How Medications Enter Breast Milk and What It Means for Your Baby

When you’re breastfeeding and need to take medicine, it’s natural to worry: is this drug going to hurt my baby? You’re not alone. Nearly half of all breastfeeding mothers stop earlier than they want to - not because they can’t make enough milk, but because they’re afraid the medicine they’re taking might harm their baby. The truth? Most medications are safe. But understanding how they get into breast milk - and how much actually reaches your baby - can help you make smarter, calmer choices.

How Medications Get Into Breast Milk

Breast milk isn’t just filtered blood. It’s made inside tiny sacs in your breast called alveoli, surrounded by capillaries that bring nutrients and drugs from your bloodstream. Medications cross from your blood into milk mainly through passive diffusion - about 75% of the time. That means if a drug is small enough, fat-soluble, and not tightly bound to proteins in your blood, it slips easily through the walls of the milk-producing cells.

Think of it like a sponge soaking up water. Small molecules (under 300 daltons) move quickly. Lithium, for example, is tiny (74 daltons) and ends up in milk at up to 10% of your dose. On the other hand, heparin is huge (15,000 daltons) and barely makes it into milk at all - less than 0.1%. That’s why insulin, which is also large, is safe to use while breastfeeding.

The rest of the transfer comes from two other paths. About 15% of drugs use special transporters in the breast cells - like nitrofurantoin and acyclovir. These are like taxis that carry specific drugs across. And 10% are actively pumped in by the cells themselves, often because the drug is a weak base. That’s why drugs like amitriptyline (used for depression and nerve pain) show up in milk at 2 to 5 times higher levels than in your blood. Why? Because breast milk is slightly more acidic than your blood. Weak bases get trapped there - a phenomenon called ion trapping.

What Makes a Drug More Likely to Reach Your Baby

Not all drugs behave the same. Three big factors determine how much ends up in your milk:

• Molecular weight: Under 300 daltons? High chance of transfer. Over 800? Almost none.
• Lipid solubility: The more fat-soluble a drug is, the better it moves into milk. Diazepam, for example, has a milk-to-plasma ratio of 1.5-2.0. That means your baby gets more in milk than in your blood. Gentamicin, a water-soluble antibiotic, has a ratio of just 0.05-0.1 - barely any transfer.
• Protein binding: If a drug sticks tightly to proteins in your blood (like warfarin at 99%), it can’t easily leave the bloodstream. That’s why warfarin is considered safe. Sertraline, though 98.5% bound, still gets through because a small free fraction is enough to cross.

Timing Matters More Than You Think

When you take your pill isn’t random - it can cut your baby’s exposure in half. The best time? Right after you breastfeed. Why? Because your blood concentration peaks about 1-2 hours after taking a dose, then drops. If you wait 3-4 hours before nursing again, your baby gets much less.

A 2019 study showed this simple trick reduces infant exposure by 30-50%. That’s huge. It doesn’t mean you have to pump and dump. Just time it right. For long-acting drugs like diazepam, which can stay in your system for days, spacing doses after feedings helps prevent buildup in your baby.

What About Antidepressants and Psychiatric Medications?

This is where fear runs highest. Many moms stop breastfeeding because they’re told SSRIs like sertraline (Zoloft) are risky. But here’s the data: sertraline transfers at only 1-2% of your maternal dose. The American Academy of Pediatrics rates it as “usually compatible” - the safest category. Infant serum levels are typically under 10% of what’s needed to treat an adult. Most babies show no symptoms.

But some do. About 8.7% of infants exposed to SSRIs may become fussy, have trouble feeding, or sleep less. That doesn’t mean you should stop. It means you should watch. If your baby seems unusually irritable or isn’t gaining weight, talk to your doctor. Check their serum levels at 2 weeks. If they’re under 10% of your therapeutic dose, you’re likely fine.

The European Medicines Agency has raised concerns about serotonin syndrome, but only a handful of case reports exist. In real-world use, the benefits of treating maternal depression almost always outweigh the risks. Untreated depression can be far more harmful to both you and your baby than sertraline.

Antibiotics, Painkillers, and Common Drugs

Antibiotics are the most common class of drugs used during breastfeeding - about 28.5% of nursing mothers take them. Amoxicillin? Safe. Gentamicin? Very low transfer. Ciprofloxacin? Limited data, but generally considered low risk. The key is to avoid tetracycline in infants under 8 - but even that’s rare because it’s not usually prescribed to nursing moms.

Painkillers? Acetaminophen and ibuprofen are top choices. Both pass into milk in tiny amounts. You’d need to take 10-20 doses a day to even come close to a harmful level for your baby. Opioids? Use with caution. Codeine and tramadol can convert to stronger forms in your body and may cause breathing problems in babies. If you need them, use the lowest dose for the shortest time.

What About Hormones and Birth Control?

Estrogen is the big red flag. Birth control pills with more than 50 mcg of ethinyl estradiol can slash your milk supply by 40-60% within just 72 hours. That’s why doctors recommend progestin-only pills, IUDs, or implants during breastfeeding. They don’t affect supply the same way.

Bromocriptine? That’s a different story. It’s designed to stop lactation - used after miscarriage or if a mom chooses not to breastfeed. It works by suppressing prolactin. If you’re trying to breastfeed, never take it.

Nuclear Medicine and Special Cases

If you need a scan - like a bone scan or PET scan - you might be told to stop breastfeeding for days. But that’s not always true. For a VQ scan using Tc-99m MAA, you’ll need to pump and dump for 12-24 hours. But for an FDG-PET scan? You can breastfeed right after. Only 0.002% of the dose ends up in milk. That’s less than a drop.

Radioactive iodine (I-131) for thyroid treatment? That’s one of the few absolute no-gos. It concentrates in the baby’s thyroid and can cause permanent damage. You’ll need to stop breastfeeding permanently.

What’s Safe? What’s Not?

Here’s a quick guide based on expert consensus:

Medication Safety Categories During Breastfeeding

Category	Transfer Level	Infant Exposure	Examples
Level 1 (Safest)	None or negligible	<1% of maternal dose	Insulin, heparin, acetaminophen, ibuprofen
Level 2 (Usually Compatible)	Low	1-5% of maternal dose	Sertraline, amoxicillin, ciprofloxacin, diazepam (with monitoring)
Level 3 (Cautious Use)	Moderate	5-10% of maternal dose	Lithium, fluoxetine, codeine
Level 4 (Probably Risky)	High	10-20% of maternal dose	Chloramphenicol, cyclosporine
Level 5 (Contraindicated)	Very high	>20% of maternal dose	Radioactive iodine-131, bromocriptine

What You Should Do Right Now

1. Don’t stop breastfeeding without checking. Over 98% of medications are safe or safe with monitoring.

2. Take your medicine right after feeding. Wait 3-4 hours before the next feed to let levels drop.

3. Watch your baby. Look for unusual sleepiness, fussiness, poor feeding, or rashes. These are rare, but if they happen, call your pediatrician.

4. Use trusted resources. The InfantRisk Center’s LactMed app (version 3.2, updated 2023) gives real-time, science-backed info. So does the CDC’s breastfeeding guidelines.

5. Ask your doctor to check the dose. For antidepressants or long-acting drugs, ask if your baby’s exposure is under 10% of your therapeutic level.

Final Reality Check

You’re not risking your baby’s health by taking most medications. You’re risking your own mental and physical health by avoiding them. Breastfeeding is powerful - but it shouldn’t cost you your well-being. The data is clear: the vast majority of drugs are compatible with breastfeeding. The real danger isn’t the medicine. It’s the fear that keeps you from taking it.

The next time someone tells you to stop breastfeeding because of a pill, ask them: “What’s the evidence?” Chances are, you can keep going - safely.