There are numerous synonyms for the phrase "become worse." Some popular options include "deteriorate," "decline," "degenerate," "worsen," and "dwindle." Additionally, "regress," "sink," and "slump" can also be used to describe a downward trend or shift in circumstances. In some contexts, you might choose to use "weaken," "atrophy," or "debase," depending on the specifics of the situation. Regardless of which term you select, these synonyms all convey a sense of decline, loss, or diminishing quality, making them useful for describing everything from health conditions to economic trends. Remember to choose the word that suits your intended tone and emphasis.