Death Becomes Her (1992)

Helen, a writer, and Madeline, an actress, have hated each other for years. Madeline is married to Ernest, who was once Helen's fiance. After she recovers from a mental breakdown, Helen vows revenge by stealing back Ernest and plotting to kill Madeline. Both rivals have secretly drunk a miracle cure for aging; they accidentally discover, when each tries to eliminate the other, that they have become immortal and that "life" will never be the same again.