Fan Casting Death Becomes Her (1958)

When a novelist loses her man to a movie star and former friend, she winds up in a psychiatric hospital. Years later, she returns home to confront the now-married couple, looking radiant. Her ex-husband's new wife wants to know her secret, and discovers that she has been taking a mysterious drug which grants eternal life to the person who drinks it. The actress follows suit, but discovers that immortality has a price.