It can be complicated and confusing trying to navigate the realm of medications while breastfeeding. Which drugs are compatible with breastfeeding and which ones might post a problem is a common concern amongst parents. While pharmacology is a massive topic and a subject I would never claim to be an expert in, I do want to give you a bit of insight on how medications transfer into milk and what factors you can look to help you determine a medication's safety.

How Do Medications Get Into Breastmilk?

Drugs that enter the milk get in there via the circulatory system. Your cells need the oxygen that is carried in the red blood cells, making your veins & capillaries the perfect highway for nutrient distribution. Any drug that does NOT enter the blood plasma will not get into your milk. And even when a medication gets into the plasma, it doesn't mean that it will make it into the milk.

Let's take a look at the cellular components responsible for milk production and drug transfer.

The cells that make milk (Lactocytes) directly touch the cells of the Capillaries. Transfer of oxygen and nutrients to the Lactocytes needs to be extremely rapid, so only two cells separate the "stuff" in the capillaries and the "stuff" in the Alveolus.

If you're not familiar with the term, Alveolus, it is the round, bubble-like compartment made up by Lactocyte. Bunches of Alveolus form to create lobes of Alveoli, which looks like a cluster of grapes. In the picture I made for this blog, you can see how the capillaries surround the Alveoli and how the cells of the capillaries are in direct contact with the Lactocytes of the Alveolus.

Now that we've gotten that out of the way...

Why do not all drugs get into the milk?

As mentioned above, for a medication to get into the milk compartment (the hollow space of the Alveolus where milk is secreted and stored), the drug must first enter the blood supply. Topical medication or oral sprays, for example, don't cross into the bloodstream in high volume; therefore, it will never get into your breastmilk.

The drugs that make into the plasma transfer into the milk by way of diffusion, which is passive transport of particles from high concentrations to low concentrations. Which means that because the drug is in high levels in the capillaries, it will inevitably attempt to get into the milk compartment because it has less medication in it to reach equilibrium. But the transfer is not simple, and many obstacles stand in the way.

Drugs need to get through or around the Lactocytes.

In the first few days after delivery, gaps exist between Lactocytes which allow higher volumes of medication to transfer into your breastmilk. The door is wide open, and drugs can walk right through. After a few days, however, these junctions tighten and prevent easy access. There is little concern about medication transfer to babies in the early days, however, because the total volume of milk that a baby gets is generally low despite the high concentration of a drug. Once the gaps close, it takes a high level of a drug to get through the cells of both the capillaries and alveolus to pose a risk of adverse reactions.

There are two ways drugs get through the Lactocyte, gobbled up in milk fat or directly transferred through the cell.

The specific job of the Lactocytes is to produce milk, and this occurs by the hard work of different cellular organelles. As the drug makes its way through the Lactocyte, it can be absorbed into the milk components directly.

Drugs that don't make it into the milk itself gets into the milk compartment by continuing on its journey of diffusion through the cell without any help.

Another barrier to the drugs trip into the milk is that it must remain intact after being absorbed into the alveolar cell's bilayers. It's easy to picture cells being like a box with its walls having little holes that allow things to pass through. The cellular organelles neatly tucked inside and everything else passes through the top and bottom. The cell walls, however, are protected by a thin membrane called the Lipid Bilayer. When you get a close-up view of it, you see it's much more like a dense jungle than anything else and navigating through it is quite the challenge.

Drugs must fit a certain set of criteria to make it into the milk.

If the journey through the cells wasn't daunting enough, many factors play a role in how readily a drug can transfer into breastmilk. There are many more criteria than I am going to mention today, but the top three are size, solubility ( ability to dilute), and the drug's half-life ( how much time it takes for half the mediation to be metabolized and eliminated).

Drugs with smaller molecular weight are more easily able to get into the milk because, well they're smaller. They can pass through the bilayer more effortlessly and, they can better sneak through the Lactocyte. Generally speaking, drugs smaller than 800 Dalton will have no issue with moving into the milk, whereas drugs over 25,000 Dalton haven't a chance.

Remember that membrane that protects the cell called the Lipid Bilayer? Well, it does a darn good job keeping things out unless that thing is lipid soluble. Lipid-Soluble drugs dissolve easily in lipids (fats) making the bilayer better for them with regards to transferring through the cell than if it were not. But being fat soluble doesn't come without its own set of challenges. Because they get absorbed into fat very quickly, lipid-soluble drugs often get deposited in our body fat or brains before it can ever make its way into the milk.

Last important factor of today's blog is the half-life of a drug. Drugs with shorter half-lives are more compatible with breastfeeding than drugs with longer half-lives because the drug gets out of one's body more quickly. The longer the half-life, the more time for a drug to build up in the plasma which increases the opportunity for the drug to transfer into the milk.

I know that was a lot of information and I doubt anyone was expecting a science lesson today! Here's a quick recap:

- Drugs must enter the plasma in high enough volumes to transfer into your breastmilk.

- Medicines that have molecular weights smaller than 800 Dalton get into milk the easiest.

- Medications that are lipid soluble pose a greater chance of getting into breast milk.

- Drugs with shorter half-lives are preferred when available.

I will cover other relevant information on drugs and breastfeeding in the future, but I just wanted to provide you all with the basic information you guys can use to help investigate drug safety while breastfeeding. I use an app called Lactmed when looking into a drug's safety. It's a free app that has TONS of medications in the database and will give you not only the possible impact on your baby but also provides potential side effects on lactation in general. We don't want to take meds that may decrease supply or create weird side effects for us! The Toxnet Website is the sister site for the app and is perfect for searching drugs on the web.

Also, most lactation professionals will have a book titled Medications and Mother's Milk which is the go-to book on drugs & breastfeeding compatibility. I know I use mine frequently when families ask if a particular drug is safe for breastfeeding or not. Because it's inappropriate for me to say "Sure yes, go ahead" I find that this book is excellent for providing the information families need to hear to make their decision.

As always, I hope you've found this post helpful and please leave feedback in the comments below!