Drugs in Pregnancy and Lactation: Tenth Edition

CHLORZOXAZONE

Muscle Relaxant

PREGNANCY RECOMMENDATION: Limited Human Data—No Relevant Animal Data

BREASTFEEDING RECOMMENDATION: No Human Data—Probably Compatible

PREGNANCY SUMMARY

The available human pregnancy experience does not suggest a risk of teratogenicity.

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. The relatively low molecular weight (about 170) suggests that the drug will cross the placenta.

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.

BREASTFEEDING SUMMARY

No reports describing the use of chlorzoxazone during lactation have been located. The molecular weight (about 170) is low enough that excretion into breast milk should be expected. The effect of this potential exposure on a nursing infant is unknown. However, other muscle relaxants are excreted in milk and do not appear to harm a nursing infant (e.g., see Baclofen and Carisoprodol).