Oral decongestants such as Sudafed should be avoided if possible, as it contains Pseudoephedrine which when taken orally can reduce milk supply. (abckidsinc.com)
Further, the effects of pseudoephedrine for nasal congestion have not been widely studied in very young children. (babble.com)
For this reason, oxymetazoline is a probably a better choice than oral systemic decongestants such as pseudoephedrine during breastfeeding. (infantrisk.com)