Widely used antidepressants known as selective serotonin reuptake inhibitors (SSRIs) may delay a woman’s breast milk production soon after giving birth, a new study hints.

In a survey of 431 women who gave birth at one U.S. medical center, researchers found that seven of the eight women on SSRIs did not have their breast milk come in within the typical 72 hours of giving birth. On average, their full breast milk production was delayed by about a day compared with other mothers.

In a separate part of the study, the investigators also found that SSRI medication affected the functioning of human cell samples from the breast tissue lining, and appeared to alter breast milk production in lab mice.

After giving birth, women produce a precursor to breast milk called colostrum until their full breast milk comes in; if that shift does not happen within 72 hours, researchers consider it “delayed secretory activation.”

The concern with this is that some infants may start to become dehydrated, and that some mothers, worried and frustrated, may give up on breastfeeding, said Dr. Nelson Horseman, a professor of molecular and cellular physiology at the University of Cincinnati in Ohio, and the senior researcher on the study.

Still, Horseman said the findings should not prompt pregnant women on SSRIs to stop taking the drugs - which include medications like sertraline (Zoloft), paroxetine (Paxil) and fluoxetine (Prozac).

For one, the results are based on only eight women who were taking the medications, and larger studies are still needed to confirm the findings. Moreover, patients on prescription drugs are generally warned against stopping on their own.

“Separate from our findings,” Horseman said in an interview, “it’s clear that women on these medications should not stop taking them without talking with their doctors.”

However, he added, women should be aware that delayed breast milk production may be a “natural side effect” of SSRIs.

If they have concerns, Horseman said, expectant or new moms can ask for help from their doctors or a lactation consultant - specialists in breastfeeding issues who work in some hospitals and also in private practice.

The study, which is published in the Journal of Clinical Endocrinology and Metabolism, builds on evidence from the same researchers’ earlier work showing the importance of serotonin in breast milk secretion.

The hormone is involved in regulating mood, and SSRIs are believed to treat depression by increasing serotonin availability in the brain. In breast tissue, Horseman explained, serotonin appears to inhibit milk production; SSRIs, therefore, may extend the amount of time it takes for a new mother’s breast milk to come in.

Horseman said that future research should look not only at the effects of SSRIs on the initial stages of breast milk secretion, but also any potential effects on long-term breastfeeding.

“We’d like to know if there are any changes in the lactation process in women who start using these drugs after they begin breastfeeding - as they might for postpartum depression,” Horseman said.

SOURCE: Journal of Clinical Endocrinology and Metabolism, February 2010.