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

When you're breastfeeding, every pill, injection, or patch you take doesn't just affect you-it can reach your baby. It’s a reality that worries new parents: Is this medication safe for my child? The truth is, most medications are. But knowing which ones are, and how they move through breast milk, isn’t common knowledge. Too many mothers are told to stop breastfeeding unnecessarily because of a lack of clear, evidence-based guidance. You don’t need to choose between your health and your baby’s. You just need the right information.

How Medications Get Into Breast Milk

Medications don’t magically appear in breast milk. They travel from your bloodstream, through the cells lining your milk-producing glands, and into the milk itself. This happens mostly by passive diffusion-meaning drugs move from areas of higher concentration (your blood) to lower concentration (your milk). It’s not a perfect filter. Small molecules under 200 daltons slip through easily. Drugs that dissolve well in fat (high lipid solubility) also cross more readily. If a drug is tightly bound to proteins in your blood (over 90%), it’s less likely to enter milk because it’s stuck to those proteins and can’t move freely.

The timing matters too. Drug levels in your milk usually mirror what’s in your blood. So if you take a pill, you’ll see a peak in milk concentration within an hour or two, then it drops. That’s why timing your dose can make a big difference. Taking your medication right after you nurse means your baby gets the least amount possible during the next feeding. If you’re on a once-daily drug, take it after the bedtime feeding, when your baby will sleep the longest.

There’s one exception: the first few days after birth. Your milk is colostrum-thick, sticky, and low in volume. But the cells in your breasts haven’t fully sealed yet. That means more drugs can leak through. The good news? Your baby is only drinking about 30 to 60 milliliters a day during this time. By day five, your milk volume increases, the cells tighten up, and exposure drops even if the drug concentration stays the same.

Why Some Drugs Are Riskier Than Others

Not all drugs behave the same way. Some get concentrated in breast milk because of something called ion trapping. This happens when the pH of your milk (about 7.2) is slightly lower than your blood (7.4). Weakly basic drugs-like lithium, certain antidepressants, and barbiturates-become charged in milk and get trapped there. That can lead to milk-to-plasma ratios of 2:1 or even 10:1. That means your baby could be getting more of the drug than you’d expect.

Half-life is another big factor. If a drug sticks around in your body for more than 24 hours, it’s more likely to build up in your milk over time. Drugs with short half-lives (like ibuprofen or amoxicillin) clear out quickly. That’s why they’re preferred. Oral absorption by the baby also matters. If a drug isn’t well absorbed in the gut-like many antibiotics-it won’t reach the baby’s bloodstream even if it’s in the milk. That’s why penicillin and cephalosporins are considered low risk: they’re poorly absorbed and don’t cause side effects in infants.

Topical medications are usually safer than oral ones-unless you’re applying them directly to your nipple. Creams, patches, and sprays on your arms or back rarely transfer enough to affect your baby. But if you put a lidocaine patch on your chest, or use a steroid cream on your nipple, you’re giving your baby direct access. Always wash your hands after applying anything to your breasts, and wipe off the nipple before feeding if you’ve applied something there.

The L1 to L5 Risk Classification System

Dr. Thomas Hale, a leading expert in breastfeeding pharmacology, created the most widely used system to rate medication safety during lactation: L1 to L5.

• L1 (Safest): Drugs like acetaminophen, ibuprofen, and many antibiotics (penicillin, amoxicillin). These have been studied in hundreds of mothers with no reported adverse effects.
• L2 (Safer): Drugs like sertraline, fluoxetine, and metformin. There’s limited data, but no serious issues reported. These are often recommended when needed.
• L3 (Moderately Safe): Drugs like diazepam, fluoxetine (in high doses), and some antidepressants. Use with caution. Monitor baby for drowsiness or irritability.
• L4 (Possibly Hazardous): Drugs like lithium, cyclosporine, and some chemotherapy agents. Only use if benefits clearly outweigh risks. Close monitoring required.
• L5 (Contraindicated): Drugs like radioactive iodine, chemotherapy drugs like methotrexate, and ergotamine. These are rarely used during breastfeeding because they can cause serious harm.

Most medications fall into L1 or L2. Only about 1% of all drugs require you to stop breastfeeding. The American Academy of Pediatrics says the benefits of breastfeeding almost always outweigh the risks of medication exposure. Still, it’s easy to get scared. That’s why so many mothers are wrongly told to stop.

What Medications Are Most Commonly Used?

A 2022 study found that over half of breastfeeding mothers take at least one medication. The top three categories:

• Analgesics (28.7%): Ibuprofen and acetaminophen are the go-tos. They’re safe, effective, and clear quickly from milk.
• Antibiotics (22.3%): Amoxicillin, cephalexin, and clindamycin are all L1. Even if your baby gets a tiny amount, it won’t hurt them. Diarrhea is rare and usually mild.
• Psychotropics (15.6%): Sertraline is the most studied and preferred antidepressant. Fluoxetine is effective but has a long half-life, so it can build up. Avoid benzodiazepines like alprazolam unless absolutely necessary.

Other common ones? Birth control pills-progestin-only are fine. Estrogen-containing pills can reduce milk supply, so they’re avoided in the first few months. Thyroid meds like levothyroxine are safe. Even insulin doesn’t pass into milk because it’s too large.

Where to Find Reliable Information

Don’t rely on Google, Facebook groups, or even your doctor’s memory. Use trusted, updated databases.

LactMed, maintained by the U.S. National Library of Medicine, is the gold standard. It has data on over 4,000 drugs, including herbal supplements and vitamins. It’s free, updated weekly, and used by over a million people a year. It gives you detailed info: how much gets into milk, infant exposure levels, potential side effects, and alternatives. The downside? It’s technical. If you’re not a clinician, it can feel overwhelming.

Medications and Mothers’ Milk by Dr. Hale is the most practical book for providers. It uses the L1-L5 system and gives clear, actionable advice. It covers about 1,300 drugs, but it’s easier to understand than LactMed.

MotherToBaby (run by OTIS) offers free phone and chat consultations with specialists. They handle about 15,000 breastfeeding medication questions a year. You can call them directly-no referral needed. Their advice is based on real-world data from thousands of mothers.

There’s also a mobile app called LactMed On-the-Go with 45,000 downloads. It’s perfect for quick checks at the pharmacy or during a clinic visit.

What to Do When You Need a Medication

Here’s a simple four-step plan:

1. Ask if the medication is truly necessary. Can you wait? Can you use a non-drug option? Sometimes, rest, hydration, or physical therapy can replace pills.
2. Check LactMed or Hale’s guide. Don’t guess. Look it up. If your provider doesn’t know, ask them to check.
3. Time your doses. Take the medication right after you nurse. If you take it twice a day, give it after the morning and afternoon feeds. Avoid nighttime doses unless it’s a once-daily drug.
4. Watch your baby. Look for changes: excessive sleepiness, poor feeding, rash, irritability, or diarrhea. If you see any, call your pediatrician. But don’t assume every fussiness is from the medicine. Babies are unpredictable.

And remember: if you’re on a long-term medication, like an antidepressant or thyroid pill, don’t stop cold turkey. Work with your provider to switch to a safer option if needed, but don’t quit breastfeeding unless you have to.

What’s Changing in the Field

The science is moving fast. Until recently, pregnant and breastfeeding women were left out of most drug trials. That’s changing. In 2022, the FDA started pushing pharmaceutical companies to include lactating women in clinical studies. By 2030, we may see personalized breastfeeding pharmacology-using your genes to predict how much of a drug will end up in your milk.

Right now, only 12 out of 85 FDA-approved biologic drugs (like Humira or Enbrel) have enough data to be considered safe. That’s a gap. But the MilkLab study, led by the InfantRisk Center, has already measured drug levels in breast milk from over 1,250 mothers. That’s real-world data, not theory.

And the numbers speak for themselves: fewer than 2% of infants experience any clinically significant side effect from medication exposure through breast milk. That’s lower than the risk from a cold or a flu shot.

You Don’t Have to Choose

You can be a healthy mom and a breastfeeding mom. You can take the medication you need without harming your baby. The fear around medications and breastfeeding is outdated, exaggerated, and often based on misinformation.

Ask questions. Use LactMed. Talk to a lactation consultant. If your doctor says to stop breastfeeding because of a medication, ask them to show you the evidence. Chances are, they’re relying on an old guideline or a myth.

More than 50% of breastfeeding mothers take medication. Most of them never even think twice about it. You don’t have to either. With the right info, you can keep doing what you love-nourishing your baby-while taking care of yourself.