A bit earlier...
http://en.wikipedia.org/wiki/Use_of_per ... s_in_sport
By 1889, "dope" was used in connection with the preparation of a thick viscous preparation of opium for smoking, and during the 1890s this extended to any stupefying narcotic drug. In 1900, dope was also defined as "a preparation of drugs designed to influence a racehorse's performance
http://en.wikipedia.org/wiki/Use_of_per ... s_in_sport