How was heroin discovered?

Kind of a difficult question to answer. Opium, morphine, and laudanum have been used to get high for centuries, so when heroin hit the market as a cough suppressant and "non-addictive morphine substitute"...it probably didn't take people very long to figure out that it could be used for non-medical purposes. Heroin was first synthesized in 1874 and was completely illegal in the US by 1924; take your answer from that.