Is Right Bundle Branch Block a Contraindication to Flexeril?
Yes, right bundle branch block (RBBB) is an absolute contraindication to Flexeril (cyclobenzaprine) according to the FDA drug label, which explicitly lists "heart block or conduction disturbances" as contraindications. 1
FDA-Mandated Contraindications
The FDA label for cyclobenzaprine clearly states that the drug is contraindicated in patients with "arrhythmias, heart block or conduction disturbances, or congestive heart failure." 1 RBBB falls under the category of conduction disturbances, making this an absolute contraindication regardless of whether the RBBB is symptomatic or asymptomatic.
Why This Matters Clinically
Cyclobenzaprine is structurally similar to tricyclic antidepressants and shares their cardiac effects. 1 The concern with using cyclobenzaprine in patients with conduction disturbances stems from:
Risk of worsening conduction delays: Patients with RBBB already have impaired conduction through the right bundle branch, and adding a medication with anticholinergic and membrane-stabilizing effects could further compromise cardiac conduction 1
Potential progression to complete heart block: While isolated RBBB may be benign in many cases 2, the addition of a pro-arrhythmic medication creates unnecessary risk, particularly if the patient has bifascicular block (RBBB plus left anterior or posterior hemiblock) which carries higher risk of progression to complete AV block 2, 3
Clinical Decision-Making Algorithm
If RBBB is documented on ECG:
- Do not prescribe Flexeril under any circumstances 1
- Consider alternative muscle relaxants without cardiac conduction effects (e.g., methocarbamol, tizanidine)
- Document the contraindication clearly in the medical record
If no recent ECG is available:
- Obtain a 12-lead ECG before prescribing Flexeril in patients with risk factors for conduction disease (age >60, history of cardiac disease, hypertension, diabetes) 3, 4
Common Pitfalls to Avoid
Do not assume isolated RBBB is "benign enough" to ignore this contraindication: Even though asymptomatic isolated RBBB may not require specific cardiac treatment 2, the FDA contraindication exists because cyclobenzaprine can precipitate serious cardiac events in patients with any conduction disturbance 1
Do not rely on patient symptom status: The contraindication applies to both symptomatic and asymptomatic RBBB, as the FDA label does not distinguish between these presentations 1
Avoid the temptation to use "just a short course": The contraindication is absolute and does not provide exceptions for duration of therapy 1