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

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The muscle relaxant effects of botulinum toxin diminish or wear off after several months because a new presynaptic terminal "sprouts" from the axon that was originally affected by the toxin. This process is called collateral sprouting and it allows the affected muscle to regain some of its function. 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