Breastfeeding and Medications: What Drugs Pass Into Breast Milk and What’s Safe

When you're breastfeeding, every pill, injection, or cream you take doesn't just affect you-it can reach your baby through your milk. It’s a reality that worries many new mothers: is this medication safe for my child? The truth is, most drugs are. But knowing which ones are, and which ones aren’t, isn’t something you should guess at. It’s science-and it’s clearer than most people realize.

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). But not all drugs do this equally.

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

• Molecular weight: Drugs under 200 daltons slip through easily. Most common medications fall under this limit.
• Lipid solubility: Fatty drugs like antidepressants or sedatives 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. That’s why drugs like warfarin or aspirin in low doses are low risk.
• Half-life: The longer a drug stays in your system, the more chance it has to build up in milk. Short-acting drugs are safer.

There’s also something called ion trapping. Because breast milk is slightly more acidic than your blood, weakly basic drugs-like some antidepressants or antihistamines-can get pulled into milk and stay there. This can make their concentration in milk two to ten times higher than in your blood. But even then, the actual amount reaching your baby is often tiny.

In the first few days after birth, when your body is producing colostrum, the gaps between milk cells are wider. That means more drugs can pass through. But here’s the catch: you’re only making about 30-60 mL of colostrum a day. Your baby isn’t drinking enough to get a meaningful dose. By day five, when mature milk comes in, those gaps close, and milk volume increases-but so does your baby’s ability to process small amounts of drugs.

What’s Safe? The L1-L5 System Explained

Dr. Thomas Hale, a leading expert in lactation pharmacology, created the most trusted system for classifying drugs during breastfeeding. It’s called the L1-L5 scale:

• L1: Safest - Drugs like acetaminophen, ibuprofen, and most penicillins. No adverse effects reported in infants.
• L2: Probably Safe - Drugs like sertraline, fluoxetine, and amoxicillin. Limited data, but no major concerns.
• L3: Possibly Risky - Drugs like lithium, certain anticonvulsants, or benzodiazepines. May need monitoring or timing adjustments.
• L4: Possibly Hazardous - Drugs like cyclosporine or chemotherapy agents. Use only if benefits clearly outweigh risks.
• L5: Contraindicated - Drugs like radioactive iodine, bromocriptine, or certain cancer drugs. Avoid completely.

Here’s the big takeaway: over 90% of commonly prescribed medications fall into L1 or L2. That means the vast majority of prescriptions you get while breastfeeding are fine. The American Academy of Pediatrics confirmed this in their 2013 report: “The vast majority of medications are compatible with breastfeeding.” And according to Dr. Ruth Lawrence, a pioneer in the field, fewer than 1% of medications require you to stop nursing.

The Most Common Medications and What We Know

Let’s look at what mothers actually take-and what the data says about safety.

• Analgesics: Acetaminophen and ibuprofen are L1. They’re safe, even for newborns. Avoid aspirin in high doses-it can cause Reye’s syndrome in infants.
• Antibiotics: Penicillins, cephalosporins, and azithromycin are all L1. Even clindamycin and metronidazole (L2) are considered safe with no need to interrupt feeding.
• Psychotropics: Sertraline (Zoloft) is the go-to antidepressant for breastfeeding moms. It has the lowest transfer rate among SSRIs. Fluoxetine (Prozac) has a long half-life and can build up in babies-so it’s L2, not L1. Lamotrigine for epilepsy is also well-studied and safe.
• Thyroid meds: Levothyroxine is L1. Your baby needs thyroid hormone too-and you’re giving it to them naturally.
• Birth control: Progestin-only pills are safe. Estrogen-containing contraceptives can reduce milk supply, so avoid them in the first few months.

What about opioids? Codeine and tramadol are risky because some people metabolize them into morphine faster than normal. This can lead to dangerous levels in breast milk. Oxycodone and hydrocodone are L3-use the lowest dose for the shortest time. Always watch your baby for excessive sleepiness, poor feeding, or breathing problems.

How to Minimize Your Baby’s Exposure

Even when a drug is safe, you can reduce exposure even further. Here’s how:

1. Time your doses. Take your medication right after you breastfeed, not before. That gives your body time to clear the drug before the next feeding.
2. Use the shortest half-life option. If you have a choice between two drugs, pick the one that leaves your system faster.
3. Choose low-dose, low-absorption options. For example, topical creams (like hydrocortisone) are safer than pills-unless you’re applying them to your nipple.
4. Avoid long-acting or extended-release formulas. They keep drug levels high for longer, increasing the chance of infant exposure.
5. Monitor your baby. Watch for unusual sleepiness, fussiness, poor feeding, or rash. Most reactions are mild and stop when you stop the drug.

For mothers taking daily meds, timing matters. If you take a pill once a day, take it after the evening feeding. That’s when your baby sleeps the longest. If you take it three times a day, take it right after each feeding.

Reliable Resources You Can Trust

Don’t rely on Google or advice from well-meaning friends. Use science-backed tools:

• LactMed (NIH): Free, online, updated daily. Covers over 4,000 drugs, 3,500 with infant exposure data. Used by over 1.2 million people a year. It’s technical, but you can search by drug name and get clear risk ratings.
• Medications and Mothers’ Milk by Dr. Hale: The gold standard for clinicians. Uses the L1-L5 system and gives practical advice. Updated every two years.
• MotherToBaby: A free service by OTIS. Call or chat with specialists who answer breastfeeding medication questions. They handle 15,000 inquiries a year.
• LactMed On-the-Go app: A mobile version of LactMed. Downloaded over 45,000 times since 2023. Great for quick checks at the pharmacy or doctor’s office.

Many doctors don’t know these resources exist. A 2021 survey of 500 lactation consultants found that 78% saw at least one mother per month who was wrongly told to stop breastfeeding because of a medication. That’s not just sad-it’s preventable.

What About Herbal Supplements and CBD?

LactMed now includes over 350 herbal products and 200 dietary supplements. But here’s the problem: most haven’t been studied in breastfeeding women. St. John’s Wort? It can cause irritability and colic in babies. Chamomile? Safe in tea, but concentrated extracts? Unknown. CBD? No data on long-term effects. The FDA doesn’t regulate supplements the way it does drugs. If you’re taking them, assume they’re risky until proven otherwise.

What’s Changing in the Next Few Years

The field is moving fast. In 2022, the FDA urged drug makers to include breastfeeding women in clinical trials. Right now, only 12 of the 85 FDA-approved biologics (like Humira or Enbrel) have enough data to be considered safe. That’s changing.

The InfantRisk Center’s MilkLab study has measured actual drug levels in breast milk from over 1,250 mothers. They’ve published 15 papers since 2018. And by 2030, experts predict we’ll be using genetic testing to predict exactly how much of a drug your baby will get-based on your DNA. That’s personalized breastfeeding pharmacology.

For now, the message is simple: don’t stop breastfeeding because you’re on medication. Talk to your doctor. Check LactMed. Use the right timing. And remember-your baby gets more from your milk than just nutrients. They get protection, comfort, and connection. That’s worth protecting.