(CNN) -- A new study published Monday in the journal Pediatrics found that long-term use of acetaminophen by pregnant women was associated with twice the risk of attention-deficit hyperactivity disorder in children. However, some experts believe the data do not support those findings.
Acetaminophen is the active ingredient in Tylenol as well as in hundreds of other over-the-counter and prescription medicines. Acetaminophen is commonly prescribed to pregnant women to relieve pain and fever, and the CDC said an estimated 65 percent of pregnant women in America use the drug.
The lead author of the new study said pregnant women who are using the medicine for fever or pain relief "should not refrain from short-term acetaminophen use."
However, the author said pregnant women who need the drug for longer periods of time should consult their physicians. "Longer periods" were classified as 29 days or more.
"We found that using acetaminophen for 29 days or more during pregnancy gave a 220 percent increase in risk for ADHD in the child," the lead researcher wrote.
However, researchers also found that pregnant women who used acetaminophen for less than seven days during pregnancy had a decreased risk of ADHD in their children.
In an interesting twist, researchers said fathers who used the pain reliever for 29 or more days prior to conception also had twice the number of children with ADHD.
"It could be that fathers who use a lot of acetaminophen have a higher genetic risk for ADHD," the lead researcher said, or that long-term use of acetaminophen could lead to changes in sperm.
Researchers concluded that they had not found a cause-effect relationship between the drug and ADHD, and they said they believe further study is needed.
Dr. Alison G. Cahill, a member of the American College of Obstetricians and Gynecologists' Committee on Obstetric Practice, was not part of the study and said the findings don't "add anything to our medical knowledge."
She said one of the design flaws of the study was the way in which ADHD diagnoses are assessed. She said researchers should have used the same careful and measured approach to assess all children; however, researchers calculated the number of children diagnosed with ADHD by their medical records.
"As you can imagine, there can be some disconnect between the making of that diagnosis and how it's actually coded," Cahill said.