DMSA (meso-2,3-dimercaptosuccinic acid) a prescription drug and a heavy-metal chelating agent, is shown to act both as a sulfur source and a capping agent in the aqueous synthesis of CdS quantum dots under mild conditions. Release of sulfur from DMSA depends on the solution pH and the reaction temperature. Combination of 70 degrees C and pH 7.5 was determined as the best reaction conditions for a well-controlled reaction. Changing the SH/Cd ratio from 2.5 to 7 provides QDs emitting from blue to orange with 6-9% quantum yield with respect to Rhodamine 2B. Viability tests performed with HeLa and MCF-7 cell lines indicate a very low cytotoxicity. Mild reaction conditions and biocompatibility makes these particles valuable candidates for bio applications.