Flexaril (Cyclobenzaprine) Pregnancy Category
Flexaril (cyclobenzaprine) is classified as FDA Pregnancy Category B, indicating that animal reproduction studies have failed to demonstrate a risk to the fetus, but there are no adequate and well-controlled studies in pregnant women. 1
Understanding FDA Pregnancy Categories
The FDA established pregnancy categories to help healthcare providers assess medication risks during pregnancy:
- Category A: Studies in pregnant women show no risk to the fetus
- Category B: Animal studies show no risk, but no adequate studies in pregnant women
- Category C: Animal studies show adverse effects, but benefits may outweigh risks
- Category D: Evidence of human fetal risk exists
- Category X: Contraindicated in pregnancy due to proven fetal risk
Cyclobenzaprine Safety Data
According to the FDA drug label, reproduction studies have been performed in rats, mice, and rabbits at doses up to 20 times the human dose and have revealed no evidence of impaired fertility or harm to the fetus due to cyclobenzaprine. However, because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed. 1
Special Considerations
Potential Risks
A case report has suggested that maternal use of cyclobenzaprine during late pregnancy may potentially cause ductal closure and persistent pulmonary hypertension in neonates. The proposed mechanism includes inhibition of norepinephrine and serotonin reuptake, which can inhibit prostaglandin and nitric oxide - factors involved in maintaining ductal patency. 2
Breastfeeding
It is not known whether cyclobenzaprine is excreted in human milk. Because cyclobenzaprine is closely related to tricyclic antidepressants (some of which are known to be excreted in human milk), caution should be exercised when cyclobenzaprine is administered to a nursing woman. 1
Clinical Implications
When considering cyclobenzaprine use in pregnancy:
- Weigh the potential benefits against possible risks
- Use the lowest effective dose (5 mg TID may be as effective as 10 mg TID with fewer side effects) 3
- Consider alternative non-pharmacological approaches when appropriate
- Monitor closely if used in late pregnancy due to potential risk of ductal closure 2
Important Note
The FDA has replaced the ABCDX drug pregnancy categories with more descriptive information under the Pregnancy and Lactation Labeling Rule as of 2015. However, cyclobenzaprine was categorized under the previous system as Category B. 4