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

When you're breastfeeding, taking a pill for pain, depression, or an infection isn't just about you. It’s about your baby too. Every time you swallow a medication, a small amount can end up in your breast milk. But here’s the truth most people don’t tell you: breastfeeding medications are almost always safe. Less than 2% of babies show any real side effects from what passes through milk. The fear? It’s often bigger than the risk.

How Medications Get Into Breast Milk

Medications don’t travel to breast milk by magic. They move through your bloodstream, then cross into milk the same way oxygen and nutrients do-mostly by passive diffusion. The bigger the molecule, the harder it is to get through. Drugs under 200 daltons (like ibuprofen or amoxicillin) slip in easily. Larger ones, like heparin or insulin, barely make it at all.

Lipid solubility matters too. If a drug loves fat, it’s more likely to enter milk. That’s why some antidepressants and anti-anxiety meds show up in higher amounts. But here’s the twist: just because a drug is in milk doesn’t mean your baby absorbs it. Most babies have immature guts. They don’t absorb much orally, especially if the drug is poorly absorbed or broken down by stomach acid.

Protein binding is another gatekeeper. If 90% or more of a drug is stuck to proteins in your blood, it can’t float freely into milk. That’s why drugs like warfarin or phenytoin rarely cause issues. Half-life matters too. A drug that sticks around for 24 hours or more might build up in milk over time. Shorter half-life drugs like acetaminophen clear out fast-so timing your dose helps.

In the first few days after birth, your milk is colostrum. It’s thick, low in volume (only 30-60 mL a day), and your breast cells aren’t fully sealed yet. That means more drugs *could* get through. But because so little milk is being consumed, the actual amount your baby gets is tiny. By day five, milk volume increases, but so does the barrier. So even though more milk means more volume, the concentration of drugs often drops.

The L1 to L5 Risk System You Can Trust

Dr. Thomas Hale created the most used system for judging medication safety during breastfeeding: L1 to L5.

- L1 (Safest): Drugs like acetaminophen, ibuprofen, and penicillin. No reported adverse effects in infants. Used safely for decades.

- L2 (Probably Safe): Fluoxetine, sertraline, amoxicillin-clavulanate. Limited data, but no harm shown in large groups of babies.

- L3 (Probably Safe with Caution): Some SSRIs, metformin, and certain antibiotics. May cause mild side effects in rare cases-like fussiness or loose stools. Often still recommended because benefits outweigh risks.

- L4 (Possibly Hazardous): Lithium, cyclosporine, some chemotherapy drugs. Risk is real. Use only if no alternatives and under close monitoring.

- L5 (Contraindicated): Radioactive isotopes, certain cancer drugs, ergotamine. These can cause serious harm. Breastfeeding must stop.

You won’t find L5 drugs in your medicine cabinet unless you’re in a hospital. And even then, doctors often find ways to pause treatment or pump and dump temporarily.

What Medications Are Most Commonly Used?

Over half of breastfeeding moms take at least one medication. The top three? Pain relievers, antibiotics, and mental health drugs.

- Analgesics (28.7%): Ibuprofen and acetaminophen are first-line. They’re in L1. Codeine? Not so much. It turns into morphine in your body, and some moms metabolize it too fast-leading to dangerous levels in milk. Avoid codeine if you can.

- Antibiotics (22.3%): Amoxicillin, cephalexin, azithromycin-all L1 or L2. Even metronidazole, once feared, is now considered safe at standard doses. Only avoid if your baby has a known allergy or gets diarrhea that doesn’t clear.

- Psychotropics (15.6%): Sertraline and escitalopram are the go-tos for depression and anxiety. They have the lowest transfer rates and least side effects in babies. Fluoxetine? It lingers. It’s L2, but some babies get jittery or have trouble sleeping. Switching to sertraline often helps.

You might hear horror stories about SSRIs causing “autism” or “developmental delays.” No study backs that up. The real risk? Not treating your depression. Untreated maternal mental illness harms babies more than any medication ever could.

When Timing Matters

You don’t need to pump and dump unless you’re on a drug like L5. But timing your dose can cut your baby’s exposure by half.

Take your medication right after you breastfeed. That gives your body time to clear it before the next feeding. For example, if your baby sleeps 6 hours at night, take your pill right after the bedtime feeding. By morning, most of the drug is gone.

For drugs taken multiple times a day, like antibiotics, take them just before the feeding with the longest gap. That way, the peak concentration in your milk happens when your baby isn’t nursing.

Avoid long-acting or extended-release versions if possible. A quick-release pill taken twice a day is safer than a once-daily slow-release version.

Topical and Inhaled Meds Are Safer

If you’re using a cream, spray, or inhaler, you’re already ahead. Only a tiny amount enters your bloodstream. That means almost nothing reaches your milk.

Corticosteroid creams for eczema? Safe. Inhalers for asthma? Safe. Even lidocaine patches? Safe. Just avoid applying creams directly to the nipple before feeding. Wash it off first.

The same goes for eye drops and nasal sprays. You’d need to take dozens of doses to get enough into your blood to affect milk.

What Resources Should You Use?

Don’t guess. Don’t rely on Google. Use trusted, science-backed tools.

- LactMed (from the NIH): Free, updated daily, covers over 4,000 drugs and 350 herbs. It’s technical, but you can search by drug name and get details on infant exposure, half-life, and risk level. Over 1.2 million people use it every year.

- Medications and Mothers’ Milk by Dr. Hale: The gold standard. Uses the L1-L5 system. Easier to read than LactMed. Great for quick decisions.

- MotherToBaby: A free service run by OTIS. Call or chat with a specialist. They handle 15,000 calls a year. Ask about antidepressants, pain meds, or antibiotics-they’ve seen it all.

Most lactation consultants and pediatricians don’t know the details. But these tools? They’re designed for you.

What to Watch For in Your Baby

Most babies show no signs at all. But if you’re nervous, look for:

- Unusual sleepiness or fussiness

- Poor feeding or weight gain

- Diarrhea or rash

- Jaundice that won’t clear

These are rare. But if you notice them, call your doctor. Don’t stop breastfeeding without checking first. Often, switching to a different drug fixes it.

What About Herbal Supplements and CBD?

LactMed now includes over 350 herbal products and 200 supplements. But here’s the catch: they’re not regulated. There’s no guarantee of purity, dose, or safety.

Chamomile? Probably fine in tea. But high-dose supplements? Unknown. CBD? No reliable data. The FDA warns against it during breastfeeding. Even if it’s “natural,” it can cross into milk.

Stick to food. If you want to take something for sleep or anxiety, talk to your doctor first. Don’t self-prescribe.

The Big Picture: Breastfeeding Wins

The American Academy of Pediatrics says this clearly: “The benefits of breastfeeding should be weighed against the potential risks of drug exposure.” In almost every case, the benefits win.

Breast milk lowers your baby’s risk of infections, allergies, obesity, and sudden infant death. For you, it cuts the risk of breast cancer, ovarian cancer, and type 2 diabetes.

If you need a medication, you don’t have to choose between your health and your baby’s. You can have both. The data is clear. The tools are ready. The advice is simple: check LactMed, talk to a specialist, and keep nursing.

What’s Coming Next

In 2022, the FDA started pushing drug companies to include breastfeeding women in clinical trials. Right now, only 12 of 85 FDA-approved biologics (like Humira or Enbrel) have any breastfeeding data. That’s changing.

The InfantRisk Center’s MilkLab project has already measured drug levels in over 1,250 mothers. Soon, we’ll have personalized predictions-using your genetics to say exactly how much of a drug will end up in your milk.

By 2030, doctors may test your DNA before prescribing. If you’re a fast metabolizer of certain drugs, they’ll adjust your dose or choose a different one. No more guessing. No more fear.

You’re not alone. You’re not breaking the rules. You’re doing what millions of moms do: taking care of yourself so you can take care of your baby. And that’s not just safe. It’s powerful.