There are several synonyms for the common phrase "going off the deep end." This expression generally refers to someone acting irrationally or losing their composure, often to an extreme degree. One possible synonym is "losing it," which is a more informal way of saying someone is becoming unhinged. Another similar phrase is "going crazy," which indicates that someone is exhibiting insane or erratic behavior. In more clinical terms, someone might be described as having a "psychotic break," meaning they are experiencing a severe mental health episode. Regardless of the phrasing, all of these synonyms imply that someone has reached a breaking point and is no longer functioning normally.