Medical experts urge caution over use of acetaminophen-based painkillers during pregnancy

People who are pregnant should be cautious about taking acetaminophen as it may alter fetal development, according to a coalition of international public health experts.

In a consensus statement published on Thursday in the scientific journal Nature Reviews Endocrinology, the group stresses that while acetaminophen is common — it's found in hundreds of pain-relief medications, including Tylenol, Midol, Nyquil and some Benadryl products — there's growing evidence it comes with risks when used during pregnancy.

There is increasing "experimental and epidemiological research" which suggests that prenatal exposure to acetaminophen might alter fetal development, reads the statement.

The experts, from a range of countries including the U.S., Sweden, Denmark, Brazil and Scotland, recommend that pregnant women should be cautioned at the start of their pregnancy to forgo taking acetaminophen unless its use is medically indicated.

Women should also consult with a physician or pharmacist if they're uncertain about whether to use painkillers with this ingredient and before using it on a long-term basis, the group wrote, and should minimize their exposure "by using the lowest effective dose for the shortest possible time."

Outside experts also agree people who are pregnant need to be aware of the possible risks.

"I think the reason it's important to talk about this … is because there's sort of this widespread perception that there is a negligible risk to acetaminophen," said Dr. Tali Bogler, the chair of family medicine obstetrics at St. Michael's Hospital in Toronto, during an interview with CBC News.

But the news shouldn't be "cause for alarm," she added.

"I think it should be a cause for discussion and more awareness about acetaminophen use as a drug in pregnancy," Bogler said.

WATCH | Why some experts are urging caution around acetaminophen use in pregnancy:

Drugs long thought to be of 'minimal risk'

The team behind the paper noted that drugs containing acetaminophen are widely used by pregnant women, with various international regulators long considering the products to be of "minimal risk" when used as directed during pregnancy.

"In general, doctors say it is usually safe to take acetaminophen (Tylenol) for fever and pain," reads advice from B.C.'s online health portal, for instance.

Pregnant women typically use pain-relief drugs with acetaminophen to treat headache, muscle pain, back pain and infection, the authors of the journal statement wrote, though in some instances those painkillers are used to treat high fever and severe pain that, left untreated, could potentially affect the developing fetus or the mother.

Chemicals that disrupt the endocrine system can interfere with certain hormones "that are essential for healthy neurological, urogenital and reproductive development," the team said.

"We are concerned about increasing rates of neurological, urogenital and reproductive disorders," their statement reads. "We are witnessing disturbing increases in the number of children with cognitive, learning and/or behavioural problems."

The relationships between prenatal exposure and adverse neurodevelopmental outcomes have been investigated in 29 observational studies involving more than 220,000 mother-child pairs from different areas of the world, according to the group.

Of those studies, 26 identified "positive associations" with exposure to acetaminophen and a range of outcomes for the child, including attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and language delays.

But there isn't clear evidence actually linking drugs like Tylenol to the development of conditions like autism, said obstetrician-gynecologist and University of Toronto assistant professor Dr. Modupe Tunde-Byass, who feels that's where the consensus statement's messaging falls short.

Medical professionals need to urge caution to pregnant patients without causing them "undue anxiety," she added.

More research needed, experts say

Jonathan Zipursky, an internist and clinical pharmacologist at Sunnybrook Health Sciences Centre in Toronto, stressed that the paper isn't calling for pregnant women to stop using drugs like Tylenol altogether.

"They're calling for more research in the future to clarify the safety of Tylenol in pregnancy," he said. "And in addition, how to use Tylenol in the safest way possible during pregnancy."

Acetaminophen-based drugs are still less of a concern than non-steroid anti-inflammatory drugs (NSAIDs), which Health Canada has issued warnings about as recently as this summer.

In June, the federal department announced it had completed a safety review confirming that the use of drugs such as acetylsalicylic acid (aspirin), ibuprofen, naproxen, diclofenac and celecoxib, starting from approximately 20 weeks of pregnancy or later, may cause rare but serious kidney problems in an unborn baby.

Tunde-Byass said in some cases, pregnant women may still need to rely on lower doses of acetaminophen for pain relief and can consult with their health-care provider for guidance.

"My advice in this moment in time is do other things first," she said. "Try other things. Try exercise, pay attention to your posture, try a warm compress first... if that does not help, then you can consider taking Tylenol."