The majority of Americans have used Tylenol at one point or another. In fact, many likely have it in their bathroom cabinets. The over-the-counter pain reliever is employed for all kinds of discomfort, from fevers to cramps to headaches. But this pharmaceutical household name has recently been tied to autism spectrum disorder, with President Donald Trump warning pregnant women against its adverse effects in the womb. But for years, Tylenol has been recommended to ease aches and pains during pregnancy, has something changed?

Acetaminophen (C8H9NO2), the main ingredient in Tylenol, is a planar molecule containing a central benzene (C6H6) ring surrounded by other molecules. While its exact mechanism of action has not been cemented, it is generally considered to act in line with other NSAIDs (non-steroidal anti-inflammatory drugs). NSAIDs inhibit two different kinds of cyclooxygenase enzymes: COX-1 and COX-2. COX enzymes are used in the creation of prostaglandins, lipids that cause inflammation by dilating blood vessels and attracting white blood cells to injured areas. These COX enzymes convert arachidonic acid into prostaglandins, thus NSAIDs that block them reduce production and feelings of pain. However, acetaminophen tends to act on COX enzymes within the central nervous system– the brain and spine– rather than those of the peripheral– the rest of the body. It also exhibits an inhibitory effect on the hypothalamus, which is responsible for regulating body temperature and thus controlling fevers. As such, Tylenol can reduce high temperatures and sensations of pain but have less of an anti-inflammatory effect compared with other NSAIDs, such as ibuprofen. COX-1 is known as a housekeeping enzyme, meaning it is crucial to general body functions, like producing prostaglandins that help to protect the lining of the stomach and stimulate platelet aggregation. COX-2 is slightly more specialized as an inducible enzyme, meaning it is activated by a specific stimulus. Areas of injury or disease tend to stimulate COX-2, where it produces prostaglandins involved in inflammation, such as fever and swelling. 

All drugs pose risks, but according to the American College of Obstetricians and Gynecologists (ACOG), Tylenol is a safe option for fever and pain relief in pregnant women. It is recommended to be used judicially or as little for as short a time as possible. This is because chronically overusing acetaminophen can cause liver and kidney damage, as well as anemia. Still, the dangers of untreated maternal fevers are arguably more severe than those of Tylenol. In the first 12 weeks, fevers sharply increase the risk of neural tube defects, the structure that eventually gives way to the spinal cord and brain. Spina bifida, the exposure of the spine and its protective meninges, and anencephaly, missing portions of the skull and spinal cords, are two prevalent outcomes. Untreated pain can also result in poor sleep quality, increased blood pressure, and a hostile maternal environment due to stress– all incredibly dangerous for the fetus.

Though numerous studies have explored the association between Tylenol and an increased risk of autism, according to the FDA, a clear or even casual relationship between the two has not been established. A risk is more evident with chronic acetaminophen use, which can damage a still-developing fetal liver that is unable to metabolize drugs easily. Commissioner of Food and Drugs Martin A. Makary emphasizes that acetaminophen is the safest over-the-counter option during pregnancy, especially compared with ibuprofen and aspirin. Though Health and Human Services Secretary Robert F. Kennedy Jr. has vowed to launch a public service campaign on the autism-acetaminophen debate, Donald Trump’s proclamation to pregnant women in his September news conference: “Fight like hell not to take [Tylenol]” is unsubstantiated, to say the least.