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

When you’re breastfeeding and need to take a medication, it’s natural to worry: is this drug going to hurt my baby? The truth is, most medications are safe. But not all. And figuring out which ones are okay isn’t as simple as just avoiding pills. It’s about understanding how drugs move from your bloodstream into your milk, what factors make some drugs riskier than others, and how to use them wisely.

How Medications Get Into Breast Milk

Medications don’t magically appear in breast milk. They travel through your body like everything else-through your blood. From there, they pass into the milk-producing cells in your breasts, mostly by passive diffusion. Think of it like water seeping through a sponge: the drug moves from where it’s more concentrated (your blood) to where it’s less concentrated (your milk).

Not all drugs make the trip equally. Four key things determine how much gets through:

• Molecular weight: Drugs under 200 daltons slip through easily. Larger molecules, like heparin or insulin, barely get in.
• Lipid solubility: Fats love drugs. The more a drug dissolves in fat, the more likely it is to enter milk. That’s why some antidepressants and anti-anxiety meds show up in higher amounts.
• Protein binding: If a drug is tightly glued to proteins in your blood (over 90%), it can’t float freely into milk. Warfarin and many NSAIDs are like this-very little gets through.
• Half-life: The longer a drug stays in your system, the more it can build up in milk. A drug with a 2-hour half-life clears quickly. One with a 24-hour half-life? That’s a different story.

There’s also something called ion trapping. Your blood is slightly more alkaline (pH 7.4) than breast milk (pH 7.2). Weakly basic drugs-like lithium, certain antidepressants, or barbiturates-get pulled into the milk and stuck there. This can make milk concentrations two to ten times higher than in your blood.

Right after birth, your body makes colostrum. The gaps between the cells in your breasts are wider, so more drugs can slip through. But here’s the good news: you’re only making about 30-60 mL a day in those first few days. By the time you’re producing mature milk (500-800 mL daily), those gaps have closed. So even if a drug passes through easily early on, the total amount your baby gets is still tiny.

The L1 to L5 Risk System-What Doctors Actually Use

Dr. Thomas Hale, a pioneer in breastfeeding pharmacology, created the most trusted system for classifying medications during lactation. It’s simple: five categories, from safest to most dangerous.

• L1: Safest - No documented risk. Examples: acetaminophen, ibuprofen, most penicillins.
• L2: Safer - Limited data, but no negative reports in many babies. Examples: sertraline, fluoxetine, amoxicillin.
• L3: Moderately Safe - Either limited data or mild side effects reported. Use if benefit outweighs risk. Examples: citalopram, azithromycin, metformin.
• L4: Possibly Hazardous - Evidence of risk, but maybe acceptable in life-threatening situations. Examples: lithium, cyclosporine, some anticonvulsants.
• L5: Contraindicated - Proven risk. Avoid completely. Examples: radioactive iodine, chemotherapy drugs, bromocriptine.

This system isn’t perfect, but it’s the best tool we have. And it’s not about whether a drug is in milk-it’s about whether it’s harmful in milk. For example, lithium is in L4 because it can build up in a baby’s system and cause toxicity. But sertraline? Even though it shows up in milk, babies rarely have any side effects. That’s why L2 drugs are often the go-to for depression during breastfeeding.

Common Medications and What the Data Shows

Most women take meds while breastfeeding. Studies show over half do. Here’s what’s most common-and what we know about safety.

• Analgesics (28.7%): Acetaminophen and ibuprofen are L1. Both are safe, with minimal transfer. Avoid aspirin in large doses-it can cause Reye’s syndrome in infants.
• Antibiotics (22.3%): Penicillins, cephalosporins, and macrolides like azithromycin are L1 or L2. Even metronidazole, once thought risky, is now considered safe at standard doses. Only tetracycline (L3) needs caution due to tooth discoloration risk.
• Psychotropics (15.6%): Sertraline is the top choice for depression-it’s L2, has low transfer, and studies show no developmental issues. Fluoxetine is L2 but has a long half-life, so it can build up. Paroxetine is also low-risk. Avoid benzodiazepines like diazepam (L3) if possible-they can cause drowsiness in newborns.
• Thyroid meds: Levothyroxine is L1. It’s safe and necessary if you have hypothyroidism.
• Birth control: Progestin-only pills (mini-pill) are safe. Estrogen-containing pills can reduce milk supply-avoid them in the first few months.

One big myth: “If it’s safe for babies, it’s safe for breastfeeding.” That’s mostly true. If a drug is approved for infants, it’s usually fine in milk. But not always. For example, codeine is safe for babies over 1 year, but in breastfeeding, it can turn into morphine in your body and overdose your baby. That’s why codeine is now L4 and discouraged.

When and How to Take Medications to Minimize Risk

Timing matters more than you think. You don’t have to stop breastfeeding. You just need to time it right.

• Take your dose right after feeding. That gives your body time to clear the drug before the next feed.
• If you take a drug once a day, take it after your baby’s longest sleep stretch-usually at night.
• For drugs taken multiple times a day, take them just before a feed. This lets the drug peak and start clearing before the next feeding.
• Avoid extended-release versions unless necessary. They keep drug levels high longer.
• For topical meds (creams, patches), avoid applying them to the breast or nipple. If you do, wash it off before feeding.

There’s no need to “pump and dump.” Unless you’re on chemotherapy or radioactive iodine, dumping milk doesn’t help. The drug clears from your blood naturally. Pumping just makes your body produce more milk-and more drug transfer.

Where to Get Reliable Info-And What to Avoid

Not all sources are created equal. Here’s what works:

• LactMed (NIH): Free, updated daily, covers over 4,000 drugs and 350 herbal products. Used by over a million people a year. The downside? It’s technical. Great for doctors, confusing for moms.
• Hale’s Medications and Mothers’ Milk (2022): The go-to for clinicians. Uses the L1-L5 system. Clear, practical, and focused on what you can actually do.
• MotherToBaby (OTIS): Free phone and chat service. Staffed by specialists. Handles 15,000 calls a year. Perfect for quick, personalized advice.
• InfantRisk Center: Offers a free app with real-time data and even research from their MilkLab study-measuring actual drug levels in milk from real moms.

Avoid Google searches, Reddit threads, or well-meaning but untrained friends. One 2021 survey found that 78% of lactation consultants see at least one case a month where a mom was told to stop breastfeeding over a medication that was actually safe.

What About Newer Drugs? Biologics, Cancer Meds, and What’s Coming

Most data comes from older, generic drugs. Newer ones? Not so much.

Of the 85 FDA-approved biologic drugs (like Humira, Enbrel), only 12 have enough data to say they’re safe in breastfeeding. That’s a gap. But progress is coming. In 2022, the FDA urged drug companies to include breastfeeding women in trials. That should improve data over the next 5-7 years.

For cancer drugs, most are still L5. But some targeted therapies are showing promise. One 2023 study found that a few oral cancer drugs had very low transfer rates. More research is needed, but the future is looking less scary.

By 2030, experts predict we’ll be using personalized lactation pharmacology. That means testing a mom’s genes to predict exactly how much of a drug will end up in her milk. Accuracy could hit 85-90%. That’s not science fiction-it’s the next step.

Bottom Line: You Can Breastfeed While Taking Meds

Fewer than 1% of medications require you to stop breastfeeding. Less than 2% of babies have any serious side effects from drug exposure through milk.

Don’t let fear silence your health. If you need a medication, talk to your doctor. Use LactMed or call MotherToBaby. Don’t guess. Don’t quit breastfeeding unless a professional tells you to.

Medications aren’t the enemy. Ignorance is. With the right info, you can take care of yourself-and keep giving your baby the best start possible.