Hydrocortisone (cortisol) is a normal component of breastmilk that passes from the mother's bloodstream into milk and might have a role in intestinal maturation, the intestinal microbiome, growth, body composition or neurodevelopment, but adequate studies are lacking.[1] Concentrations follow a diurnal rhythm, with the highest concentrations in the morning at about 7:00 am and the lowest concentrations in the late afternoon and evening.[2,3] Maternal stress can increase breastmilk cortisol levels.[4] Cortisol in milk may protect against later infant obesity, especially in girls;[5] however, in another study, milk glucocorticoid levels were positively associated with percent fat mass, adiposity and head circumference at 1 year of age.[6] Hydrocortisone has not been studied in breastmilk after exogenous administration in pharmacologic amounts. Although it is unlikely that dangerous amounts of hydrocortisone would reach the infant, a better studied alternate corticosteroid might be preferred. Maternal use of hydrocortisone as an enema would not be expected to cause any adverse effects in breastfed infants. Local maternal injections, such as for tendinitis, would not be expected to cause any adverse effects in breastfed infants, but might occasionally cause temporary loss of milk supply. See also Hydrocortisone, Topical. Hydrocortisone concentrations in breastmilk are not affected by storage for 36 hours at room temperature, during multiple freeze-thaw cycles, nor Holder pasteurization (62.5 degrees C for 30 minutes).[7-9]