Paroxetine in Breast Milk

To the Editor: Zachary N. Stowe, M.D., and colleagues expressed concern in their article (1) that the manner in which we have reported serum levels might be misleading: “The reporting of ‘0 ng/ml’ in infant serum…could be misinterpreted by clinicians as suggesting a complete absence of infant exposure to medication.” We welcome the opportunity to clarify the terms used to describe small amounts of drug in breast-fed infants’ sera, as we have done previously (2). Assays have a limit of quantifiability. In the report cited (1), it was 2 ng/ml, which means that the limit of the assay for reliable quantification was 2 ng/ml (not truly a limit of detection). Levels below 2 ng/ml frequently are detectable but not are reliably quantified by analytical readouts. Our report of 0 ng/ml means that no amount was detected above the baseline (within instrumental limitations). We use three categories of exposure in our developmental studies of infants (0, not detectable; <2 ng/ml, less than reliably quantifiable but detectable; and ≥2 ng/ml, a reliably quantifiable numeric amount). We have stated that the unknown neurochemical effects of even these small amounts of drug or metabolite remain a concern (3).

More significant than the format of reporting assay results is the pharmacochemical assessment of paroxetine exposure. The principal paroxetine metabolite, a methylated and conjugated catechol, was not found in maternal and baby sera or breast milk measurements (1). Although this metabolite is neurochemically inactive, serum concentrations provide data for assessing material balance and treatment adherence. Data for levels of paroxetine and its metabolite have been reported in children (4).

We are concerned that the readership may misinterpret a statement in the article by Dr. Stowe et al. (1). Parents were asked if “the pediatrician had been informed of maternal paroxetine use.” The pediatrician must be part of the decision-making team that evaluates the risks and benefits of breast-feeding during pharmacotherapy. Our policy is to discuss the risk-benefit analysis with the baby’s pediatrician and document this conversation in the record (3). This procedure allows an opportunity for resolution if the pediatrician disagrees with the plan, as well as a chance to update him or her about new data in this rapidly evolving field.

We applaud the publication of serum level data for infants whose mothers took paroxetine during breast-feeding and appreciate the significance of the contribution of Dr. Stowe et al. (1) to the literature.