What is the onset of action for Botulinum neurotoxin?

• A. 12-72 hours, though can be as soon as 2 hours
• B. 2-6 hours
• C. 3-5 days
• D. 1-2 weeks

Answer: (A)

Onset of action for Botulinum neurotoxin hinges on how long it takes for the toxin to bind, be internalized, and cleave SNARE proteins at the neuromuscular junction so acetylcholine release is blocked. Because these steps require time for toxin diffusion and enough SNARE disruption to reduce motor nerve signaling, clinical effects don’t appear immediately. In practice, noticeable weakening of injected muscles typically develops within about 12 to 72 hours, with most patients seeing early effects within the first couple of days and progression over a few days. In rare cases with higher doses or very rapid local diffusion, effects can begin as soon as around 2 hours, but that is not the usual course.