There are several synonyms for the phrase "be persistently painful," each expressing a slightly different nuance. One synonymous phrase is "be chronically sore," which emphasizes the long-term and ongoing nature of the pain. Another option is "be relentless agony," which adds a sense of severity and intensity to the discomfort. "Be continually aching" is another possible synonym, highlighting the constant nature of the pain. Finally, "be incessantly throbbing" suggests a persistent pulsing sensation, which is especially applicable to headaches and other types of pain that have a rhythmic quality. Regardless of the specific synonym used, each conveys the idea of a persistent and unrelenting experience of discomfort.