Medications and Lactation: What PNPs Need to Know

Maternal Plasma Concentrations

Drugs enter, and in almost all cases exit, breast milk in a direct relation to the concentration of the medication in maternal plasma. The higher the milk/plasma (M/P) ratio, the greater the amount of the drug found in breast milk. When the maternal plasma concentration of a medication falls below its concentration in breast milk, equilibrium forces drive the medication out of the breast milk and back into the maternal plasma for elimination (Hale, 2002). The M/P ratio gives an estimate of a drug's distribution between maternal milk and plasma. A M/P ratio greater than 1.0 suggests that the drug may be sequestered in breast milk in high concentrations. For example, a M/P ratio of 4.0 means that the concentration of the medication in breast milk is four times higher than it is in maternal plasma (Riordan & Auerbach, 1998), whereas a M/P ratio less than 1.0 is considered low and is indicative that only low drug concentrations will be transferred into breast milk. While it is best to choose drugs with low M/P ratios, a high M/P ratio does not necessarily contraindicate the use of a particular medication by a breastfeeding mother (Hale, 2002). This is because the M/P ratio does not provide all of the information about the absolute amount of a drug that can be transferred to the infant from breast milk (Hale, 2002). Even if the medication has a high M/P ratio, if the maternal plasma concentration of the drug is minute, then the absolute amount (i.e., dose) of a drug entering breast milk will likely be small enough to be considered subclinical for the infant (Hale, 2002). Thus, higher M/P ratios can be misleading because they can provide the impression that large amounts of a medication are transferred into breast milk, and this may not be the case if the maternal dosage is low.

The pKa of a drug is the pH at which a drug exists equally in ionic and nonionic states. Drugs that have a pKa higher than 7.2 may be sequestered in breast milk to a slightly higher degree than those with a lower pKa (Hale, 2002). This occurs when the nonionized molecule of a high pKa (i.e., weakly basic) drug enters breast milk, is quickly ionized, and then is prevented from diffusing out of breast milk back into the maternal circulation. Thus, the drug is "trapped" in its ionized form and accumulates in the breast milk at high M/P ratios. Since drugs that have a higher pKa tend to have higher M/P ratios, medications with a lower pKa are favored for breastfeeding women (Hale, 2002).

Cite this: Medications and Lactation: What PNPs Need to Know - Medscape - Nov 01, 2003.

Table. Drug information resources
Source	Contact Information
The Lactation Center	University of Rochester Medical Center 585-275-0088
Crozer-Chester Medical Center Drug Information Center	One Medical Center Boulevard Upland, PA 19013 610-447-2851
Brigham and Women's Hospital Boston, MA	617-732-5500; ask for clinical pharmacologist
Drugs in Lactation Web site (Hale)	www.neonatal.ttuhsc.edu/lact
Hale, T. W. (2002). Medications and Mother's Milk (10th ed.). Amarillo, TX: Pharmasoft	Ordering information: Pharmasoft Publishing Single copies $24.95, plus shipping 21 Tascocita Circle Amarillo, TX 79124-7301; tel: 800-378-1317; fax: 806-376-9901; www.iBreastfeeding.com
Drug Topics Red Book (106th ed.) (2002). Montvale, NJ: Medical Economics	Ordering information: www.medecbookstore.com
Briggs, G. G., Freeman, R. K., & Yaffe, S. J. (2002). Drugs in Pregnancy and Lactation (6th ed.)	Ordering information: Lippincott Williams & Wilkins, 530 Walnut Street Philadelphia, PA 19106

Commenting is limited to medical professionals. To comment please Log-in.

Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.