The Journey of Medication into Breast Milk
Nearly all medications taken by a mother will pass into her breast milk to some degree, but most do so in such tiny amounts that they have no effect on the infant [1.3.1, 1.7.1]. The transfer from maternal plasma to milk is a complex process governed by several pharmacological principles. The core mechanism for most drugs is passive diffusion, where the medication moves from an area of higher concentration (maternal blood) to one of lower concentration (milk) [1.2.3].
In the first few days postpartum, the gaps between the milk-producing alveolar cells are larger, allowing easier passage for many drugs [1.2.3, 1.2.7]. As lactation becomes more established, these gaps close, making transfer more difficult. The infant's exposure depends on the drug concentration in the milk, the volume of milk consumed, and how the infant absorbs and metabolizes the drug [1.2.7].
Key Factors Influencing Drug Excretion in Milk
The amount of a specific drug that enters breast milk is not random; it is determined by a combination of the drug's properties and the mother's physiology [1.2.2].
- Molecular Weight: Drugs with a low molecular weight (generally under 500-800 daltons) pass into milk more easily. Conversely, large-molecule drugs like heparin and insulin have great difficulty crossing into the milk compartment and are considered safe [1.2.2, 1.3.2].
- Lipid Solubility: Human milk has a higher lipid (fat) content than plasma. Highly lipid-soluble drugs can dissolve in these milk fats and may concentrate in milk [1.2.2]. Many CNS-active drugs fall into this category [1.2.3].
- Maternal Plasma Levels: The concentration of a drug in the mother's blood is a primary driver of how much enters the milk. As plasma levels rise and fall, milk levels typically follow a similar pattern [1.2.2]. Medications that are poorly absorbed by the mother or are administered topically or via inhalation usually result in very low plasma levels and, consequently, insignificant milk levels [1.2.3].
- Protein Binding: Most drugs travel in the bloodstream bound to proteins like albumin. Only the "free" or unbound portion of the drug is available to move into the milk. Drugs with high protein binding (e.g., warfarin, ibuprofen) tend to have lower concentrations in breast milk [1.2.3, 1.6.3].
- pKa and Ion Trapping: The pH of breast milk is slightly more acidic than that of plasma. Some weakly basic drugs can become ionized in this acidic environment, a process which "traps" them in the milk compartment and can lead to higher concentrations [1.2.3].
Assessing Infant Risk: The Relative Infant Dose (RID)
To standardize risk assessment, experts use the Relative Infant Dose (RID). The RID calculates the dose an infant receives via milk (in mg/kg/day) as a percentage of the mother's dose (in mg/kg/day) [1.5.2]. It provides a way to estimate the infant's level of exposure relative to the maternal therapeutic dose [1.5.1].
A general consensus in the medical community is that an RID of less than 10% is considered a reference point for unlikely risk for most medications in a healthy, full-term infant [1.5.3, 1.5.6]. However, this threshold is not absolute. For certain potent drugs, like chemotherapeutics, even a very low RID could be harmful [1.5.3]. The RID is a valuable tool but must be interpreted with caution, considering the infant's age, health, and the drug's specific properties [1.5.1].
Common Medications: A Comparison of Safety
Making informed decisions requires knowing which drugs are generally considered safe and which warrant caution or should be avoided entirely. A healthcare provider should always be consulted before taking any medication while breastfeeding [1.4.4].
| Drug Category | Safe/Preferred Options | Use with Caution / Alternatives Preferred | Generally Avoid |
|---|---|---|---|
| Pain Relievers | Acetaminophen (Tylenol), Ibuprofen (Advil) [1.4.2] | Morphine, Hydrocodone (short-term, low dose) [1.4.2] | Codeine, Tramadol, long-term Naproxen, high-dose Aspirin [1.4.2, 1.3.4] |
| Antidepressants | Sertraline (Zoloft), Paroxetine (Paxil) [1.4.2, 1.7.3] | Fluoxetine (Prozac) [1.4.2] | Not specified, depends on individual risk/benefit |
| Antibiotics | Penicillins (e.g., Amoxicillin), Cephalosporins (e.g., Keflex) [1.4.1] | Trimethoprim-sulfamethoxazole (in infants <2 months) [1.4.3] | Chloramphenicol [1.3.2] |
| Allergy Meds | Loratadine (Claritin), Nasal steroids (e.g., Flonase) [1.4.2] | Sedating antihistamines (e.g., Benadryl) [1.4.5] | Oral decongestants containing pseudoephedrine (may decrease milk supply) [1.4.7] |
| Cardiovascular | Labetalol, Metoprolol, Propranolol, Hydralazine [1.4.3] | Atenolol, Diltiazem [1.3.6] | Amiodarone [1.3.3] |
Medications Contraindicated During Breastfeeding
A small number of drugs are considered unsafe and should be avoided during lactation due to the potential for serious adverse effects in the infant. This list includes:
- Antineoplastic (Chemotherapy) Drugs: Such as methotrexate and cyclosporine, which can cause immune and bone marrow suppression [1.3.2, 1.3.3].
- Drugs of Abuse: Including cocaine, heroin, and PCP [1.3.2].
- Certain Psychotropics: Lithium is a primary concern and requires rigorous monitoring if used at all [1.3.3].
- Radiopharmaceuticals: Used for diagnostic imaging. Breastfeeding must often be temporarily stopped [1.3.2].
- Other Specific Medications: Including amiodarone, ergotamine, and oral retinoids [1.3.3].
Conclusion
The question of which drug is excreted in milk is complex; while most do, the risk to the infant is typically very low [1.7.1]. A drug's physicochemical properties—like its molecular weight, protein binding, and lipid solubility—determine the extent of its transfer into milk [1.2.2]. For the vast majority of common ailments, safe and effective medication options exist for breastfeeding mothers, including many pain relievers, antibiotics, and antidepressants [1.4.1]. However, a few drugs, such as certain cancer chemotherapies and drugs of abuse, are absolutely contraindicated [1.3.2]. The guiding principle for any lactating mother is to consult with a healthcare professional to weigh the benefits of a medication against the potential risks to her infant, ensuring both her health and her baby's safety.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before taking any medication while breastfeeding.
