Tacrolimus is used as an immunosuppressive treatment for myasthenia gravis (MG), and monotherapy without corticosteroids may be feasible in selected patients. Still, what determines how quickly patients reach minimal manifestations (MM) under this approach has not been well characterized.

To examine the time course to MM achievement and to investigate whether baseline disease severity and early tacrolimus trough concentration are associated with the time to MM achievement in acetylcholine receptor (AChR) antibody-positive MG.

This single-center retrospective cohort study included 30 AChR antibody-positive MG patients who started tacrolimus monotherapy (3 mg/day) within 1–3 months after onset, without oral corticosteroids or early additional non-oral immunomodulatory treatments. These additional treatments included intravenous methylprednisolone pulse therapy, intravenous immunoglobulin, plasmapheresis, and molecular targeted therapies. The primary outcome was the time from tacrolimus initiation to first MM achievement. Clinical courses were compared between ocular MG (OMG) and generalized MG (GMG) using the Kaplan–Meier method. A Cox proportional hazards model stratified by baseline phenotype was used for the main analysis. Exploratory quartile-based Kaplan–Meier curves were also generated according to early tacrolimus trough concentration.

Overall, 26 of 30 patients (86.7%) achieved MM, and the median time to MM achievement was 8.0 months. The time to MM was shorter in OMG than in GMG (log-rank P = 0.044). In the phenotype-stratified Cox model, higher early tacrolimus trough concentration was associated with faster MM achievement (hazard ratio [HR] 1.38, 95% confidence interval [CI] 1.15–1.66; P < 0.001), whereas higher baseline Myasthenia Gravis Composite (MGC) score was associated with slower MM achievement (HR 0.81, 95% CI 0.70–0.92; P = 0.002). Exploratory quartile-based curves showed a descriptive tendency toward earlier MM achievement in patients with higher early trough concentrations.

Early tacrolimus trough concentration and baseline MGC were linked to the time to MM achievement under steroid-free tacrolimus monotherapy in AChR antibody-positive MG. These variables warrant prospective evaluation as early indicators for treatment reassessment.