After many experiments on the action of synthetic drugs upon spirochetal diseases, Ehrlich and Hata in 1909 discovered Arsphenamine (Salvarsan, "the arsenic that saves", also known as “606”), an effective treatment for syphilis and trypanosomiasis. Arsphenamine was the first modern chemotherapeutic agent.

Manufactured by the German chemical company Hoechst, Salvarsan quickly became the most widely prescribed drug in the world. It was the first blockbuster drug, and remained the most effective drug for syphilis until penicillin became available in the 1940s. Digital facsimile of the German edition from the Internet Archive at this link. English translation, New York, 1912.

Ehrlich's discovery became the subject of the 1940 biographical film entitled Dr. Ehrlich's Magic Bullet. The film was released by Warner Bros., with some controversy over the subject of syphilis in a major studio release. 

Salvarsan first used in the treatment of rat-bite fever.