Breastfeeding and Medications: What You Need to Know About Drug Transfer Through Breast Milk

When you're breastfeeding, taking a pill for a headache, an antibiotic for an infection, or even an antidepressant can feel like a gamble. Is it safe for your baby? Will the medicine pass into your milk? Will it harm them? These aren’t just anxious thoughts-they’re real, common concerns. And the truth is, most of the time, the answer is simpler than you think.

How Medications Actually Get Into Breast Milk

Medications don’t magically appear in breast milk. They travel from your bloodstream into your milk through a process called passive diffusion. Think of it like a sponge soaking up water-drugs move from where there’s more (your blood) to where there’s less (your milk), following concentration gradients. But not all drugs do this the same way.

The key factors that determine how much of a drug ends up in your milk are simple but powerful:

• Molecular weight: Drugs under 200 daltons slip through easily. Most common medications fall well below this.
• Lipid solubility: Fatty drugs (like some antidepressants) cross more readily than water-soluble ones.
• Protein binding: If a drug is tightly bound to proteins in your blood (over 90%), it can’t easily enter milk. Warfarin and diazepam are examples-very little gets through.
• Half-life: Drugs that stick around in your body for more than 24 hours are more likely to build up in milk. Shorter half-lives mean less accumulation.

There’s also something called ion trapping. If the pH of your milk (around 7.2) is slightly different from your blood (7.4), weakly basic drugs like lithium or certain antidepressants can get trapped and concentrate in milk-sometimes at levels two to ten times higher than in your blood. That’s why lithium requires special monitoring, but most other drugs don’t.

Right after birth, your milk isn’t fully formed yet. The gaps between the cells in your mammary glands are wider, so more substances can pass through. But here’s the twist: you’re only making about 30-60 milliliters of colostrum a day in those first few days. Your baby’s total exposure is tiny. By the time you’re producing mature milk (500-800 mL/day), those gaps have closed. So even though the milk volume increases, the permeability drops.

Most Medications Are Safe-Here’s Why

It’s easy to assume that if a drug is strong enough to treat you, it must be strong enough to hurt your baby. But that’s not how it works. Babies don’t absorb everything they ingest. In fact, many drugs that show up in breast milk are poorly absorbed in the infant’s gut.

Take antibiotics like amoxicillin or cephalexin. They show up in milk, but your baby’s immature digestive system doesn’t absorb them well. Even if it did, the dose is so small-often less than 1% of the mother’s weight-adjusted dose-that side effects are rare. The same goes for most pain relievers. Acetaminophen and ibuprofen are both considered safe, with minimal transfer and even less risk.

According to the American Academy of Pediatrics, the vast majority of medications are compatible with breastfeeding. Only about 1% of all drugs require you to stop nursing. That’s not a small number-it’s a tiny fraction. And even then, it’s often temporary.

Here’s what the data says: over half of breastfeeding mothers take at least one medication. The most common? Painkillers (28.7%), antibiotics (22.3%), and psychiatric meds (15.6%). Yet fewer than 2% of babies show any clinically significant reaction. That’s not luck. It’s science.

The LactMed Database: Your Free, Reliable Go-To Resource

You don’t need to guess. You don’t need to rely on old advice from well-meaning relatives or outdated handouts. There’s a free, science-backed tool used by doctors, pharmacists, and lactation consultants around the world: LactMed.

Run by the U.S. National Library of Medicine, LactMed has data on over 4,000 drugs-including 3,500 with specific infant exposure levels. It’s updated monthly. You can search by drug name, condition, or even brand name. It tells you how much gets into milk, whether it’s absorbed by the baby, what side effects have been reported, and what alternatives exist.

It gets over 1.2 million queries a year. That’s not because people are paranoid-it’s because they want accurate info. And unlike some websites that push fear or oversimplify, LactMed gives you the full picture: the science, the numbers, the caveats.

Some providers still use Hale’s L1-L5 classification system (L1 = safest, L5 = contraindicated). It’s helpful for quick reference, but it’s not a substitute for LactMed. Hale’s guide is great for clinicians, but it only covers about 1,300 drugs. LactMed covers nearly everything you’ll ever encounter.

Timing and Dosing: Simple Tricks to Reduce Exposure

Even if a drug is safe, you might still want to minimize your baby’s exposure. That’s where timing comes in.

Take your medication right after you breastfeed-not before. That way, the drug’s concentration in your blood peaks while your baby is sleeping, not feeding. For drugs taken once a day, this is easy. For those taken multiple times a day, space them out. If you take a pill every 8 hours, take it right after the evening feeding. That gives you the longest stretch before the next feed.

Here’s another trick: choose drugs with the shortest half-life. If you need an antibiotic, amoxicillin (half-life: 1-1.5 hours) is better than erythromycin (half-life: 1.5-2 hours). For pain relief, ibuprofen clears faster than naproxen. Even small differences matter when you’re dealing with a newborn.

Topical treatments-creams, patches, sprays-are usually safer than pills. The amount that enters your bloodstream is tiny, so even less makes it into milk. But avoid putting anything directly on your nipple unless it’s specifically labeled safe for infants. Even then, wipe it off before feeding.

What About Psychiatric Medications?

This is where anxiety runs highest. Parents worry about antidepressants, anti-anxiety meds, or mood stabilizers affecting their baby’s brain development. The fear is real-but the data is reassuring.

SSRIs like sertraline and paroxetine are among the most studied. Sertraline transfers in very low amounts. Paroxetine is also low, but it can cause mild withdrawal symptoms in newborns if the mother took it late in pregnancy. That’s different from breastfeeding, though. In milk, levels are low, and side effects are rare.

Fluoxetine? It sticks around longer. It has a longer half-life and can build up in milk. If you’re on it, your provider might suggest switching to sertraline, especially if your baby is young or premature.

For bipolar disorder, lithium is the outlier. It concentrates in milk and can affect your baby’s thyroid and kidneys. If you need it, you’ll likely need regular blood tests for your baby and close monitoring. But even then, many mothers continue breastfeeding with proper care.

The bottom line? Stopping breastfeeding because of a psychiatric medication is almost never necessary. In fact, untreated depression or anxiety poses a greater risk to your baby than the medication itself.

What About Newer Drugs? Biologics, Cancer Meds, and Herbal Supplements

As medicine advances, so do the questions. Biologics-like Humira or Enbrel-are now used for autoimmune conditions. Only 12 out of 85 FDA-approved biologics have enough breastfeeding data as of 2023. But early studies show they’re large molecules that don’t absorb well in the baby’s gut. That means even if they get into milk, they’re unlikely to cause harm.

Cancer drugs? Most are not recommended during breastfeeding because of their toxicity. But some newer targeted therapies are being studied. If you’re undergoing treatment, talk to your oncologist and a lactation specialist. Sometimes, pumping and dumping for a few days after chemo is enough.

Herbal supplements? They’re not regulated like drugs. LactMed now includes over 350 herbs and 200 dietary supplements-but many have no safety data. Avoid anything labeled “natural” or “safe for nursing moms” without checking. St. John’s Wort, for example, can cause fussiness and sun sensitivity in babies. Kava and valerian can depress the central nervous system.

What to Do When You’re Prescribed a New Medication

Here’s your simple 4-step plan:

1. Ask if it’s necessary. Is this drug essential for your health? Can it wait? Could a non-drug option work?
2. Check LactMed. Go to lactmed.nlm.nih.gov. Search the drug name. Read the infant exposure and risk level.
3. Time it right. Take it right after a feeding, especially if it’s a once-daily dose.
4. Watch your baby. Look for changes in sleep, feeding, fussiness, or rash. Most issues are mild and temporary.

And if your provider says, “Don’t breastfeed,” ask: “What’s the evidence? Is there a safer alternative? Can I use LactMed to check?” Too often, mothers are told to stop breastfeeding based on outdated assumptions or fear-not science.

A 2021 survey of 500 lactation consultants found that 78% saw at least one case a month where a mother was wrongly told to stop nursing. That’s not just a mistake-it’s a failure of education.

Final Thought: Breastfeeding Is Stronger Than You Think

Your body didn’t evolve to shut down milk production every time you take a pill. It evolved to nourish your baby, even under stress. The fact that so many medications are safe isn’t an accident. It’s biology working the way it should.

You don’t need to be perfect. You don’t need to avoid every medication. You need accurate information, a trusted provider, and the confidence to keep doing what’s best for you and your baby.

Most of the time, the safest choice isn’t stopping breastfeeding. It’s taking the medicine you need-while keeping your baby close.