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The "safest" drug for relieving aches and pains, lowering fever and treating headaches in pregnancy may not be so safe after all, according to a new report — it may raise the risk of ADHD and similar disorders in their children.

Researchers found that pregnant women who frequently took acetaminophen, sold under the brand name Tylenol, were more likely to have children later diagnosed with attention deficit hyperactivity disorder (ADHD) and similar issues.

They say it doesn't mean that pregnant women must never, ever take the drug, but they said women may want to avoid using it frequently until more studies have been done.

"We aren't saying if you take one Tylenol once it will give your child hyperactivity," said Dr. Beate Ritz, an epidemiologist at the University of California Los Angeles who worked on the study. "You should just avoid chronic or long-term use."

Infections such as influenza in pregnancy are known to affect the brain development of babies — they're linked to autism, for instance. It's one of the many reasons pregnant women are urged to get flu shots.

So the researchers in this study were careful to ask women if they were taking the acetaminophen pills to reduce a fever to treat flu symptoms. Even when taking this into account, women who took acetaminophen were more likely to have children later diagnosed with ADHD.

The team used an ongoing Danish study of 64,000 children and their mothers, who were called up regularly during pregnancy and asked whether they had taken any painkillers at all. "All these women were asked had they taken any pain medications and fever medications, any medications," Ritz said.

About half had taken acetaminophen, also known as paracetamol, Ritz's team reports Monday in the American Medical Association journal JAMA Pediatrics.

"They also reported what kind of disorders they had. But there are viral infections that the woman doesn't even recognize. She just feels a little malaise." That is why it is possible that it may not be the drug itself to blame, but some infection the women had, Ritz and other experts point out.

It's also possible that women who feel pain more acutely, and take painkillers more often, may also have some sort of genetic predisposition that raises the risk of ADHD.

The researchers were not able to make a clear estimate of risk based on dose. But women who reported ever taking the drug had a 29 percent higher risk of having a child diagnosed with ADHD, and a 37 percent higher risk of a rarer diagnosis called hyperkinetic syndrome.

Still, the findings don't show a clear cause-and-effect.

"I don't think one study alone is enough to say nobody should use acetaminophen in pregnancy," said Dr. Jeff Chapa of Cleveland Clinic Children's Hospital, who was not involved in the study.

However, there are suspicions about acetaminophen. UCLA's Dr. Jorn Olsen, who designed and led the study, had set it up because of lab studies suggesting that acetaminophen might have a hormone-like effect on the developing fetus.

And it's not the first study to show something may happen in people, too. A study late last year showed that women who took acetaminophen frequently during pregnancy raised the risk of behavior problems in their children by 70 percent.

"Pregnancy is just a very sensitive period of time where the hormones are very important to development," Ritz said.

And although acetaminophen is considered a very gentle drug, it is not harmless. Large doses can damage the liver and even kill. The Food and Drug Administration has asked drug companies to limit how much they put into products.

It wasn't possible to compare acetaminophen to other painkillers and fever reducers, such as ibuprofen, because pregnant women so rarely took them, Ritz said.

Chapa raises another possibility. Maybe moms who favor acetaminophen for themselves also gave it to their children when they were very young and their brains were still developing. "Are those moms also more likely to give acetaminophen to their children after birth?" he asked.

Kate Langley, a neuroscientist at Britain's Cardiff University, notes that it's likely that more than one thing is going on with kids who develop ADHD. "ADHD is a complex disorder so it means that there are lots of different risk factors and we know a bit about some of them but we don't know a great deal," said Langley, who helped write a commentary on the findings. "We know that there are genetic and environmental and possibly prenatal (factors) but we know that none of them on their own cause ADHD."

ADHD diagnoses are on the rise. More than 1 in 10 children has been diagnosed with ADHD, according to the Centers for Disease Control and Prevention, although much of this is due to awareness and not a real increase in incidence, experts say.

Nonetheless, pregnant women need to weigh the risks, Ritz says.

"We make these tradeoffs all the time. We would not take a woman off antibiotics when she has a severe infection during pregnancy," Ritz said.

"It's the risk-benefit ratio. As much as we want to avoid every type of risk, it is not possible."

In this case, she advised, it might be worth toughing out a headache.

"The immediate take-home message is that if you are taking a lot, maybe you shouldn't, and really question whether you need to take it," Chapa added. "And if you do that's fine, but take a small quantity for a small duration. If you have chronic pain, maybe don't take acetaminophen for it."

McNeil Consumer Healthcare, which makes Tylenol, said pregnant and breastfeeding women whould consult their doctors before taking any medication.

"We are aware of the recent JAMA Pediatrics study; however, there are no prospective, randomized controlled studies demonstrating a causal link between acetaminophen use during pregnancy and adverse effects on child development," the company said in a statement.

NBC News intern Nikita Japra and Medical Fellow Hayley Goldbach contributed to this story.

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