Can Flexeril (cyclobenzaprine) cause tachycardia?

Flexeril (Cyclobenzaprine) and Tachycardia

Yes, Flexeril (cyclobenzaprine) can cause tachycardia, and it is specifically listed as one of the common manifestations of cyclobenzaprine overdose in the FDA drug label. 1

Mechanism and Risk

Cyclobenzaprine is structurally related to tricyclic antidepressants, which are known to cause cardiovascular effects including:

• Tachycardia (increased heart rate)
• Arrhythmias
• Prolongation of cardiac conduction time

The FDA drug label explicitly states that "tricyclic antidepressants have been reported to produce arrhythmias, sinus tachycardia, prolongation of the conduction time leading to myocardial infarction and stroke." 1

Evidence of Tachycardia Risk

The strongest evidence comes directly from the FDA-approved drug label for cyclobenzaprine, which clearly identifies tachycardia as one of the most common effects associated with cyclobenzaprine overdose. 1

While tachycardia is more commonly reported in overdose situations, the structural similarity to tricyclic antidepressants suggests this cardiovascular effect may occur even at therapeutic doses in susceptible individuals.

High-Risk Scenarios

Tachycardia risk may be increased in:

1. Patients taking higher doses (10 mg TID vs. lower doses) 2
2. Patients with pre-existing cardiovascular conditions
3. Patients taking other serotonergic medications (SSRIs, SNRIs, TCAs, etc.) 1, 3
4. Elderly patients or those with impaired metabolism of the drug

Serotonin Syndrome Consideration

Cyclobenzaprine can contribute to serotonin syndrome when combined with other serotonergic medications, which may present with:

• Mental status changes (confusion, agitation)
• Autonomic instability including tachycardia
• Neuromuscular abnormalities (tremor, hyperreflexia)
• Gastrointestinal symptoms 1, 3

Clinical Management Recommendations

If tachycardia develops in a patient taking cyclobenzaprine:

1. Discontinue the medication if tachycardia is significant or symptomatic
2. Consider dose reduction to 5 mg TID which has shown similar efficacy to 10 mg TID with fewer side effects 2
3. Monitor cardiac function with ECG if symptoms persist or are severe
4. Be vigilant for QRS widening as this may indicate cyclobenzaprine toxicity 1
5. Avoid combining with other medications that can cause tachycardia or have cardiac effects

Prevention Strategies

To minimize the risk of tachycardia:

• Start with lower doses (5 mg TID) which have shown similar efficacy to higher doses with fewer adverse effects 2
• Avoid combining with other medications that can cause serotonin syndrome
• Use caution in patients with pre-existing cardiovascular disease
• Monitor for symptoms of increased heart rate, especially when initiating therapy

Conclusion

Clinicians should be aware that cyclobenzaprine can cause tachycardia, particularly in overdose situations but potentially at therapeutic doses in susceptible individuals. This cardiovascular effect is consistent with the drug's structural similarity to tricyclic antidepressants and is clearly documented in the FDA drug label.