Drug distribution in human milk

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Editor, – I refer to the article 'Drug distribution in human milk' (Aust Prescr 1997;20:35-40).

I am impressed by the assessment and summary of the safety of breast feeding during maternal drug therapy. I am particularly interested to learn about the drug transfer into milk of anti-tuberculosis drugs, especially isoniazid.

Do infants born to lactating mothers with tuberculosis and who are taking isoniazid need isoniazid prophylaxis? Or do we have to readjust (decrease) the dose of isoniazid provided when they need isoniazid prophylaxis?

Surafel Kebede
General Practitioner
Police Forces Hospital
Addis Ababa, Ethiopia

K.F. Ilett, J.H. Kristensen, R.E. Wojnar-Horton and E.J. Begg, the authors of the article, comment:
Space constraints in this journal and a marked lack of data have precluded us from addressing a broad range of drugs. Thus, our article concentrated on drugs which are commonly prescribed in Australia. However, the references listed under 'Further Reading', or a telephone call to the nearest Obstetric Drug Information Service can often provide useful information on other drugs.

Dr Kebede's request for information on the transfer of antitubercular drugs into human milk is an example of a group of drugs where there is limited information. Data for pyrazinamide and p-aminosalicylic acid come from a single patient study that found an estimated infant exposure of 0.3% and 0.1% respectively of the weight-adjusted maternal dose.1 Breast feeding was considered to be safe. Rifampicin and ethambutol have each been studied in only two patients, but milk concentrations were low and breast feeding was considered safe.2,3

Both isoniazid and its active metabolite N-acetylisoniazid are excreted in milk. In one patient, maximum infant exposure was calculated to be approximately 12% of the weight-adjusted maternal dose4, or 6% of a children's dose of isoniazid (10 mg/kg). Others have calculated that a breast-fed infant would receive 9-31% of a 10 mg/kg infant dose of isoniazid.5 The American Academy of Pediatrics regards maternal isoniazid therapy as being safe to use while breast feeding.6 Some individualised reduction in dose might be necessary for infants who are also being treated with the drug. In addition, since isoniazid has a significant adverse effect profile in adults, it would seem prudent to monitor the breast-fed infant for toxic adverse effects, some of which occur more frequently in the slow acetylator phenotype.