Dextromethorphan, the major ingredient in most OTC cough medicines, has been shown to cause birth defects and fetal death in chicken embryos exposed to concentrations relative to those typically taken by humans. Researchers found that dextromethorphan causes defects so early in the development of the embryo that in many cases the woman wouldn't even know she is pregnant. Reserachers feel that a single dose is capable of causing a birth defect and that, ultimately, it could be the cause for a woman to have a miscarriage.
Dextromethorphan suppresses cough by acting on receptors in the adult central nervous system. But in embryos, the drug appears to "knock out" the receptors, thus leading to the defects. Further study is needed, but in the meantime, the authors suggests that pregnant women or women attempting to get pregnant, be advised not to use dextromethorphan-containing cough medicine.
Pediatric Research January 1998;43:1-7