Paracetamol, commonly known as Panadol and Tylenol, has long been considered a safe medication for pregnant women to take for pain, fever, and even the flu. But a new study suggests that the risk of children experiencing autism spectrum disorder symptoms can be traced back to their mom’s use of OTC painkillers.

A team of researchers in Spain studied 2,644 mother-child pairs, starting with pregnancy and going through the child’s fifth birthday. Women were asked if they ever took paracetamol during pregnancy, and if so, if their use was sporadic or persistent. And at age one and five, respectively, researchers gave children computerized tests to measure their attention and visual speed processing.

Compared to children who had not been exposed to paracetamol during pregnancy, those who were exposed scored lower on tests for hyperactivity, impulsivity, and visual speed, the amount of time it takes to respond to a visual stimulus. These effects were greater among children whose mothers persistently took the painkiller during pregnancy, and in boys especially, there was an increased risk for autism. The latter echoes a 2013 study published in Environmental Health that found autism was more prevalent in boys who were prenatally exposed to the drug.

Tylenol Tylenol tablets, which contain acetaminophen, are shown on April 14, 2015 in Chicago, Illinois. Scott Olson/Getty Images

“Although we measured symptoms and not diagnoses, an increase in the number of symptoms that a child has, can affect him or her, even if they are not severe enough to warrant a clinical diagnosis of a neurodevelopmental disorder,” lead author Claudia Avella-Garcia said in a press release.

Avella-Garcia’s co-author, Dr. Jordi Júlvez, speculated that the potential link between a mother’s paracetamol use and her fetus’ neurodevelopment could have to do with the way the painkiller acts on the cannabinoid receptors in the brain. These receptors are part of the Endocannabinoid system, which is involved in several physiological processes, like feelings of pain, appetite, and mood. It’s also the system responsible for the widely sought after runner’s high.

“Since these receptors normally help determine how neurons mature and connect with one another, paracetamol could alter these important processes,” Júlvez said. “It can also affect the development of the immune system, or be directly toxic to some fetuses that may not have the same capacity as an adult to metabolize this drug, or by creating oxidative stress.”

Júlvez added that the male brain may be more vulnerable to any harmful side effects of persistent paracetamol use, hence the higher autism rates he and his team see among boys. But according to the National Autism Association, boys are already four times more likely to develop the neurological disorder than girls. Beyond Tylenol, autism is caused by environmental and genetic risk factors, or so scientists have so far been able to establish.

Some more evidence of the link between paracetamol and disrupted early development: In February, a study published in the International Journal of Epidemiology suggested that prenatal exposure to paracetamol increased risk for asthma, one of the many medical conditions associated with autism. And back in 2013, a study from the Norwegian Institute of Public Health found that children who were exposed to the painkiller for more than 28 days of pregnancy had poorer gross motor skills, poor communication skills, and more behavioral problems. If there’s a connecting link between all of these studies it’s that very little research is devoted to understanding adverse outcomes of prenatal exposure to drugs like Tylenol.

While researchers maintain that their findings could shed some light on the number of ADHD and autism disorders diagnosed today, they recognize that more research needs to be done before they can give new recommendations on painkiller use during pregnancy. In addition to only measuring symptoms, researchers were unable to measure the exact doses women took. Additional studies will not only help narrow down a precise dosage, but it will shed light on the risks and benefits of taking Tylenol during pregnancy.

Source: Avella-Garcia CB, et al. Acetaminophen Use in Pregnancy and Neurodevelopment: Attention Function and Autism Spectrum Symptoms. International Journal of Epidemiology, 2016.