Can Flexeril (cyclobenzaprine) cause tachycardia?

Flexeril (Cyclobenzaprine) and Tachycardia

Yes, Flexeril (cyclobenzaprine) can cause tachycardia, and this is specifically listed as one of the most common effects associated with cyclobenzaprine overdose in the FDA drug label. 1

Mechanism and Risk

Cyclobenzaprine is structurally related to tricyclic antidepressants, which are known to affect cardiac function. According to the FDA drug label:

• Tachycardia is one of the most common manifestations associated with cyclobenzaprine use, particularly in overdose situations
• Tricyclic antidepressants, including cyclobenzaprine, have been reported to produce arrhythmias and sinus tachycardia 1
• The medication can cause prolongation of conduction time, which may lead to more serious cardiovascular events

Clinical Considerations

Dose-Related Effects

• At therapeutic doses (5-10 mg TID), cardiovascular side effects are less common but still possible
• The 5 mg TID regimen has been shown to be as effective as 10 mg TID with a lower incidence of sedation 2
• Higher doses significantly increase the risk of tachycardia and other cardiovascular effects

Monitoring

When prescribing cyclobenzaprine:

• Monitor for signs of tachycardia, especially in patients with pre-existing cardiovascular conditions
• Be aware that QRS duration ≥0.10 seconds may indicate severity of cardiovascular effects in overdose cases 1
• ECG monitoring is recommended if tachycardia or other cardiac symptoms develop

High-Risk Situations

Drug Interactions

Cyclobenzaprine can interact with other medications that may increase the risk of tachycardia:

• Serotonergic agents (SSRIs, SNRIs, TCAs, tramadol, bupropion, meperidine, verapamil, MAOIs) can lead to serotonin syndrome, which may present with tachycardia 1, 3
• Concomitant use with MAO inhibitors is contraindicated 1
• Use with alcohol, barbiturates, and other CNS depressants may enhance adverse effects 1

Overdose Management

In cases of overdose with tachycardia:

• Obtain an ECG and initiate cardiac monitoring immediately
• Establish intravenous access and initiate gastric decontamination if appropriate
• For QRS widening, serum alkalinization using intravenous sodium bicarbonate is recommended
• Dysrhythmias may respond to lidocaine, bretylium, or phenytoin
• Type 1A and 1C antiarrhythmics are generally contraindicated 1

Special Populations at Risk

• Patients with pre-existing cardiovascular disease
• Those with conduction abnormalities
• Patients taking other medications that affect cardiac conduction
• Elderly patients who may be more sensitive to cardiovascular effects

Clinical Decision Making

When considering cyclobenzaprine for muscle spasm:

1. Assess cardiovascular risk factors before prescribing
2. Start with lower doses (5 mg TID) in patients with cardiovascular concerns
3. Monitor for tachycardia and other cardiovascular symptoms
4. Consider alternative muscle relaxants in patients with significant cardiac history
5. Discontinue if tachycardia or other cardiac symptoms develop

Remember that while tachycardia is a recognized side effect, most patients tolerate cyclobenzaprine well at therapeutic doses, with somnolence and dry mouth being the most commonly reported adverse effects 2.