Dr. Anderson raises an interesting point regarding the calculation of the Relative Infant Dose (RID) and suggests it would be more appropriate to use a maternal dose of 200 mg of armodafinil rather than 400 mg of modafinil. Modafinil (PROVIGIL TM ) and armodafinil (NUVIGIL TM ) are marketed as two distinct drugs. Modafinil is a racemic (50:50) mix of two enantiomers, S-modafinil and R-modafinil (armodafinil) (Murillo-Rodríguez et al., 2018). Although these enantiomers are pharmacologically equipotent, they have different pharmacokinetic profiles; the terminal half-life of the R-enantiomer (15 hr) is significantly greater than the S-enantiomer (4-5 hr; Darwish et al., 2010) and is considered responsible for the majority of the pharmacological effects (Aurora et al., 2018). The U.S. Food and Drug Administration does not regard modafinil and armodafinil to be bioequivalent (Darwish et al., 2010). We measured the concentration of armodafinil in human milk and used the maternal dose of modafinil to calculate the RID (4.85%), which is consistent with the method and calculation used by Aurora et al. (2018), and cited in several reputable lactation resources, including LactMed ® , e-lactancia ® and HalesMeds ® . We note Dr. Anderson's comment regarding armodafinil, and, based on our results, it could be inferred that if a breastfeeding mother were to be treated with the drug armodafinil, then it is possible that the RID could be as high as 9.7%, which is still below the nominal safety value of 10%.Dr. Anderson further comments that the RID may be about 12% if the therapeutic contribution of the S-enantiomer is included. Our calculations do not correlate with this figure. According to the U.S. product information, modafinil total exposure could comprise 75% R-modafinil and 25% S-modafinil at steady state concentrations. If we consider this scenario, where the ratio of R-modafinil to S-modafinil is 3:1, then, theoretically, our reported C average concentration would be approximately 33% higher, that is, an increase from 1.96 mg/L (R-modafinil only) to 2.61 mg/L (R-and S-modafinil). The Theoretical Infant Dose (TID) could increase from 0.294 mg/kg/day (R-modafinil) to 0.392 mg/kg/ day (R-and S-modafinil), a dose still approximately 10 times