The series is set in 1889. Diacetyl morphine however was first synthesized at Bayer by the Chemist Felix Hoffmann on August 21st, 1897 (it was first synthesized by C.R.A. Wright in 1874) and subsequently sold, under the trade name Heroin, as a cough medicine and against pain, depression and several other ailments including stomach cancer. Only in the years before the first World War the fact that if snuffed or injected it was much more effective than Morphium was spread by word of mouth.
American character Jackson uses the off-hand remark, "Knock yourself out". While that is typically used only in US English it is a slang term that did not became popular until the 1930s, with some sources stating its earliest recorded use was 1942.