current medical guidance, and agreed more research was needed to determine a causal link.
"Findings from this study should be interpreted cautiously and should not change practice," wrote the commentary's authors, led by psychiatrist Miriam Cooper of Cardiff University. "However, they underline the importance of not taking a drug's safety during pregnancy for granted, and they provide a platform from which to conduct further related analyses exploring a potential relationship between acetaminophen use and altered neurodevelopment."
"Patients should not panic if they took Tylenol during their pregnancies, and the current practice about prescribing Tylenol should not change based on this one study," added Dr. Joanne Stone, director of maternal-fetal medicine at The Mount Sinai Hospital in New York, to CBS News in an email.
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