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

When you're breastfeeding, every pill, injection, or cream you use doesn't just affect you-it can reach your baby through your milk. It’s a common fear: "Is this medication safe for my baby?" The truth is, most medications are fine. But knowing which ones are safe, how they get into breast milk, and how to minimize risk makes all the difference.

How Medications Get Into Breast Milk

Medications don’t travel to your baby by magic. They move from your bloodstream into your breast milk through passive diffusion. That means if the drug is in your blood, some of it will naturally end up in your milk. The amount depends on a few key factors:

• Molecular weight: Drugs under 200 daltons (like ibuprofen or acetaminophen) pass easily. Anything bigger, like heparin or insulin, barely gets through.
• Lipid solubility: Fatty drugs slip through cell membranes more easily. That’s why some antidepressants and sedatives show up more in milk.
• Protein binding: If a drug is tightly bound to proteins in your blood (over 90%), it can’t easily enter milk. Warfarin and most SSRIs are highly bound, so very little gets through.
• Half-life: Drugs that stick around longer (over 24 hours) have more time to build up in milk. Shorter half-life drugs like penicillin clear faster.
• Ion trapping: In the slightly lower pH of breast milk, weakly basic drugs like lithium or certain antidepressants can get "trapped" and concentrate at 2:1 to 10:1 ratios compared to your blood.

Early on, in the first few days after birth, your milk is colostrum-thick, sticky, and low in volume. Even if a drug gets in, your baby is only drinking 30-60 mL per day. By day 5-7, milk volume increases to 500-800 mL, so timing and choice of medication become more important.

The L1 to L5 Risk Scale: What Your Doctor Should Know

Dr. Thomas Hale developed the most trusted system for rating medication safety during breastfeeding: the L1 to L5 scale. It’s not perfect, but it’s widely used by lactation consultants and doctors.

• L1 (Safest): No known risk. Examples: ibuprofen, acetaminophen, most penicillins, levothyroxine.
• L2 (Safer): Limited data, but no adverse effects reported. Examples: sertraline, fluoxetine (in low doses), metformin, azithromycin.
• L3 (Moderately Safe): Possible risk, but benefits may outweigh risks. Examples: amitriptyline, citalopram, tramadol, codeine (use with caution).
• L4 (Possibly Hazardous): Evidence of risk, use only if no alternatives. Examples: lithium, cyclosporine, doxycycline (long-term use).
• L5 (Contraindicated): Proven risk. Examples: chemotherapy drugs, radioactive iodine, bromocriptine.

Here’s the key: 98% of commonly prescribed medications fall into L1 or L2. Only about 1% of all drugs require you to stop breastfeeding. That’s not a statistic you hear often-but it’s the truth.

Most Common Medications and What the Data Shows

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

• Analgesics (28.7%): Ibuprofen and acetaminophen are top choices. They’re low in milk, short-acting, and safe for newborns. Avoid aspirin in infants under 6 months-rare but risky.
• Antibiotics (22.3%): Penicillins, cephalosporins, and macrolides like azithromycin are all L1 or L2. Even clindamycin and metronidazole (once thought risky) are now considered safe at standard doses.
• Psychotropics (15.6%): Sertraline is the gold standard for depression and anxiety. It’s low in milk, and studies show no developmental delays in exposed infants. Fluoxetine has a longer half-life and can build up, so it’s often avoided unless necessary. Benzodiazepines like lorazepam are okay occasionally, but avoid long-term or high doses.

For thyroid meds like levothyroxine, you can take your full dose-your baby gets less than 1% of your dose, and it’s not absorbed well. Birth control? Progestin-only pills are fine. Estrogen-containing pills can reduce milk supply, so they’re not recommended in the first 6 weeks.

When Timing Matters: How to Reduce Baby’s Exposure

It’s not just about the drug-it’s about when you take it.

• Take single-dose medications right after breastfeeding, before your baby’s longest sleep stretch (often after the night feeding).
• For multiple daily doses, take them right before feeding so your body has time to clear the drug before the next feed.
• Avoid extended-release or slow-release versions unless necessary-they keep drug levels steady, meaning more is in your milk over time.
• For topical meds (creams, patches), wash your hands after applying and avoid putting them on your nipple. If you do, wipe it clean before feeding.

One simple trick: if you’re on a daily medication, take it after your baby goes to sleep. By morning, levels in your milk are much lower. This works especially well for antidepressants and pain meds.

Reliable Resources: Where to Check Before You Take Anything

Don’t guess. Don’t rely on old advice or internet forums. Use trusted tools:

• LactMed (NIH): Free, updated daily, covers 4,000+ drugs and 350 herbal products. Used by over 1.2 million people a year. It’s technical, but you can search by drug name and get clear summaries.
• Hale’s Medications and Mothers’ Milk: The go-to reference for clinicians. Uses the L1-L5 system and gives practical advice like "take after feeding" or "avoid in preemies."
• MotherToBaby: A free service (run by OTIS) where you can call or chat with a specialist. They handle 15,000+ calls a year and answer questions about antidepressants, pain meds, and more.
• LactMed On-the-Go app: A mobile version of LactMed with 45,000 downloads. Great for quick checks at the pharmacy or doctor’s office.

Here’s what most doctors don’t tell you: if a drug is safe for your baby to take directly, it’s almost always safe in breast milk. That’s why antibiotics like amoxicillin or antifungals like clotrimazole are low risk-they’re given to newborns all the time.

What to Watch For in Your Baby

Most babies show no signs of exposure. But if you’re on a new medication, keep an eye out for:

• Unusual sleepiness or fussiness
• Changes in feeding patterns (less interest, more frequent feeds)
• Diarrhea or vomiting
• Unusual rashes

These are rare. Only 1-2% of infants experience any noticeable effect-and even then, it’s usually mild and goes away when the drug clears.

Don’t panic if your baby seems a little off. Talk to your pediatrician. Most of the time, it’s unrelated. But if you’re worried, pause the medication for 24-48 hours (with medical advice) and see if things improve.

What’s Changing in 2025?

The field is evolving fast. The FDA now encourages drug makers to include breastfeeding women in clinical trials. That’s a big deal-until recently, most new drugs had zero data for nursing moms.

Biologics (like Humira or Enbrel) are now being studied more. Only 12 out of 85 FDA-approved biologics have enough data to give clear safety advice. But early results are promising-most don’t cross into milk in meaningful amounts.

By 2030, personalized lactation pharmacology could be real. Researchers are testing maternal DNA to predict how fast you’ll process a drug, which could tell you exactly how much your baby will get. Imagine a test that says: "Take this pill at 10 PM, your baby will get 0.01% of your dose-perfectly safe."

For now, we’re not there yet. But we have better tools than ever.

Final Takeaway: You Don’t Have to Choose

You don’t have to pick between being healthy and breastfeeding. The vast majority of medications won’t hurt your baby. Stopping breastfeeding because of a medication is almost always unnecessary.

Here’s your action plan:

1. Don’t stop a medication without checking with a specialist.
2. Use LactMed or MotherToBaby before taking anything new.
3. Time your doses to minimize exposure.
4. Watch your baby-but don’t assume every cry or sleep change is drug-related.
5. Ask your doctor: "Is this safe for breastfeeding?" If they don’t know, ask for a referral to a lactation consultant or pharmacist.

Breastfeeding is powerful. Medications are tools. You don’t have to give up one to protect the other. With the right info, you can do both.