The generic drug acetaminophen, also known as paracetamol, is the active ingredient in the popular over-the-counter pain reliever and fever reducer, Tylenol, and also an ingredient in versions of other drugs such as Theraflu, Excedrin and Mucinex. Many people take it without a problem.

But there have been concerns about whether it's safe to take acetaminophen during pregnancy after studies found that when people took it while pregnant, it raised the risk of autism and sleep and attention problems in their children.

There was no increased risk of neurodevelopmental disorders in the offspring of women who took acetaminophen during their pregnancy.

A new comprehensive study, the largest to date, challenges these findings.

More than 2.4 million Swedish children were included in a study which found that there was no increased risk of neurodevelopmental disorders in the offspring of women who took acetaminophen during their pregnancy.

The study was unique because it used a sibling comparison design. This method eliminates the possible influences of variables like family differences (such as parents' disciplinary style) or environmental factors (such as exposure to substances like pesticides) that might affect an apparent relationship between acetaminophen use during pregnancy and risk of neurodevelopmental conditions.

“Users of acetaminophen differ from non-users in a number of ways and standard statistical analyses without a sibling control can't control for all of the differences,” co-senior author Brian Lee, PhD, an associate professor in Drexel's Dornsife School of Public Health, fellow at the A.J. Drexel Autism Institute, and research affiliate at the Karolinska Institute, said in a press release.

The researchers followed each sibling up to 26 years after birth. Although they did find a small increased risk of autism, ADHD and intellectual disability in the overall population, they reported no increased risk of any of these conditions when one sibling was exposed to acetaminophen while in the uterus before birth — and the other sibling was not.

Since siblings share a considerable portion of their genetic backgrounds, as well as similar exposure to environmental factors during their development, comparing siblings helped to control these shared elements that ordinarily are hard to measure during epidemiological studies which focus on the incidence, distribution and control of diseases.

The research team also took note that the study's parents were already cautious about the drug's use. As a result of their caution, only 7.5 percent of the children were exposed to acetaminophen during pregnancy.

“This study's findings may be welcome news for birthing people who use acetaminophen as a pain or fever management option, since there are few safe alternatives for relief available,” co-senior author of the study, Renee M. Gardner, of Sweden's Karolinska Institute, said in a press release.

“We hope that our results provide reassurance to expectant parents when faced with the sometimes fraught decision of whether to take these medications during pregnancy when suffering from pain or fever.”

The authors stress that all patients should continue to follow guidance from their physician on whether acetaminophen is safe for them and their future children.

The study is published in JAMA, Journal of the American Medical Association.