Aims To assess the effects of pseudoephedrine on breast blood flow, temperature and milk production, and to estimate the likely infant dose during breastfeeding.Methods Eight lactating women (mean age 35 years and weight 69 kg) participated in a single-blind randomized crossover study of 60 mg pseudoephedrine hydrochloride vs placebo. Breast blood flow and surface temperature were measured from 0 to 4 h following the dose, and change in plasma prolactin was measured as the difference between predose and 1 h postdose concentrations. Milk production was measured for 24 h following placebo and pseudoephedrine. Infant dose of pseudoephedrine for a 60-mg dose administered four times daily to the mother was quantified as the product of average steady-state drug concentration in milk and an estimated milk production rate of 0.15 l kg(-1) day(-1) and expressed relative to the maternal weight-adjusted dose.Results There were no physiologically significant changes in breast blood flow or temperature between the placebo and pseudoephedrine periods. The mean change in plasma prolactin was slightly (13.5%), but not significantly lower (t = 1.245, P = 0.253) after pseudoephedrine (1775 mU l(-1)) compared with placebo (2014 mU l(-1)). However, the mean milk volume was reduced by 24% from 784 ml day(-1) in the placebo period to 623 ml day(-1) in the pseudoephedrine period (difference between means 161 ml day(-1) (95% CI: 63, 259 ml day(-1)); t = 3.9, P = 0.006). Assuming maternal intake of 60 mg pseudoephedrine hydrochloride four times daily, the estimated infant dose of pseudoephedrine was 4.3% (95% CI, 3.2, 5.4%) of the weight-adjusted maternal dose.Conclusions A single dose of pseudoephedrine significantly reduced milk production. This effect was not attributable to changes in blood flow, but depression of prolactin secretion may be a contributing factor. At the maximum recommended pseudoephedrine doses, the calculated infant dose delivered via milk was <10% of the maternal dose, and is unlikely to affect the infant adversely. The ability of pseudoephedrine to suppress lactation suggests a novel use for the drug.