Tiopronin is a prescription thiol drug used primarily in the treatment of severe homozygous cystinuria. Patients with cystinuria excrete high levels of cystine in their urine and are at risk for kidney stone formation. Tiopronin is used as a second-line therapy to control the rate of cystine precipitation and excretion, and prevent kidney stone formation. It is used after a failure of the non-pharmacological first line treatment consisting of increased fluid intake, restriction of sodium and protein, and urinary alkalinization. As cystinuria is a relatively rare disease, tiopronin is classified as an orphan drug and is not patented in the United States. It is similar to d-penicillamine in use and efficacy, but offers the advantage of far less adverse effects. Tiopronin is dosed on an individual basis using close monitoring of urinary cystine concentrations and urinary output. Tiopronin is a chelating agent. It works by removing extra cystine (the cause of kidney stones) from the urine, which keeps the kidney stones from forming. It works by reacting with urinary cysteine to form a more soluble, disulfide linked, tiopronin-cysteine complex.