Pro-drug which is metabolized to methimazole that is equivalent to 60% of drug. Excretion into breast milk is insignificant (Low 1979, Johansen 1972, Rylance 1987).No adverse effects reported in breastfed infants (Rylance 1987). Plasma levels in those infants were shown to be very low (Rylance 1987). Daily dose up to 30 mg has shown to be safe for the infant (Lamberg 1984, Verd 1998, Bartalena 2005) at both short term (hormone serum level) and long term (psychomotor development).follow-up. The American Academy of Pediatrics rates us as compatible with breastfeeding