Muscle relaxant effects of botulinum toxin appear to diminish or wear off after several months because

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Botulinum toxin blocks the release of acetylcholine from the presynaptic terminal, which leads to muscle relaxation. However, over time, a new presynaptic terminal "sprouts" from the axon that was originally affected by the toxin, leading to the recovery of muscle function. This is why the muscle relaxant effects of botulinum toxin wear off after several months. The immune system does not destroy the toxin, the muscle does not hypertrophy and generate more muscle fibers, the liver does not metabolize the toxin, and the toxin is not excreted intact in the urine.

Muscle Relaxant Effects of Botulinum Toxin Appear to Diminish or Wear