What muscle relaxants are safe to use during pregnancy?

Muscle Relaxants Safe in Pregnancy

Based on the available evidence, cisatracurium and rocuronium are the only muscle relaxants classified as pregnancy category B and are considered the safest options when neuromuscular blockade is medically necessary during pregnancy. 1

Neuromuscular Blocking Agents (for anesthesia/critical care)

Preferred Agents

• Cisatracurium and rocuronium are pregnancy category B drugs and should be prioritized when neuromuscular blockade is required to save the mother's life or avoid severe hypoxia for both mother and fetus 1
• These agents have been used extensively in pregnant patients for obstetrical and non-obstetrical surgeries 1
• All other neuromuscular blocking agents or their metabolites cross the placental barrier, with the exception of cisatracurium 1

Agents to Avoid

• Vecuronium, atracurium, and pancuronium are pregnancy category C and should be avoided for long-term infusion, especially in the first trimester 1
• Vecuronium has demonstrated residual clinical effects in newborns 1
• Atracurium and mivacurium should be avoided in perioperative settings 1
• Rocuronium and vecuronium may be safer alternatives to atracurium and mivacurium, though rocuronium remains the preferred category B option 1

Critical Caveat for Streptomycin

• Streptomycin is absolutely contraindicated in pregnancy due to the risk of fetal hearing loss 1
• Streptomycin may interact with muscle relaxants to cause postoperative respiratory muscle weakness 1

Oral Muscle Relaxants (for musculoskeletal conditions)

Evidence Summary

The evidence for oral muscle relaxants in pregnancy is extremely limited and concerning:

• Cyclobenzaprine shows increased risk for multiple birth defects including cleft palate (OR 4.79), d-transposition of great arteries (OR 6.97), anorectal atresia (OR 6.91), and coarctation of aorta (OR 5.58) in a large case-control study 2
• Carisoprodol has minimal human data (only 15 published cases), though one case report of high-dose exposure (2800 mg/day) throughout pregnancy showed no developmental toxicity, only mild infant sedation during breastfeeding 3
• Metaxalone explicitly states in FDA labeling that it "should not be used in women who are or may become pregnant and particularly during early pregnancy unless, in the judgement of the physician, the potential benefits outweigh the possible hazards" 4
• Thiocolchicoside is contraindicated in pregnancy, though one small observational study of 18 inadvertent exposures found no major birth defects 5

Clinical Recommendation

Given the concerning safety signals with cyclobenzaprine, lack of adequate safety data for other oral muscle relaxants, and FDA warnings, oral muscle relaxants should be avoided during pregnancy. 4, 2 Alternative approaches including physical therapy, acetaminophen, and topical analgesics should be prioritized for musculoskeletal pain management in pregnant patients.

Key Clinical Pitfalls

• Do not assume all muscle relaxants are equivalent in pregnancy - only cisatracurium and rocuronium have category B designation 1
• Avoid first trimester exposure to category C agents when possible, as this is the period of organogenesis 1
• Multidisciplinary management is essential - involve high-risk obstetrics, anesthesia, and allergy specialists when neuromuscular blockade is required 1
• The decision to use neuromuscular blocking agents must prioritize maternal survival - they may be justified to save the mother's life or prevent severe hypoxia affecting both mother and fetus 1
• Insufficient evidence exists regarding whether muscle relaxant use significantly increases adverse maternal or fetal outcomes compared to the general population 1