There are several synonyms for the phrase "undergo agony." The most common synonym is "suffer," which refers to experiencing pain or distress in the physical or emotional sense. Another synonym is "endure," which means to bear or withstand difficult circumstances. "Tolerate" and "bear" are also synonyms that imply a sense of enduring hardship. "Experience pain" and "go through hardship" are other possible synonyms. Additionally, "feel anguish" and "be tormented" are more intense synonyms that connote extreme levels of suffering. Regardless of the synonym used, the phrase "undergo agony" conveys a sense of intense pain and discomfort.