Fetal Risk Summary
The reproductive effects of the centrally acting muscle relaxant, chlorzoxazone, have not been studied in animals. Moreover, no published reports of its use in human pregnancy have been located.
No reports describing the placental transfer of chlorzoxazone have been located. The molecular weight (about 170), however, is low enough that transfer to the fetus should be expected.
In a surveillance study of Michigan Medicaid recipients involving 229,101 completed pregnancies conducted between 1985 and 1992, 42 newborns had been exposed to chlorzoxazone during the 1st trimester (F. Rosa, personal communication, FDA, 1993). One (2.4%) major birth defect was observed (two expected), a cardiovascular defect (0.5 expected). Earlier data, obtained from the same source between 1980 and 1983, totaled 264 1st trimester exposures with 17 defects observed (17 expected). These combined data do not support an association between the drug and congenital defects.
Breast Feeding Summary
No reports describing the use of chlorzoxazone during lactation have been located. The molecular weight (about 170), however, is low enough that excretion in milk should be expected. The effects of this potential exposure on a nursing are unknown.