Why it wears off, and why on a clock

The nerve is not destroyed and the muscle is not damaged. The ending sprouts new terminals and rebuilds SNAP-25, and over about three to four months the original junction takes over again. That rebuild is why the effect is reliably temporary and why the timing is so consistent between people. In facial use the duration typically runs three to four months, and in a thick muscle like the masseter a single session peaks near three months and can read for six to twelve.