A new report out Monday finds a troubling link: when women take the common painkiller acetaminophen when they are pregnant, their kids may be more likely to have behavioral problems such as hyperactivity.
But doctors have immediately questioned the inevitable headlines, and say this study does not show the pills affect unborn babies.
And even if it did, the overall rates are low. Just 5 percent of the kids in the study had behavior problems.
Acetaminophen, sold generically and under the brand name Tylenol, is a top choice for pregnant women to use to control pain and fevers — it's considered a safe painkiller.
But a few studies have suggested that it might affect the brain development of unborn children. A study in 2013 found that women who took acetaminophen frequently during pregnancy raised the risk of behavior problems in their children by 70 percent.
Related: Can Tylenol Cause ADHD?
British doctors set out to find out more, studying the records of more than 7,700 women who took part in a study when they were pregnant in the early 1990s.
About half used acetaminophen, also called paracetamol, when they were pregnant. And while just 5 percent of children had behavioral problems at 6 or 7 years old, those who did were more likely to have been born to moms who remembered having used acetaminophen while pregnant, Evie Stergiakouli of the University of Bristol and colleagues reported.
"Maternal prenatal acetaminophen use at 18 weeks of pregnancy was associated with higher odds of offspring conduct problems and hyperactivity symptoms while maternal acetaminophen use at 32 weeks of pregnancy was associated with higher odds of offspring having emotional symptoms conduct problems hyperactivity symptoms and total difficulties," the researchers wrote in the Journal of the American Medical Association's JAMA Pediatrics.
"Children exposed to acetaminophen prenatally are at increased risk of multiple behavioral difficulties, and the associations do not appear to be explained by unmeasured behavioral or social factors linked to acetaminophen use insofar as they are not observed for postnatal or partner's acetaminophen use," they added.
Dr. Hal Lawrence, CEO of the American College of Obstetricians and Gynecologists, said pregnant women shouldn't worry.
"ACOG and ob-gyns across the country have always identified acetaminophen as one of the only safe pain relievers for women during pregnancy," he said in a statement.
Related: Studies Find Tylenol/ADHD Link
"This new study, and other studies that have been conducted in the past, show no clear evidence that proves a direct relationship between the prudent use of acetaminophen during any trimester and developmental issues in children."
While acetaminophen is considered a very gentle drug, it is not harmless. Large doses can cause liver damage and even kill. The Food and Drug Administration has asked drug companies to limit how much they put into products.
The researchers did try to address some of the questions doubters had. They asked the women, for instance, about symptoms that may have caused them to take pain medication. Infections such as influenza can affect brain development — for instance, they are linked to autism. It's one of the many reasons pregnant women are urged to get flu shots.
But the surveys didn't ask one key question, which is how often or at what doses the women took the pills. If the risk of behavior problems went up the more often women took the pills, that would be more of a smoking gun.
"This is all critical information that is missing in order to begin to ascertain a cause and effect," Lawrence said.
"Patients should not be frightened away from the many benefits of acetaminophen. However, as always, any medication taken during pregnancy should be used only as needed, in moderation, and after the expectant mother has consulted with her doctor."
British experts agreed.
"It is important to highlight that from these results we cannot determine a direct link between paracetamol usage and any behavioral problem," said Dr. Tim Overton, spokesman for the Royal College of Obstetricians and Gynaecologists.
"Women should not be alarmed by the results of this study and we recommend that pregnant women continue to follow current guidance and take the lowest effective dose for the shortest possible time when necessary."
The researchers say that given how common acetaminophen use is, that women should be aware of the findings. They say there are potential explanations of how pain pills might affect a developing fetus.
"The mechanism might involve disrupted endocrine function, which is important for neurodevelopment," they wrote.
Behavioral problems such as attention deficit hyperactivity disorder (ADHD) are being diagnosed more frequently. More than 10 percent of U.S. children, or about 5.8 million kids aged 5 to 17 years old, are now diagnosed with ADHD.