Key Facts

The decision to prescribe during breastfeeding should consider the importance of the drug to the mother, the safety of the drug for the infant, and the benefits of continuing breastfeeding. Unnecessary avoidance of medication or unnecessary cessation of breastfeeding should be avoided.

Principles of Drug Transfer into Breast Milk

• Most drugs enter breast milk by passive diffusion
• Factors promoting transfer: high lipophilicity, low protein binding, low molecular weight (<500 Da), non-ionised form, high maternal plasma levels
• Colostrum: more permeable than mature milk — first few days higher transfer, but volumes are small
• Timing: drug levels in milk mirror plasma levels — feed just before next dose (trough level) to minimise exposure

Pharmacokinetic Considerations

• Infant oral bioavailability: many drugs poorly absorbed orally by infant (e.g. aminoglycosides, heparin) → minimal systemic exposure even if present in milk
• Infant metabolism: neonates have immature hepatic and renal function → impaired drug clearance
• Premature infants: most vulnerable — immature organ function, higher susceptibility

Pathophysiology

• Drug in maternal plasma → crosses alveolar epithelium of mammary gland by passive diffusion → enters breast milk
• Milk:plasma ratio: <1 for most drugs (lower concentration in milk than plasma)
• Relative infant dose (RID) = (dose via milk / maternal dose) adjusted for weight — RID <10% generally safe

Safe Drugs in Breastfeeding

• Analgesics: paracetamol, ibuprofen (preferred NSAID — low milk transfer, short half-life)
• Antibiotics: most penicillins, cephalosporins, macrolides (erythromycin preferred)
• Antihistamines: loratadine, cetirizine (non-sedating preferred)
• Asthma: inhaled corticosteroids, salbutamol — minimal systemic absorption
• Antihypertensives: labetalol, nifedipine, enalapril
• Antidepressants: sertraline (preferred — low milk transfer), paroxetine
• Antiepileptics: lamotrigine, valproate, carbamazepine — generally compatible with monitoring
• Anticoagulants: warfarin, LMWH — do not cross into milk significantly

Drugs to AVOID

• Cytotoxic drugs: all — immunosuppression, bone marrow toxicity
• Amiodarone: long half-life, iodine content → neonatal thyroid suppression
• Ergotamine: vomiting, diarrhoea, seizures in infant
• Radioactive iodine: concentrated in breast milk → thyroid damage
• Gold salts: accumulation risk
• Retinoids: potential toxicity

Drugs Requiring CAUTION

• Codeine: risk of morphine toxicity in CYP2D6 ultra-rapid metaboliser mothers — fatal cases reported; avoid
• Lithium: significant transfer; monitor infant lithium levels, TFTs, renal function
• Benzodiazepines: sedation, poor feeding; prefer short-acting if essential
• Carbimazole: monitor infant TFTs
• Metronidazole: high-dose courses — express and discard for 12–24 hours

Red Flags

• Infant drowsiness, poor feeding, respiratory depression — consider drug exposure via milk
• Codeine in breastfeeding — risk of fatal opioid toxicity (CYP2D6 ultra-rapid metaboliser)
• Mother requiring cytotoxics — breastfeeding must stop
• Radioactive iodine — stop breastfeeding (may need to stop permanently depending on dose)

Before Prescribing

• Consult BNF breastfeeding appendix: check safety of specific drug
• LactMed database (NIH): evidence-based resource for drug safety in lactation
• UKTIS: specialist advice for complex cases

Monitoring

• Infant observation: feeding, alertness, growth, stool pattern
• Infant drug levels: rarely needed; consider for lithium, anticonvulsants
• Infant TFTs: if mother on carbimazole or amiodarone
• Infant FBC: if mother on drugs with haematological effects

Special Tests

• CYP2D6 genotyping: maternal genotyping before codeine use in breastfeeding — identifies ultra-rapid metabolisers (MHRA warning)
• Milk:plasma ratio: research data — available in LactMed

General Principles

• Benefits of breastfeeding almost always outweigh medication risk — do NOT stop breastfeeding unnecessarily
• Use drugs with established safety in breastfeeding
• Lowest effective dose for shortest duration
• Time feeds to minimise exposure: feed just before next maternal dose (trough level)
• Monitor infant: for any unusual symptoms (sedation, poor feeding, rash)
• Topical/inhaled routes: preferred where possible — minimal systemic absorption

Practical Prescribing

• Pain: paracetamol + ibuprofen — both safe; avoid codeine (MHRA)
• Infection: penicillins, cephalosporins, erythromycin — safe; avoid tetracyclines, chloramphenicol, ciprofloxacin
• Depression: sertraline (first choice); avoid fluoxetine (long half-life)
• Hypertension: labetalol, nifedipine, enalapril — safe; avoid atenolol (concentrates in milk)
• Contraception: progesterone-only methods from 3 weeks postpartum; combined OCP from 6 weeks (may reduce milk supply)

When Breastfeeding Must Stop

• Cytotoxic chemotherapy
• Radioactive iodine treatment
• High-dose methotrexate
• Certain biological therapies (case-by-case)

Express and Discard

• Some drugs require temporary cessation: express and discard milk to maintain supply
• Example: after radiocontrast with gadolinium (express for 24 hours)

Referral Criteria

• UKTIS: complex prescribing decisions in breastfeeding
• Lactation consultant: breastfeeding support during medication changes
• Paediatrics: if infant shows signs of drug exposure

• The vast majority of drugs are compatible with breastfeeding
• Unnecessary cessation of breastfeeding causes more harm than most drug exposures
• Fatal codeine toxicity in breastfed infants: extremely rare but preventable — avoid codeine
• Appropriate prescribing and monitoring ensures safe breastfeeding for mother and infant
• Evidence base is growing — fewer drugs now considered contraindicated than previously thought

Drug Safety in Breastfeeding — Quick Reference

Key Resources for Prescribing in Breastfeeding