Only some of the small stones can be dissolved by medications, such as cystine stones and some uric acid stones, which can be treated by dissolving the stones using, for example, sodium citrate, alpha-mercaptopropionylglycine and acetylcysteine. Stones are categorized according to their composition and are commonly found as calcium oxalate stones, phosphate stones, urate stones, carbonate stones, and cystine stones. The smaller urate stones and cystine stones can be dissolved by medication, with sodium citrate used for the former and alpha-mercaptopropionylglycine and acetylcysteine used for the latter. The rest of the stones cannot be dissolved by drugs, and the purpose of using drugs is mainly to prevent stone formation as well as to reduce the cramping pain caused by the stones.