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I've never taken or prescribed Dover's powder, but I suspect that the Ipecacuanha therein was more a safeguard against an overdose (having strong emetic effects, which we used to use in Casualty) than an expectorant.
I was a child in the 1960s, and I used to take, when suffering from bad coughs or similar, a preparation which was not called ‘Dover’s’ but which did contain, I feel sure, opium and ipecacuanha: it was a really useful remedy because, as Professor Banatvala points out, it was ‘an ideal preparation, its opium content having analgesic and soporific properties’.
As an aside, I feel sure I’ve read that when Bayer marketed both Heroin and Aspirin, virtually simultaneously in the late 1890s, that Bayer was very optimistic about sales of Heroin because it suppressed coughing, while being less certain that Aspirin had a future.