Introduction: Botulinum toxin is frequently administered serially to maintain therapeutic muscle paralysis, but repeated dose effects on muscle function are largely unknown. This study characterized the muscle response to 2 onabotulinum toxin (BoNT) injections separated by 3 months.
Methods: Animal subjects received a single toxin injection (n=8), 2 BoNT injections separated by 3 months (n= 14), or 1 BoNT and 1 saline injection separated by 3 months (n=8).
Results: The functional effect of 2 serial injections was exponentially greater than the effect of a single injection. While both groups treated with a single BoNT injection had decreased torque in the injected leg by about 50% relative to contralateral legs, the double BoNT injected group had decreased torque by over 95% relative to the pre-injection level. Both single and double BoNT injections produced clear signs of fiber-type grouping.
Discussion: These experiments demonstrate a disproportionately increased effect of a second BoNT injection.

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