Is acetaminophen harmful in pregnancy?

The short answer to that question is: probably not but we cannot be certain.

Let me beg your indulgence in an explanation about why that is the only honest answer I can give you.

The “gold standard” in assessing the risks and benefits of any medication is the “controlled trial”. In such a trial, a large group of people are randomly assigned to drug A or drug B or to Drug A or placebo. This tends to ensure that differences in outcomes between the people taking one treatment or the other are due to the treatment and not to the characteristics of the people taking them. With a large enough group and truly random assignment, differences among the subjects are assumed to be evenly spread.

Historically, drug trials have excluded pregnant women because the trial sponsors worried that they would be sued for any bad birth outcomes. While this did protect the trial sponsors, it meant that most of the time, we were totally in the dark about the safety and efficacy of drugs when prescribed to pregnant women.

The fallback has been the “observational trial.” You look at a group who took a given drug and compared their outcomes to a group who did not.

The enormous problem with observational trials is that they are often comparing apples to oranges. People who take a medicine on their own are not the same as those who do not.

Let us take acetaminophen as an example. We know that about 60% of women take it during pregnancy. Most of the trials that report more neurodevelopmental disorders (autism, ADHD) in acetaminophen users compare women who used it with those who did not.

What is the commonest reason for acetaminophen use? It is fever. Thus, acetaminophen users are much more likely to have had febrile illnesses than those who did not use it. If there is an increased incidence of autism among users, the culprit might just as easily be the febrile illness, not the drug.

Until someone – and realistically it will have to be the NIH, as no commercial firm will pay for it – does a controlled trial comparing pregnancy outcomes among women who take acetaminophen with those who take a placebo, we will be offering advice either way with imperfect data.

The closest I can find to a good study is one out of Sweden that used successive pregnancies of women who had more than one child, comparing the siblings and looking at acetaminophen use. This study found no increase in autism, ADHD or learning problems caused by acetaminophen use. Again, though, this was an observational study.

Given this, what is the best advice you can take?

First, as is true for all medications, take medication only when you need it. A temperature of 99.6 does not need treatment. A temperature of 102 is harmful to the fetus and should be treated. If you have a backache that can be relieved with heat or a backrub, skip the pills. If you are very uncomfortable, acetaminophen is clearly safer than anti-inflammatories such as ibuprofen or naproxen, that are known to cause fetal malformation, or narcotics.

Second, try to take it for as short a time as possible. The limited data we have suggests that chronic use is worse than occasional use, and that makes physiological sense.


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