Skeletal Muscle Relaxant
PREGNANCY RECOMMENDATION: Limited Human Data—Animal Data Suggest Low Risk
BREASTFEEDING RECOMMENDATION: No Human Data—Potential Toxicity
PREGNANCY SUMMARY
The animal reproduction data and human pregnancy experience suggest that the use of cyclobenzaprine in pregnancy is low risk.
FETAL RISK SUMMARY
Cyclobenzaprine is a centrally acting skeletal muscle relaxant that is closely related to the tricyclic antidepressants (e.g., imipramine). The agent is not teratogenic or embryotoxic in mice, rats, and rabbits given doses up to 20 times the human dose (1). No published reports of its use in human pregnancy have been located.
It is not known if cyclobenzaprine crosses the human placenta. The molecular weight (about 276 for the free base) is low enough that exposure of the embryo or fetus probably occurs.
In a surveillance study of Michigan Medicaid recipients involving 229,101 completed pregnancies conducted between 1985 and 1992, 545 newborns had been exposed to cyclobenzaprine during the 1st trimester (F. Rosa, personal communication, FDA, 1993). A total of 24 (4.4%) major birth defects were observed (23 expected), including (observed/expected) 5/5 cardiovascular defects, 1/1 oral clefts, and 2/2 polydactyly. No anomalies were observed in three other categories of defects (spina bifida, limb-reduction defects, and hypospadias) for which data were available. Earlier data, obtained from the same source between 1980 and 1983, totaled 168 1st-trimester exposures with 12 defects observed (10 expected). These combined data do not support an association between the drug and congenital defects.
BREASTFEEDING SUMMARY
No reports have been located on the excretion of cyclobenzaprine into milk. The molecular weight (about 276 for the free base) is low enough that excretion into milk should be expected. In addition, the closely related tricyclic antidepressants (e.g., see Imipramine) are excreted into milk and this should be considered before cyclobenzaprine is used during lactation.
Reference
1.Product information. Flexeril. Merck Sharpe & Dohme, 1993.