Increasing the Dose of Onabotulinum Toxin A From 400 to 600 Units for Post-stroke Spasticity: A Retrospective Study on Safety and Muscle Selection

In Japan, the approved maximum dosage of Onabotulinum Toxin A (BoNT-A) (BOTOX, Allergan plc, Irvine, CA, USA) for upper and lower limb spasticity was increased to 600 units in November 2022. As reports on this high dosage are rare internationally, this study aimed to confirm its safety and evaluate the muscles administered. The study included 24 stroke patients (18 men, six women) treated with 600 units of BoNT-A between December 2024 and January 2025. Medical records were reviewed for treatment history, including the number of injection cycles and target muscles. On the final injection day, patients were verbally surveyed about side effects of the dose increase (from 400 to 600 units), changes in spasticity, functional improvement, and their intent to continue the regimen. The patients had been receiving BoNT-A therapy for a mean duration of 7.5 years. Eight key muscles, including the pectoralis major, biceps brachii, hamstrings, and gastrocnemius, were injected in over 70% of cases. The study found no side effects or adverse events resulting from the dose increase in any patient. Furthermore, all 24 patients reported a greater reduction in spasticity and noted that it was easier to use their paralyzed side. All patients expressed their intention to continue receiving the 600-unit dose. In conclusion, given the long-term treatment history, lack of adverse events, and unanimous patient-reported benefits, the safety of increasing the BoNT-A dose to 600 units for limb spasticity is considered to be assured.