Many studies in vitro compared the onset of action, maximum efficacy, and duration of botulinum toxin type A (BoNT/A) preparations. In this study, we analyzed the onset of action of BoNT/A preparation free of complexing proteins in patients with upper limb spasticity post stroke up to 30 days after treatment. 75 patients affected by Biceps Brachii spasticity were enrolled. Outcome measures were instrumental muscle tone modification (myometric measurement), improvement of Modified Ashworth Scale (MAS), improvement of elbow's passive extension, and improvement of compound muscle action potential (cMAP) evaluated by electroneurography. We analyzed data at t0 (pre-injection), t1 (1 day after), t2 (7 days after), t3 (14 days after), and t4 (30 days after). All measurements decreased at t2, t3, and t4 with initial improvement at t2 and maximum improvement at t4; no statistical difference at t1 was found. This study demonstrated the onset of Incobotulinumtoxin A efficacy started after 7 days; this rapid action and efficacy of BoNT/A preparation could improve an intensive rehabilitation program after some days post-injection. Early clinical onset of action could be by the absence of complexing proteins in the preparation.