Acetaminophen, the active ingredient in medications including Tylenol and Excedrin, is the most commonly used over-the-counter pain and fever reliever during pregnancy. Now, research suggests it might increase the risk of attention deficit hyperactivity disorder-like problems in children.
The findings, published in JAMA Pediatrics on Monday, are preliminary and do not establish cause and effect. However, they do intensify questions about the risks and benefits of taking the medication while pregnant.
Aspirin and ibuprofen -- nonsteroidal anti-inflammatory drugs, or NSAIDs -- are generally not recommended as pain relievers for pregnant women, particularly during the last three months. Acetaminophen-based medications such as Tylenol, however, have generally been thought to be safe, and estimates suggest that more than 50 percent of women in the United States take acetaminophen at some point while pregnant.
"It is important we follow up [on] the potential health risks that acetaminophen may cause," Zeyan Liew, a Ph.D. candidate with the UCLA Fielding School of Public Health and an author on the study, told The Huffington Post. "ADHD incidence has been noticed to be increased in the last decades, and we are interested in searching for avoidable environmental factors that may contribute to the trend."
Liew and his co-authors looked at data on more than 64,000 women and their children taken from the Danish National Birth Cohort. They found that children whose mothers took acetaminophen while pregnant had a 13 percent to 37 percent greater risk of later being diagnosed with hyperkinetic disorder (which is similar to ADHD, but uses different diagnostic criteria), taking medications for ADHD, or displaying ADHD-like behaviors at age 7.
That link was stronger among women who took acetaminophen in multiple trimesters or who used it more frequently. For example, the risk of behavioral issues was elevated by 50 percent or more in children whose mothers took the pain reliever for more than 20 weeks while pregnant.
The findings join a small but growing body of research -- some of it led by the same researchers on the new study -- linking acetaminophen use during pregnancy with health issues such as increased asthma risk in children and undescended testis in boys. An investigation published in November linked mothers' frequent Tylenol use (at least 28 total days) during pregnancy with children's decreased motor and communication skills as well as behavioral issues.
"All these reports so far have suggested that this common painkiller and fever reducer may not be as harmless as the users would think, especially while taken during pregnancy where the fetuses are more vulnerable and susceptible to environmental exposures during a critical developmental period," Liew said.
Yet both the authors of the new study and outside experts are quick to caution that the findings are preliminary and constitute just an observed link -- not a causal relationship. The study has gaps in information, such as how many tablets the women took at any given point, a team of independent researchers with Cardiff University in Wales wrote in an editorial that ran alongside the new study.
"Findings from this study should be interpreted cautiously, and should not change practice," the Cardiff University scientists wrote, emphasizing that there are certain conditions for which medication during pregnancy is essential.
Liew agreed that there is much still to be investigated. "But personally," he said, "I think given the results of this study and previous studies showing the potential health risk, pregnant women should be more cautious."
"If possible, do not take it when it is not necessary, and try to reduce the amount of usage when needed," he said.
ADHD is among the most common brain disorders of childhood, often characterized by an inability to stay focused, impulsivity and hyperactivity. Its precise causes are unknown, although experts believe a combination of genetic and non-genetic factors are at play.