Published: 10 June 2015
Amended: 3 February 2016


Medicine Use in Lactation

This article is more than five years old. Some content may no longer be current.
For a specific medicine, see the data sheet or consumer medicine information for information about use in lactation. You can also refer to the medicine monograph on the New Zealand Formulary website.

Prescriber Update 36(2): 22-25
June 2015

Key Messages

  • Most medicines may be safely used during breastfeeding. However, where possible, the lowest effective dose for the shortest possible duration should be used.
  • Physicochemical characteristics of the medicine and breast milk composition determine the concentration of medicines in breast milk.
  • Adverse reactions more commonly observed in nursing infants include sedation, gastrointestinal upset and irritability.
  • The benefits and risks of harm of treatment in both the mother and infant should be considered as well as the benefits and risks of harm of not treating.

Medicines are often needed by women who are breastfeeding. However, there is often a lack of, or ambiguous, information about the safety of medicines transferred to the infant in breast milk. This has meant that in the past breastfeeding has been stopped unnecessarily or a different, potentially less appropriate treatment prescribed1.

Although most medicines are excreted in breast milk to some degree, the amount is usually less than 10% of the maternal dose. Medicines excreted at less than 10% are considered compatible with breastfeeding. Therefore, with a few exceptions the majority of medicines may be used2.

Almost all medicines pass into breast milk via passive diffusion. The amount available is dependent on a number of factors such as:3,4,5

  • oral bioavailability
  • plasma half-life
  • lipid solubility
  • molecular weight or size
  • ionisation
  • percentage of maternal protein binding
  • composition of the milk.

For example, breast milk is slightly more acidic than plasma (pH 7.1 compared with pH 7.4)5. Therefore, medicines with a higher pH (basic) such as β-blockers will diffuse into the milk more readily. The relatively acidic milk then changes the physicochemical structure of the medicine, shifting the equilibrium between the ionised and non-ionised forms to the ionised form. The ionised form is less able to diffuse back from the milk to the plasma, thereby becoming 'trapped' in the milk5.

In addition to being more acidic, breast milk contains more lipids and less protein than plasma. Because of breast milk lipophilicity, medicines with high lipid solubility are more likely to concentrate in the breast milk4.

Medicines with a high molecular weight (eg, insulin and heparin) do not readily transfer into breast milk as they are too large to cross mammary epithelium cell walls3. Active transport or dissolution in lipid membranes of cells is required, reducing the likelihood of these medicines passing into breast milk.

Highly maternally protein bound medicines are also less likely to pass into breast milk as the more bound a medicine is, the less efficiently it can cross cell membranes3.

As all medicines have a combination of these factors and milk composition changes with time, there is variability in medicine transfer into breast milk.

The safety of breastfeeding whilst taking medicines also depends on the age of the infant as drug metabolism and clearance is reduced in the neonate6.

An up-to-date database of medicine levels in breast milk and infant blood and possible adverse reactions in the nursing infant is available at LactMed ( The World Health Organization (WHO) has also produced a classification guide for medicines use in breastfeeding based on the WHO list of essential drugs (

Medicines with inherent toxicity or those with high infant exposure and therefore potential for significant toxicity are contraindicated during breastfeeding, and include:4

  • cytotoxic agents
  • immunosuppressive agents
  • amiodarone
  • lithium
  • ergotamine
  • gold salts
  • isotretinoin.

Radiopharmaceutical administration also requires temporary cessation of breastfeeding7.

There are a range of health issues that affect women who are breastfeeding. These most commonly include infection, depressive disorders, pain, contraception, low milk supply and atopic conditions1. Table 1 provides information on the compatibility of medicines used in the treatment of these conditions with breastfeeding.

Table 1: Compatibility of commonly used medicines with breastfeeding2,7,8

Condition Treatment Breastfeeding Recommendation Additional Information
Infection Antibiotics
(eg, amoxicillin)
Compatible Gastrointestinal flora changes possible; monitor infant for diarrhoea, vomiting, thrush
(eg, erythromycin)
Compatible Single dose of azithromycin considered safe
(eg, cephalexin)
Compatible May also affect infant gut flora (third generation more likely)
(eg, ciprofloxacin)
Avoid if possible Potential risk of arthropathies
Trimethoprim Compatible  
Nitrofurantoin Compatible Avoid nitrofurantoin if infant less than one month old or premature
Metronidazole Avoid if possible If single 2 g metronidazole dose given, discontinue breastfeeding for 12 hours
(eg, fluconazole)
Compatible If applying miconazole oral gel to nipples, apply after breastfeeding
Nystatin Compatible  
Aciclovir Compatible  
Depressive disorders Antidepressants
(eg, paroxetine)
Compatible Paroxetine and sertraline preferred due to shorter half-lives
(eg, amitriptyline)
Less preferred due to potential toxicity Amitriptyline compatible in doses up to 150 mg/day
(eg, temazepam)
Compatible in a single dose; avoid repeated doses Short-acting benzodiazepines preferred as accumulation may occur
Monitor infant for drowsiness
Pain Analgesics
Paracetamol Compatible Paracetamol analgesic of choice
(eg, ibuprofen)
Compatible Avoid breastfeeding with long-term acetylsalicylic acid treatment
(eg, codeine)
Compatible in occasional doses Monitor infant for drowsiness, apnoea, bradycardia and cyanosis
Use codeine with caution in rapid metabolisers
Tramadol Compatible  
Contraception Hormonal methods
Progesterone Compatible See data sheet
Oestrogen Avoid if possible May inhibit lactation
Allergies and hay fever Antihistamines
(eg, promethazine)
Probably compatible Occasional use probably safe
Monitor for sedation in mother and infant
(eg, loratadine)
(eg, hydrocortisone)
Compatible If applying to breasts apply after feeding
Asthma β2- adrenergics
(eg, salbutamol)
(eg, budesonide)
Other Warfarinbr Compatible  
Metformin Compatible  

Consideration should be given to the benefits and risks of harm of treatment in both the mother and child prior to initiating therapy, as well as the risks of harm of not initiating or continuing therapy. The breastfed infant should be monitored for potential adverse reactions, most commonly sedation, gastrointestinal effects (eg, diarrhoea) and irritability5.

In addition to possible effects in the breastfed infant, some medicines may affect milk production, often through hormonal regulation of lactation. For example, oestrogen-containing contraception is not recommended if the mother is breastfeeding as it reduces milk production. Domperidone and metoclopramide increase prolactin, which promotes lactation2.

Breastfeeding mothers should use the lowest effective dose of a medicine for the shortest possible duration. For medicines with a short half-life, feeding the infant immediately prior to mother taking the next dose reduces the drug exposure in the infant.

If possible, medicines with short half-lives, high maternal protein binding, low oral availability and high molecular weight should be used.

  1. Jayawickrama HS, Amir LH, Pirotta MV. 2010. GPs’ decision-making when prescribing medicines for breastfeeding women: Content analysis of a survey. BMC Research Notes doi: 10.1186/1756-0500-3-82.
  2. Amir LH, Pirotta MV, Raval M. 2011. Breastfeeding: Evidence based guidelines for the use of medicines. Australian Family Physician 40(9): 684-690.
  3. Nice FJ, Luo AC. 2012. Medications and Breastfeeding. Journal of the American Pharmacists Association 52(1): 86-94.
  4. Ilett KF, Kristensen JH. 1997. Drug distribution in human milk. Australian Prescriber 20(2): 35-40.
  5. Berlin CM, Briggs GG. 2005. Drugs and chemicals in human milk. Seminars in Fetal & Neonatal Medicine 10: 149-159.
  6. Medsafe. 2014. Drug Metabolism — The Importance of Cytochrome P450 3A4. Prescriber Update 35(1): 4–6. URL: (accessed 20 May 2015).
  7. World Health Organization. 2002. Breastfeeding and maternal medication: Recommendation for drugs in the eleventh WHO model list of essential drugs (accessed 09/04/2015).
  8. American Academy of Pediatrics. 2001. The Transfer of Drugs and Other Chemicals Into Human Milk (accessed 10/04/2015).
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