Pregnant women who take acetaminophen (paracetamol) – best known as Tylenol – might raise the risk that their child will develop behavioural problems such as attention-deficit/hyperactivity disorder (ADHD), a new study suggests.
Acetaminophen is generally considered safe in pregnancy so safe, in fact, that at least two-thirds of women turn to it while expecting, according to the American Academy of Family Physicians.
But when pregnant women in Britain used the pain reliever, it appeared to increase the risk of behaviour problems cropping up in their children by the time they turned 7, said lead researcher Evie Stergiakouli, a lecturer in genetic epidemiology at the University of Bristol.
Still, the study couldn't prove cause-and-effect, and Stergiakouli believes that women should still take the drug if needed.
In the study, taking acetaminophen between 18 and 32 weeks in pregnancy was associated with a 42 percent increased risk of behaviour problems in children and a 31 percent increased risk of hyperactivity, the researchers found.
The investigators also saw a 29 percent increased risk of emotional problems and a 46 percent increased risk of overall behavioural difficulties in children of women who used acetaminophen at 32 weeks of pregnancy.
No similar association cropped up in mothers who used acetaminophen after delivery, nor did it occur if the father used the over-the-counter drug, the findings showed.
"Only acetaminophen use during pregnancy has the potential to cause behavioural problems in the offspring," Stergiakouli said.
These findings jibe with two other large-scale studies that have suggested acetaminophen may have an adverse effect on a baby's brain during pregnancy, said Dr Andrew Adesman. He is chief of developmental and behavioural paediatrics at Cohen Children's Medical Centre in New Hyde Park, New York.
However, Adesman noted that a mom who needs the pain reliever will not automatically condemn her child to future behavioural problems.
"Although sophisticated statistical testing did indeed demonstrate an increased risk for behaviour and emotional problems following prenatal exposure to acetaminophen, it is important to remember that the overwhelming majority of children exposed prenatally to acetaminophen do not end up having any of these behavioural or emotional problems," Adesman said.
Stergiakouli believes pregnant women may still have good reason to take acetaminophen.
"There is a risk of not treating fever or pain during pregnancy, and this should be carefully weighed against any potential harm to the offspring," she said. "For example, untreated fever during pregnancy can lead to premature labour."
Dr Noel Strong, an assistant professor of obstetrics, gynaecology and reproductive science with the Icahn School of Medicine at Mount Sinai in New York City, said the new study is "not going to make me change my current practice".
"It's demonstrating an association, but it's a mild association at best," Strong said. "It still appears to me to be our safest pain medication for use in pregnancy. The study won't make me abandon that – yet."
For the long-term study, Stergiakouli and her colleagues analysed data for almost 8,000 mothers who enrolled in the study between 1991 and 1992, along with their children and partners.
Questionnaires assessed the women's acetaminophen use at 18 and 32 weeks during pregnancy, and again when their children were 5 years of age. A follow-up questionnaire assessed behavioural problems in children as reported by mothers when the kids turned 7.
About 53 percent of the mothers reported using acetaminophen at 18 weeks of pregnancy, and 42 percent said they used the pain reliever at 32 weeks, the findings showed.
About 5 percent of the children developed behavioural problems. This was similar to National Health Service estimates that 2 percent to 5 percent of school-aged British children have ADHD, but far below the 11 percent reported in the United States.
The findings were published online in the journal JAMA Paediatrics.
The researchers speculated that acetaminophen might affect foetal brain development by altering a mother's hormone levels, or by crossing the placenta and directly affecting the unborn child.
ADHD is a complex disorder involving many different potential factors, Strong said, and it's unlikely that any single element causes it.
"There are many degrees of separation and many bridges of evidence that would have to be crossed before you can establish a causal relationship," Strong said. "This needs a lot more science behind it."
These studies only show a potential association and do not prove a cause-and-effect link between acetaminophen and behaviour problems, Tylenol manufacturer McNeil Consumer Healthcare said in a statement.
"It is important to note there are no studies demonstrating a causal link between acetaminophen use during pregnancy and adverse effects on child development," the company said. "The authors of the recent study in JAMA Paediatrics stated that further studies are required to determine a causal explanation, and that the benefits of using any medication during pregnancy should be carefully weighed against any risks."