The U.S. Food and Drug Administration is strongly urging Pfizer to update the label on its antidepressant Zoloft to reflect findings of increased heart defects in babies whose mothers took the drug early in their pregnancies.

However, the drug maker, which is currently embroiled in multidistrict litigation in Pennsylvania federal court over allegations of Zoloft birth defects, claims the proposal was not prompted by new data or litigation, but was instead motivated by an FDA effort to revamp pregnancy labeling on all prescription drugs.

The proposed Zoloft label update will still emphasize that the “weight of evidence” indicates no difference in major birth defects between children of women who took the medication during their first trimester and those born to women who didn’t take it.

Additionally, the proposed label would state, “an increased risk of congenital cardiac defects, specifically septal defects . . . was observed in some published epidemiological studies” involving women who took the drug during their first trimesters, but would also include caveats about the limitations of those studies.

Pfizer has similarly argued in litigation that there isn’t enough research to suggest that taking Zoloft during pregnancy can lead to birth defects. It has argued that the background rate of birth defects has not seen a statistically significant change since the advent of selective serotonin reuptake inhibitors, or SSRIs, like Zoloft.

However, Zoloft litigation has uncovered internal memos that the drugmaker’s own scientists issued warnings for years that the antidepressant may cause congenital birth defects.

The FDA issued a final ruling in December requiring that drugmakers more clearly list the potential risks of taking a drug like Zoloft during pregnancy. The agency said the new labeling rules establish “detailed subsections that describe risks within the real-world context of caring for pregnant women who may need medication.”

About Zoloft

Zoloft has been widely prescribed to patients who suffer from symptoms of panic disorder and depression. It was originally approved for use in the United States by the FDA in 1991.

Zoloft is a member of a class of drugs known as selective serotonin reuptake inhibitors, or SSRIs. Zoloft was marketed as a safe medication as compared to other drugs that were on the market at the time of its approval.

That message proved to be effective, as Zoloft eventually became one of the most prescribed medications in the United States. At its height, tens of millions of prescriptions for Zoloft were written annually, generating more than $2.5 billion in revenue during 2006 alone.

Zoloft Lawsuits

As of July 2015, 545 Zoloft lawsuits are pending throughout the federal court system making similar allegations that drug maker Pfizer failed to warn consumers about potential birth defects as a result of women taking Zoloft during pregnancy.

Zoloft birth defect lawsuits have been filed by the families of children born with severe physical birth defects including cardiac defects, spinal malformation, pulmonary hypertension, lung, abdominal, and cranial defects among others. All of these observed birth defects allegedly stem from exposure to the antidepressant during pregnancy.

The first bellwether trial is expected to begin in January 2016 and the outcomes of these early Zoloft bellwether trials will not be binding on other Zoloft lawsuits but may help determine how the parties want to proceed.