Knowledge of how a drug enters breast milk is important to understanding how a particular medication can affect your baby, but it is only part of the picture. It is also critical to evaluate how a baby’s body metabolizes the drug. Some medications, though they enter the baby’s system via breast milk, are still not able to circulate throughout his body. Others, though only small amounts may be ingested, are fully utilized and should be used with greater caution. Therefore, understanding what happens in a baby’s body once the drug enters via breast milk will help in selecting the best medication with the least risk to your baby. Understanding a drug’s oral bioavailability and half live, as well as evaluating the amount of breast milk consumed and age of baby are all important factors.
What is oral bioavailability?
A drug’s ability to pass into breast milk is only part of the picture. It is also important to know what happens to the medication once it enters the baby’s system. A drug’s oral bioavailability is the amount of medication that can reach a baby’s systemic circulation. This is the only amount of a drug that will usually have an impact. Some drugs are so poorly absorbed in the infant they are unlikely to cause any problems what so ever. Therefore, choosing drugs with poor bioavailability is a good strategy. (Others, though not well absorbed, can become concentrated in the gastrointestinal system and can cause diarrhea, thrush, etc.)
What drugs have poor bioavailability in infants?
According to Breastfeeding and Human Lactation, 4th ed. by Riordan and Wambach, there are a variety of medications with poor bioavailability, thus having a low risk in babies. These medications are as follows:
- Large molecular weight proteins
- Aminoglycoside antibiotics
- Third generation cephalosporins
- Inhaled beta agonists
- Inhaled steroids
- Most topical preparations
- One-time injection of local anesthetic
- Acute use of medications (as the overall dose transferred to infant over time is so low)
A drug’s “half life” is the time it takes a drug to reduce its plasma concentration by half. So, from the time of ingestion, how long does it take the drug to reduce by half? Some drugs have a short half life, just a few minutes or hours. Others have a much longer half life – a few days even. With breastfeeding, drugs with a shorter half life are better. It takes five half lives to fully clear a drug from maternal plasma – reducing 50% each time.
A drug with a short half life of say 1-3 hours, could be strategically taken between feeds. Nursing your baby and then taking the medication could mean the drug is already reduced by half before your baby nurses again. (Think: even at the maximum amount of the drug in maternal serum it is probably less than 1% that the baby will ingest via breast milk. Add to this the ability to time feedings and the amount ingested by the baby will be even less.)
If given the choice between a medication that you take every few hours versus a long-lasting dose ingested less frequently, select the dose you can take more often.
Age and Stage of Baby
The ability of a baby to metabolize a drug is different based on his age. Newborns generally metabolize a drug more slowly than a six month old. Older infants, 6-18 months, can usually metabolize and handle drugs much more efficiently. Caffeine is a good example. It’s half-life in a newborn is 97.5 hours, but 2.6 hours by the time a baby reaches 6 months.
Not only is the age of the baby important, but the quantity of breast milk ingested. Though more of a medication is able to pass into breast milk during the first few days postpartum, the amount of milk a baby is able to drink is so small he is not ingesting a greater volume of a medication. Also, a toddler who is only drinking milk a couple times a day is receiving much less of a medication via breast milk than a two month old solely fed on breast milk.
How much of a drug is considered safe?
For most drugs, ingesting 10% of the maternal dose is considered safe. 1 With some drugs (for example, fluconazole and metronidazole) the dose can be much higher than this. And usually, the dose ingested is less than 1%. Thus, most medications truly are compatible to take while breastfeeding. This is especially true in light of the 1000s of studies that show the detrimental side effects of formula.