Drug usage was *NOT* declining, it was going up after the civil war. Drugabuse.gov is another source that contradicts your claim =>
The Institute of Medicine estimated that by 1900, perhaps 300,000 Americans were addicted to opiates.
http://www.drugabuse.gov/international/question-2-what-history-opioid-addiction-in-united-states
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The population of the US in 1900 was 76M, which works out to an addiction rate of 0.4%. That is less than the 400,000 estimated addicts in a population of 50M in 1880. That's an addiction rate of 0.8%.
300,000 is < 400,000, and 0.4% is < 0.8%. So addiction WAS declining between 1880 and 1900.
The Institute of Medicine estimated that by 1900, perhaps 300,000 Americans were addicted to opiates. "Estimated" is not a real number. It's a number pulled out of your @$$.
So addiction WAS declining between 1880 and 1900.
Extrapolating with an absence of real data. Civil war soldiers (those 400,000 addicts who created the initial market) died off. A 20 year old in 1861 would be ~60 by 1900. With war injuries I suspect the attrition rate would be higher than the normal population, especially if they managed to feed their habit.
Anyone who seriously argues that adding numerous cocainoids and opiates to the market is going to result in a decrease in addiction is a person not to be taken seriously. Narcotic laced Patent medicines exploded during this period, and only an idiot would believe this was resulting in lower usage.
You don't have any real numbers.