Dimercaptosuccinic acid

Dimercaptosuccinic acid, more commonly known by the abbreviation DMSA is a chelation therapy agent used to treat heavy metal poisoning. It is also used as a tracer in radiology when combined with technetium.
Its most common side effects are nausea and diarrhea. It should not be taken for more than 19 days in a row, and if a further course is necessary, a period of two weeks off the drug is necessary. 
Dimercaptosuccinic acid at Wikipedia