It is common for pharmacists to receive enquiries on whether a medication is compatible with breastfeeding. Sometimes, literature reviews present opposing viewpoints, much to the pharmacists’ confusion. The good news is that most medications are compatible with breastfeeding. Many factors influence how drug enters the human milk, such as lipid solubility and molecular size of the drug. Other factors would also determine if the infant may experience adverse effects from the drug, such as the blood level attained in the maternal circulation and the half-life in the maternal and infant's plasma compartment.

Here are some general guidelines to consider when choosing drugs for breastfeeding mothers:

1. Determine the drug bioavailability.

Many drugs such as aminoglycosides, 3rd generation cephalosporin, magnesium salts are poorly absorbed from the gastrointestinal tract. Therefore, it is unlikely that the infant will absorb a significant amount of these drugs though milk.

2. Determine if the milk to plasma ratio is high (i.e. >1).

Determine the dosage of the drug so that there is a high milk to plasma ratio, and also find out if there were any reports of adverse reactions on infants at that level of dosage. Even if the drug has high milk to plasma ratio, if the maternal plasma level of the medication is low (e.g. propranolol), the absolute amount of the drug entering milk will still be low and therefore unlikely to have a significant effect on the infant.

3. Choose drugs with short half-life.

Drugs with short half-life tend to clear out of the mother's circulation faster and do not accumulate within the infant's plasma. However, be careful of drugs that have active metabolites (e.g. diazepam, morphine) or antibiotics with post-antibiotics effects (e.g. metronidazole).

4. Choose drugs with high protein binding.

Drugs with high affinity to protein are often sequestered in the maternal circulation, and thus do not transfer as readily to the milk and the infant.

Below are some reliable databases for pharmacists to assess the safety profile of medications on breastfeeding mothers:

1. LactMed1

LactMed is a free online database which provides information on drugs and chemicals that are compatible with breastfeeding mothers. It also contains useful information, like the level of such substances in breast milk and infant blood, and the possible adverse effects it could have on the infant. The database is updated on a monthly basis and the information is derived from evidence-based literatures.

2. Breastfeeding and Maternal Medication2

Published by the World Health Organisation, this database lists the safety profile and compatibility of essential drugs with breastfeeding mothers, such as anaesthetics, antidotes, anticonvulsants and anti-infectives.

3. Drugs in Pregnancy and Lactation3

A bible that all pharmacists should have in Singapore, this is a common reference which is available in local drug information centres. The co-author of the book, Gerald G. Brigg, is a working mother and practising clinical pharmacist in California. An online version for the 6th Edition is available free.

It is also important for pharmacists to consider the following caveats in using references to decide if a medication is compatible with breastfeeding. Firstly, information presented in the databases is not meant to be a substitute for professional judgment. Recommendations should be made after assessing the risks and benefits of using the drug, and each recommendation should be tailored to the individual.

Secondly, do not overly rely on prescribing information leaflets. Such leaflets contain package inserts from pharmaceutical manufacturers, and almost all of them indicate that the medication should not be taken while pregnant or breastfeeding. Most drug manufacturers generally have not done studies on lactating women and simply noted that there is no available information, even if such studies have been done by others. Warning statements on the prescribing information are designed to protect themselves, rather than providing accurate information and should only be taken with a grain of salt. MIMS

Read more:
Taking medicine while breastfeeding: Practical considerations
3 truths about breastfeeding and why mothers stop doing it
Educated mothers breastfeed longer, researchers say

1. LactMed. Available at: Last accessed on 10 October 2016.
2. Breastfeeding and Maternal Medication. Recommendations for Drugs in the Eleventh WHO Model list of Essential Drugs. World Health Organisation, 2002. Available at: Last accessed on 10 October 2016.
3. Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk Sixth Edition. Gerald G. Briggs BPharm FCCP (Author), Roger K. Freeman MD (Author), Sumner J. Yaffe MD (Author). Available at: Last accessed on 10 October 2016.