Tiopronin, sold under the brand name Thiola, is a medication used to control the rate of cystine precipitation and excretion in the disease cystinuria.

[5][6] Tiopronin is indicated, in combination with high fluid intake, alkali, and diet modification, for the prevention of cystine stone formation in people 20 kilograms (44 lb) and greater with severe homozygous cystinuria, who are not responsive to these measures alone.

[1][2] Tiopronin may present a variety of side effects, which are broadly similar to those of D-penicillamine and other compounds containing active sulfhydryl groups.

[8] Tiopronin works by reacting with urinary cysteine to form a more soluble, disulfide linked, tiopronin-cysteine complex.

[11] It may also be used for Wilson's disease (an overload of copper in the body), and has also been investigated for the treatment of arthritis,[12][13] though tiopronin is not an anti-inflammatory.