Obstetrics and Gynaecology chapters
Drugs in breastfeeding
Which drugs pass into breast milk, how much the baby actually receives, the drugs that must be avoided during lactation, and how to time a dose around a feed.
- Most drugs enter milk in very small amounts, so the baby usually receives far less than a treatment dose and breastfeeding can continue.
- A few must be avoided: radioactive iodine, cytotoxic and immunosuppressive drugs, amiodarone, chloramphenicol, tetracyclines, and lithium needs care because milk levels match maternal blood.
- If the mother must take a drug, give it just after a feed so that levels fall before the next one.
On this page
Overview
Nearly every drug a mother takes can be found in her milk, but the concentration is usually low and the dose reaching the baby is far below a treatment dose. Needless advice to stop feeding does harm, because formula feeding carries higher infant illness rates. The exam asks which few drugs are genuinely unsafe, which have recognised named effects on the baby, and how dosing is timed.
The general rule is that breastfeeding should continue; only a short list of drugs forces a stop.
Physiology in one pass
- Prolactin from the anterior pituitary drives milk synthesis; levels rise up to 20-fold in pregnancy and lactation, and peak within about 45 minutes of suckling.
- Suckling stops release of prolactin-inhibiting factor (dopamine) from the hypothalamus. A dopamine agonist therefore stops lactation.
- Oxytocin from the posterior pituitary contracts myoepithelial cells, giving the let-down (milk-ejection) reflex. It is triggered by suckling and even by seeing or hearing the baby, and is blocked by stress. The same oxytocin causes the "after-pains" of the uterus.
- Milk contains lactose, protein, fat and water, with IgA for mucosal protection. It is low in vitamin K, hence vitamin K for the newborn.
What decides how much drug enters milk
| Factor | Effect |
|---|---|
| Lipid solubility | Lipophilic drugs enter milk more readily |
| Molecular size | Large molecules such as heparin and insulin do not pass in meaningful amounts |
| Protein binding | Heavily bound drugs stay in maternal plasma |
| Milk pH | Milk is slightly more acidic than plasma, so weak bases concentrate a little |
| Maternal half-life | Long-acting drugs accumulate, as with diazepam |
| Infant handling | Preterm and newborn babies clear drugs slowly, so the same milk level matters more |
"Small, fatty, unbound, long-acting: that drug gets into the milk."
Drugs by level of concern
Avoid during breastfeeding
| Drug | Reason |
|---|---|
| Radioactive iodine and radionuclides | Taken up by the infant thyroid; feeding is stopped, and withheld for a period even after small doses |
| Cytotoxic and immunomodulating drugs | Marrow suppression and unknown long-term harm |
| Amiodarone | Large amounts in milk; the baby's thyroid needs monitoring |
| Chloramphenicol | Possible marrow suppression |
| Tetracyclines | Milk level is roughly 70% of maternal serum; risk of permanent tooth staining |
| Heroin and other drugs of misuse | Pass into milk and prolong neonatal dependence |
Use with care and watch the baby
| Drug | Effect on the infant |
|---|---|
| Lithium | Milk concentration equals maternal serum and the drug is cleared only by the kidney, so the baby can receive a lot |
| Codeine | Normally safe, but a mother who is an ultra-rapid CYP2D6 metaboliser makes far more morphine; neonatal sedation and death have been reported |
| Phenobarbitone and other sedatives | Drowsiness, poor sucking; diazepam accumulates |
| Isoniazid | Reaches milk levels close to maternal blood; pyridoxine supplements are needed |
| Propylthiouracil | Rarely suppresses infant thyroid function |
| High-dose prednisolone | Low maternal doses are acceptable; very high doses are avoided |
| Theophylline | Moderate transfer; irritability possible |
| Combined oral contraception | High oestrogen doses may reduce milk supply, so a progestogen-only method is preferred early on |
Generally acceptable
Penicillins, ampicillin, cephalosporins, erythromycin, digoxin, propranolol, spironolactone, thiazides, warfarin and low molecular weight heparin, thyroxine, insulin, tolbutamide, paracetamol and occasional aspirin doses, and moderate caffeine. Nicotine levels in milk are low. Warfarin and low molecular weight heparin are both acceptable after delivery.
For postnatal mood disorder, tricyclic antidepressants have the better evidence of safety in breastfeeding; SSRIs at usual doses are probably acceptable. Small amounts of benzodiazepine reach milk, and feeding may actually soften neonatal withdrawal in a baby exposed in pregnancy.
Aspirin in occasional analgesic doses is tolerated in lactation; it is the baby given aspirin directly who risks Reye syndrome.
Practical prescribing rules
- Prescribe only where there is a clear need, choose a drug with a short half-life and poor transfer, and use a single drug.
- Time the dose just after a feed, ideally some hours before the next one, so that maternal and milk levels have fallen.
- Prefer topical or inhaled routes where they work: inhaled salbutamol and inhaled steroid are continued normally.
- Watch the baby for drowsiness, poor feeding, loose stools or rash and review if these appear.
- Where a drug must be given that is unsafe, milk can be expressed and discarded while treatment lasts.
- National advice changes; check the current guideline before giving an unfamiliar drug to a nursing mother.
When lactation must be stopped
- Dopamine agonists (bromocriptine, cabergoline) inhibit prolactin and suppress lactation, but cause drowsiness, low blood pressure, headache and gut upset, and rare serious cardiovascular events after delivery have been reported.
- By the second week, fluid restriction with a firm, supportive brassiere works as well as bromocriptine, so this non-drug approach is the method of choice.
- In HIV, advice depends on setting: where safe alternatives exist, women are advised not to breastfeed; in low-resource settings, the World Health Organization advises antiretroviral treatment with exclusive breastfeeding, since mixed feeding carries the highest transmission risk and not feeding raises child mortality. National policy can change, so check the current guideline (official government or WHO page, checked 11 Nov 2026).
Mixed breast and bottle feeding carries the highest risk of mother-to-child HIV transmission, which is why exclusive feeding is advised where feeding is chosen.
Expect: the short list of drugs that stop breastfeeding (radioiodine and radionuclides, cytotoxics and immunosuppressants, amiodarone, chloramphenicol, tetracyclines, drugs of misuse); lithium because its milk level equals maternal serum; codeine with ultra-rapid CYP2D6 metabolism; isoniazid with pyridoxine deficiency; tetracycline with stained teeth; timing a dose right after a feed; prolactin for synthesis and oxytocin for ejection, with dopamine agonists suppressing lactation; fluid restriction and support preferred over bromocriptine; progestogen-only rather than combined contraception early in lactation; and vitamin K for the newborn because milk is low in it.
Quick revision
- Most drugs reach milk in trace amounts; keep feeding unless there is a real reason not to.
- Stop for radioiodine, cytotoxics, immunosuppressants, amiodarone, chloramphenicol, tetracyclines.
- Lithium: milk level equals maternal serum, renal clearance only, so be cautious.
- Codeine can be dangerous if the mother is an ultra-rapid metaboliser.
- Isoniazid needs pyridoxine cover for the baby.
- Give the maternal dose immediately after a feed.
- Prolactin makes milk, oxytocin ejects it; dopamine agonists switch lactation off.
- Fluid restriction plus firm support is the preferred way to suppress lactation.
- Progestogen-only contraception is preferred to combined while breastfeeding.
- Breast milk is low in vitamin K, so the newborn receives vitamin K.
Written by The House Job from standard references (Obstetrics by Ten Teachers; Basic and Clinical Pharmacology by Katzung).
FAQ
Does a mother on antibiotics have to stop feeding?
Usually not: penicillins, cephalosporins and erythromycin reach milk in very low amounts, while tetracyclines and chloramphenicol are the ones to avoid.
Why is lithium treated differently from most drugs?
Its concentration in milk is the same as in maternal serum and it depends entirely on renal clearance, so the baby can receive a substantial amount.
How should a necessary dose be timed?
Straight after a feed and well before the next one, so that maternal blood and milk levels are falling when the baby next feeds.
What is the preferred way to stop milk production?
Fluid restriction with a firm, supportive brassiere; dopamine agonists work but carry side effects and are no longer first choice.