No information is currently available on the safety of methylephedrine, a component of various cold medications available in South Korea. With previous approval by an Institutional Review Board, 349 women inadvertently exposed to methylephedrine during the 1st trimester of pregnancy and an age- and gravidity-matched control group, were enrolled in a prospective cohort study. Study outcomes, for example gestational age at birth, birth weight and major and minor malformations were evaluated in 282 cases and 280 controls. Exposure to methylephedrine was at a gestational age of 4.0 weeks (median), at doses ranging from 52.5 to 1,575 mg/day, for a median duration of 3 (range: 1-30) days. No differences were observed between cases and controls in any of the pregnancy outcomes studied. There were 4/265 (1.5%) babies born with major malformations in the case group and 4/260 (1.5%) in the control group. In conclusion, inadvertent exposure to methylephedrine as a component of over-the counter oral cold remedies in early pregnancy was not associated with an increased rate of adverse pregnancy outcomes. Co-exposure with acetaminophen, cigarette smoking or alcohol did not appear to modify the outcomes.