# Drugs in Lactation Safety

> NEET-PG Obstetrics and Gynaecology notes on drugs in lactation, covering milk transfer, relative infant dose, drugs to avoid and safe common choices.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/drugs-in-lactation-safety
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Hale's Medications and Mothers' Milk; Williams Obstetrics; Dutta's Textbook of Obstetrics
- How to cite: "Drugs in Lactation Safety", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/drugs-in-lactation-safety

## Direct answer

Most maternal drugs reach breast milk by passive diffusion, so small, lipid-soluble and weakly protein-bound molecules transfer most readily. Lactation safety depends on how much the infant actually absorbs, and the relative infant dose is the usual yardstick. Only a short list of drugs must be avoided outright, and most common medicines are compatible with breastfeeding.

## What you must remember

- **Transfer principles:** passive diffusion favours low molecular weight, low protein binding and high lipid solubility; basic drugs accumulate in milk through ion trapping.
- **Relative infant dose:** the infant's dose from milk as a percentage of the weight-adjusted maternal dose; values below about 10 per cent are generally considered acceptable.
- **Radioactive iodine:** contraindicated, because it concentrates in breast tissue and is secreted into milk.
- **Cytotoxic drugs:** methotrexate and cyclophosphamide are avoided because of toxicity in the infant.
- **Lithium:** generally avoided, or used only with close infant monitoring, because of toxicity risk.
- **Codeine:** avoided, because some infants metabolise it rapidly and receive high morphine levels through milk.
- **Bromocriptine:** suppresses prolactin and lactation, so it is used only when breastfeeding is not intended.
- **Usually compatible:** paracetamol, amoxicillin, most cephalosporins and standard antihypertensives such as nifedipine and enalapril.

## A checklist for a lactating mother who needs a drug

A mother with a new prescription asks whether she can keep breastfeeding. The sequence below answers from the drug's properties and the infant's likely exposure, rather than from habit or from pregnancy labelling alone.

1. **Check the drug's profile:** molecular weight, protein binding, half-life and oral bioavailability in the infant determine transfer and absorption.
2. **Look for absolute avoidance:** radioactive iodine, cytotoxic drugs and a few others are stopped or avoided during lactation.
3. **Estimate the relative infant dose:** the infant's share is compared with the weight-adjusted maternal dose, and values below about 10 per cent are generally acceptable.
4. **Time the dose:** taking the drug just after a feed, or choosing a shorter-acting alternative, lowers the infant's peak exposure.
5. **Monitor the infant:** sedation, poor feeding, rash or altered stools prompt review when a drug with transfer concerns is used.
6. **Consult a current reference:** lactation references are revised often, and the decision should rest on the latest source.

## Where the exam tries to catch you

Vignettes punish reflexive stopping of breastfeeding and the choice of drugs that look harmless but accumulate in the infant.

- **Trap:** Stopping breastfeeding for every drug because the pregnancy label says caution. — **Correct:** Pregnancy and lactation risks differ, and many drugs cautioned in pregnancy are compatible with breastfeeding.
- **Trap:** Choosing codeine for pain in a breastfeeding mother. — **Correct:** Codeine is avoided because some infants metabolise it rapidly, producing toxic morphine levels; paracetamol is preferred.
- **Trap:** Treating bromocriptine as a neutral option for a breastfeeding woman. — **Correct:** Bromocriptine suppresses prolactin and lactation, so it is used when breastfeeding is not intended.

## Frequently asked questions

### Which drug is contraindicated in breastfeeding because it concentrates in milk?
Radioactive iodine is contraindicated in breastfeeding because it concentrates in breast tissue and is secreted into milk. Breastfeeding is stopped during therapy, and the nuclear medicine team sets the interval for resumption. The decision is never made by the mother alone.

### Why is codeine avoided in breastfeeding mothers?
Codeine is avoided because some mothers and infants are ultra-rapid metabolisers of CYP2D6, producing high morphine levels in milk. This can cause respiratory depression in the infant. Paracetamol is a safer analgesic choice.

### What does relative infant dose mean?
Relative infant dose is the infant's dose from breast milk expressed as a percentage of the maternal dose adjusted for weight. Values below about 10 per cent are generally considered acceptable. It is the standard yardstick in lactation pharmacology.

### Which common analgesic is compatible with breastfeeding?
Paracetamol is compatible with breastfeeding and is preferred for pain and fever. Ibuprofen is also generally used for short courses. Mothers should still review any drug with a current lactation reference before continuing.