[5] Common side effects include headache, feeling tired, dizziness, and dry mouth.

[10] Cyclobenzaprine is used, in conjunction with physical therapy, to treat muscle spasms that occur because of acute musculoskeletal conditions.

[11] After sustaining an injury, muscle spasms occur to stabilize the affected body part, which may increase pain to prevent further damage.

[medical citation needed] Side effects such as sedation and ataxia are also less pronounced with nonbenzodiazepine antispasmodics.

[21][22] In general, the National Committee for Quality Assurance recommends avoiding the use of cyclobenzaprine in the elderly because of the potential for more severe side effects.

[12] Rare but potentially critical complications are cardiac arrest, abnormal heart rhythms, severe low blood pressure, seizures, and neuroleptic malignant syndrome.

[12] The potential harm is increased when central nervous system depressants and antidepressants are also used; deliberate overdose often includes other drugs.

At least one study also found increased risk of serotonin syndrome when cyclobenzaprine was taken with the serotonergic drugs duloxetine or phenelzine.

[27] Cyclobenzaprine is a centrally acting muscle relaxant with a chemical structure that is very similar to those of tricyclic antidepressants like amitriptyline and imipramine.

It is very similar in chemical structure to tricyclic antidepressants like amitriptyline and imipramine, which are likewise dibenzocycloheptenes.

[6] Cyclobenzaprine differs from amitriptyline in structure only by the presence of a single double bond within the tricyclic ring system.

[37] Two Phase 3 clinical trials reported that an experimental sublingual formulation of cyclobenzaprine, TNX-102 SL, was able to reduce pain and improve sleep quality in patients with fibromyalgia.

[38][39] A separate Phase 3 clinical trial evaluated TNX-102 SL in patients with military-related post-traumatic stress disorder (PTSD) and reported the drug did not provide a sustained, significant improvement in overall PTSD severity, but it did demonstrate improvements in sleep quality during the 12-week trial.