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You ingest the medication, it is broken down in your stomach then broken down further in your small intestines. It is then absorbed into your bloodstream and as the blood flows by the breast/mammary glands the components in the blood go through a filter to get into the breast/mammary gland. This filter keeps the amount of medication allowed in the breast/mammary glands lower than what is in the bloodstream.
The filter in the breast/mammary gland won't allow large molecular weight components into the breast. If they're too large they won't be able to fit through the filter.
For most medications around 1 - 3% of the dose gets into the milk. There are a lot of variables here, but that is a general rule.
Human milk by design has a low protein level (humans need brain food vs muscle building) so if a medication has a high rate of protein binding, a lesser volume will be in the milk.
Oral bioavailability - can this medicine be absorbed via the digestive tract? If it can't then what is in the milk won't be absorbed into the baby's system.
How long is the medication's half life? (the time frame in which medication levels peaks in the body, and then starts to fall.)
Shorter timeframes are preferred, but not required.
We also look at the category of medications, are these medications prescribed to infants regularly?
Are there equal alternatives that are safer?
Is there a holistic based option with fewer side effects?
What are the risks of using formula vs parents own milk, for babies health & parents milk production?
How long does this medication need to be used? (example: There is an antibiotic that is contraindicated for long term use, but is compatible for short term.)
Weighing out the risks & benefits for the parent taking this medication, are there alternatives, can it wait a few months or is it needed immediately?
A recent survey from a university in Michigan found that more than 50% of the information from physicians regarding medications and human milk was inaccurate. Emergency Dept MD's, surgeons, and dentists ranked the highest for inaccurate information. (Lactation isn't their specialty so they often have less contact with lactating parents.)
The takeaway is to seek out more information, you can call an IBCLC* in your community, and/or you can use one of the resources below to get research based information.
Infant Risk Center
Dr. Thomas Hale and team at the Infant Risk Center are the leading experts studying human milk & medications. The Infant Risk Center is at Texas Tech University and they have an app for parents called Mommy Meds.
They also staff a call center for questions about less common medications, or more unusual situations. They can be reached Monday - Friday 8:00AM - 5:00 CST at (806) 352-2519
Mother to Baby
The University of California San Diego does a lot of human milk research. They actively study medications and human milk and can be found here:
E-Lactancia
The National Institute of Health