When something "runs its course," it means it has come to a natural end or conclusion. Synonyms for this phrase include "runs its natural course," "takes its natural course," "reaches its natural conclusion," and "comes to an end." Other possible substitutes are "expires," "runs out," "burns out," "fades away," "dies out," "subsides," "dissipates," or "abates." These synonyms are particularly useful when describing the life cycle of a phenomenon, such as a disease, trend, or social movement. By using alternative phrases, one can add variety and nuance to their writing, making it more engaging and expressive.