Are muscle relaxants (muscle relaxers) safe during pregnancy?

From the Guidelines

Muscle relaxers are not recommended during pregnancy unless absolutely necessary and the benefits outweigh the risks. The primary concern with muscle relaxants during pregnancy is their potential to cross the placenta and affect fetal development, particularly in the first trimester when organ formation occurs 1. Most muscle relaxants, such as cyclobenzaprine (Flexeril), carisoprodol (Soma), and methocarbamol (Robaxin), are classified as pregnancy category C or D, meaning animal studies have shown adverse effects on the fetus, and there are no adequate studies in pregnant women.

Some key points to consider when evaluating the safety of muscle relaxers during pregnancy include:

• The potential for adverse effects on the fetus, such as bradycardia, hypoglycemia, premature labor, and metabolic abnormalities 2
• The risk of intrauterine growth retardation, particularly with propranolol and atenolol 2
• The limited experience with certain muscle relaxants, such as sotalol and flecainide, during pregnancy 2
• The importance of consulting with an obstetrician before taking any medication during pregnancy to determine the best course of treatment for individual cases.

If muscle spasms are severe and require treatment during pregnancy, acetaminophen is typically the first-line option for pain relief, and physical therapy, gentle stretching, massage, heat therapy, and proper body mechanics are safer alternatives that should be tried first. For severe cases where medication is necessary, limited use of certain muscle relaxants like cyclobenzaprine at the lowest effective dose for the shortest duration possible may be recommended by an obstetrician.

From the FDA Drug Label

Pregnancy Category C Reproduction studies performed in rats at a dose of 3 mg/kg, equal to the maximum recommended human dose on a mg/m2 basis, and in rabbits at 30 mg/kg, 16 times the maximum recommended human dose on a mg/m2 basis, did not show evidence of teratogenicity Tizanidine at doses that are equal to and up to 8 times the maximum recommended human dose on a mg/m2 basis increased gestation duration in rats. Prenatal and postnatal pup loss was increased and developmental retardation occurred. Postimplantation loss was increased in rabbits at doses of 1 mg/kg or greater, equal to or greater than 0. 5 times the maximum recommended human dose on a mg/m2 basis. Tizanidine has not been studied in pregnant women. Tizanidine should be given to pregnant women only if clearly needed.

Muscle relaxers, such as tizanidine, should be used with caution in pregnancy. The FDA drug label indicates that tizanidine is a Pregnancy Category C medication, meaning that it may pose a risk to the fetus. While animal studies have not shown evidence of teratogenicity, they have shown increased gestation duration, prenatal and postnatal pup loss, and developmental retardation. Tizanidine has not been studied in pregnant women, and it should only be given to pregnant women if clearly needed 3.

From the Research

Safety of Muscle Relaxers in Pregnancy

• The use of muscle relaxants during pregnancy and the postpartum period requires caution due to pharmacodynamic and pharmacokinetic differences in their action during this period 4.
• There is limited evidence on the comparative efficacy and safety of different skeletal muscle relaxants for spasticity and musculoskeletal conditions, including during pregnancy 5, 6.
• Some skeletal muscle relaxants, such as baclofen, tizanidine, and dantrolene, have been found to be effective in patients with spasticity, but their safety during pregnancy is not well established 6.
• Cyclobenzaprine has been evaluated in the most clinical trials and has consistently been found to be effective for musculoskeletal conditions, but its safety during pregnancy is not well established 6, 7.
• The choice of a skeletal muscle relaxant should be based on its adverse-effect profile, tolerability, and cost, as well as consideration of the potential risks and benefits during pregnancy 5, 7.

Adverse Effects and Risks

• Skeletal muscle relaxants have been associated with adverse effects such as dizziness, drowsiness, and dry mouth, which may be more pronounced during pregnancy 6, 7.
• Some skeletal muscle relaxants, such as dantrolene, have been associated with rare serious hepatotoxicity, which may be a concern during pregnancy 6.
• The potential adverse effects of skeletal muscle relaxants should be communicated clearly to the patient, and their use during pregnancy should be carefully monitored 7.