Over the years, far too many women have been wrongly told they had to stop breastfeeding. The decision about continuing breastfeeding when the mother takes a drug, for example, is far more involved than whether the baby will get any in the milk. It also involves taking into consideration the risks of not breastfeeding, for the mother, the baby and the family, as well as society. And there are plenty of risks in not breastfeeding, so the question essentially boils down to:
Does the addition of a small amount of medication to the mother's milk make breastfeeding more hazardous than formula feeding?
The answer is almost always: Almost never.
Breastfeeding with a little drug in the milk is almost always safer. In other words, being careful means continuing breastfeeding, not stopping.
Remember that stopping breastfeeding for a week may result in permanent weaning since the baby may then not take the breast again. On the other hand, it should be taken into consideration that some babies may refuse to take the bottle completely, so that the advice to stop is not only wrong, but often impractical as well. On top of that it is easy to advise the mother to pump her milk while the baby is not breastfeeding, but this is not always easy in practice and the mother may end up painfully engorged.
Breastfeeding and Maternal Medication
Most drugs appear in the milk, but usually only in tiny amounts. Although a very few drugs may still cause problems for infants even in tiny doses, this is not the case for the vast majority.
Nursing mothers who are told they must stop breastfeeding because of a certain drug should ask the physician to make sure of this by checking with reliable sources and/or prescribing an alternative safe medication.
In this day and age, it is rarely a problem to find a safe alternative. If the prescribing physician is not flexible, the mother should seek another opinion, but not stop breastfeeding.
Why do most drugs appear in the milk in only small amounts? Because what gets into the milk depends on the concentration in the mother's blood and the concentration in the mother's blood is often measured in micro- or even nano-grams per millilitre (millionths or billionths of a gram), whereas the mother takes the drug in milligrams (thousandths of grams) or even grams. Furthermore, not all the drug in the mother's blood can get into the milk. Only the drug that is not attached to protein in the mother's blood can get into the milk. Many drugs are almost completely attached to protein in the mother's blood. Thus, the baby is not getting amounts of drug similar to the mother's intake, but almost always, much much less on a weight basis. For example, in one study with paroxetine (Paxil), the baby got less than 0.3% of the drug for each kilogram of his weight than the mother did (the mother got over 300 micrograms per kg per day, whereas the baby got about 1 microgram per kg per day).