Montelukast is a commonly used drug in allergies,  and as you know, eveyone has allergies.  This data shows that levels in milk are really quite low (RID = 0.68)  and there is no obvious risk to a breastfeeding infant.  TWH

Title: Transfer of Montelukast into Human Milk During Lactation.

Authors: Datta P, Rewers-Felkins K, Baker T, Hale TW.

Breastfeed Med. 2017 Jan/Feb;12:54-57.

Abstract:

The aim of this study was to determine levels of montelukast in human milk and to develop a simple, sensitive analytical method using mass spectrometry. Milk samples were collected from seven breastfeeding mothers, age 26-35 years, at 0, 1, 2, 4, 8, and 12 hours after oral ingestion of 10 mg montelukast. The samples were analyzed using a new Liquid Chromatography-Tandem Mass Spectrometry method. Area under the milk concentration time curve from zero to the time of the last sample (12 hours) was estimated by the linear trapezoidal rule. Average montelukast levels (C_avg) in milk were 5.3 ng/mL, and the relative infant dose was 0.68% of the maternal dose. The maximum concentration (C_max) observed at 4 hours (Tmax) was 9.7 ng/mL. The exposure to the infant seems to be very low, far below therapeutic ranges in an infant. Our data suggest that montelukast is probably safe to use in a breastfeeding mother.

Table:  Pharmacokinetic parameters of montelukast in breastfeeding women (n=7).

Parameter (units)               Value (Range)

     AUC^a (ng·h/mL)               63.88 (34.50 – 109.52)

       C_avg^b (ng/mL)                  5.32 (2.87 – 9.12)

      C_max^c (ng/mL)                 9.74 (4.93 – 16.16)

       T. _max^d (h)                       4 (2 – 4)         

      Infant dose (μg/kg/day)  0.79 (0.43 – 1.36)

       RID^e (%)                        0.68 (0.37 – 0.96) 

^a Area under the Drug Concentration-Time Curve, ^b Average Drug Concentration across the Dose Interval, ^c Maximum Drug Concentration across the Dose Interval, and ^d Time at which maximum concentration is observed, ^e Relative Infant Dose for montelukast in Milk %.

Figure: Mean milk concentration-time profile of montelukast in human milk following the oral administration of montelukast 10 mg tablets to breastfeeding mothers (n=7)

PMID 28002678