Sublingual Cyclobenzaprine for Fibromyalgia: Pharmacokinetic Rationale, Clinical Evidence, and Place in Therapy

Fibromyalgia (FM) is a chronic nociplastic pain syndrome characterized by widespread pain, nonrestorative sleep, fatigue, and cognitive dysfunction, affecting approximately 2-6% of U.S. adults. Although oral cyclobenzaprine has been used off label for decades, its clinical utility has been limited by a narrow benefit-to-harm ratio, with the number needed to treat for symptomatic improvement approximating the number needed to harm for at least one adverse event at conventional doses. This review examines the mechanistic, pharmacokinetic, and clinical evidence supporting the August 2025 FDA approval of sublingual cyclobenzaprine (TNX-102 SL; Tonmya), the first new pharmacologic treatment approved for FM in more than 15 years.

Emerging evidence suggests that cyclobenzaprine's therapeutic effects in FM are primarily mediated through antagonism of 5-HT2A and α1-adrenergic receptors, influencing central pain processing and sleep architecture rather than peripheral muscle relaxation. The sublingual formulation produces a unique pharmacokinetic profile, with peak plasma concentrations occurring 4-5 h after bedtime administration, aligning drug exposure with the middle of the sleep period. In the pivotal phase 3 RELIEF and RESILIENT trials, TNX-102 SL demonstrated significant improvements in daily pain compared with placebo, along with benefits in sleep quality, fatigue, and Patient Global Impression of Change scores. Meta-analyses encompassing four clinical trials have reported modest efficacy comparable to established agents such as duloxetine and pregabalin. The approval of TNX-102 SL represents a significant advance in FM management by providing a therapy specifically designed to target sleep-related mechanisms implicated in nociplastic pain. By shifting adverse effects from systemic manifestations to predominantly local oral effects, the sublingual formulation may offer improved tolerability compared with oral cyclobenzaprine. TNX-102 SL appears particularly well suited for patients with sleep-predominant FM symptoms who have not responded adequately to or cannot tolerate existing first-line treatments.