As is often the case these days, I've discussed similar questions quite often, the answers to which are linked here, but I felt it worth at least a brisk response tailored to this particular version thereof, especially as my own thoughts have evolved.

The core complication with discussing opium in China is that there is more than one approach, although these approaches are not mutually contradictory. One is a sort of 'objective' or 'material' approach, employed quite a bit by Dikötter et al. in Narcotic Culture (2004), which tries to assess opium consumption in largely quantitative and scientific terms, and argue on that basis how many opium consumers there were and how severe their consumption was. The other is a more 'subjective' or 'cultural' approach that takes a purely qualitative, cultural view of opium, exemplified by Yangwen Zheng's The Social Life of Opium in China (2005), and mainly concerns discourses around opium consumption. Note, of course, that even the same work can employ both – Narcotic Culture includes plenty of discursive analysis as well as data, and so too does Man-Houng Lin's China Upside Down. Both have their uses and limitations, which may be more or less apparent depending on your route of enquiry, and by extension, each draws a slightly different conclusion.

If we look at things purely from a hard, material standpoint, then the trajectory of opium consumption in China basically just went upward from 1800 to 1910, and fluctuated a bit until the 1950s when the Communist Party began a relatively successful crackdown on drug usage. Up until the 1870s this was driven by an increase in opium imports, from about 4000 chests per year in 1800 to a peak of 80,000 in 1880. Thereafter, imported opium was increasingly outcompeted by domestic opium production within China itself, peaking at 540,000 chests per annum in 1906, with the collapse of the Qing in 1911 and the instability of the ROC making subsequent output hard to quantify with certainty. How many opium smokers that translates to is hard to quantify, as we would need to answer a lot of questions to define what a 'smoker' even is (as just one example, is just smoking on special occasions enough to count, or do you have to do it with some degree of regularity?), and we also don't have a good estimate for average consumption, which may range anywhere from eight to twenty grams per day depending on which contemporary observers' estimates you go by, what their metrics were, and based on what contexts. Now, we can give a relative assessment of the quantities of opium consumption over time: for instance, there were probably about 150x as many smokers in 1906 as in 1800, if we presume that the average consumption amount was the same in both years (not necessarily a self-evident assumption to make), given 4000 imported chests a year in 1800, and 50,000 imported and 540,000 domestically-produced chests for a total of 590,000 in 1906. But that's not the same as an absolute number relative to population, and even that would, moreover, only be the first step towards assessing the economic, societal, and cultural effects of opium use.

The economic side of things, which is what you angle at, has been the subject of considerable debate aimed squarely at the material dimensions thereof. Traditionally, the argument went that the opium trade directly drained China of silver and wreaked havoc on its monetary economy, and latterly opium addiction destroyed productivity. The revisionist scholarship has disputed both of these points. On the economics front, while there is no firm consensus as to what did cause the 'silver drain', there is agreement as to it not being opium. Man-Houng Lin argues that it related primarily to downturns in Latin American precious metal production in the 1820s, while Richard von Glahn and Werner Burger have argued that it instead relates to failures in Qing monetary policy and the over-minting of copper coinage, in conjunction with an outflow of silver, but on a substantially smaller scale than previously believed (von Glahn proposes a silver outflow in 1818-54 of less than half what Lin calculated). To be honest, I have to admit defeat on ever really understanding the economics, but the broad conclusion is that the Qing's early 19th century economic crisis had far more to do with the vicissitudes of monetary policy than merely the introduction of opium, especially as Qing China started running an annual trade surplus again from the late 1850s until the 1880s, despite year-on-year increases in opium imports in the same period. On the productivity front, this is where it gets more complicated and contentious. Dikötter et al. argued that the anecdotal correlation between opium and lack of productivity by foreign observers can be explained by virtue of opium being a recreational drug: people smoked when they had nothing to do; they didn't stop doing things because they smoked. Also highlighted was data that showed that at the local level, opium consumption decreased during periods of economic downturn, suggesting that opium users were able to moderate their consumption in line with financial stress. Dikötter et al.'s arguments verge on the polemical, but there are, I would argue, some valuable takeaways, including the incredible difficulty of attempting to ascribe any degree of quantified economic impact to opium and opium alone.

When we look at opium from a cultural standpoint, the situation becomes a little more fluid. We can point to the existence of a moral panic over opium on the part of some elites in the late 1830s and early 1840s, but this died down in the wake of the Opium War as other matters became the new hot-button political issues of their time. There were some flare-ups of grassroots anti-opium action in the following decades, most notably on the part of the Taiping, in conjunction with considerable argument for prohibition coming from the missionary community and portions of the foreign diplomatic community as well. But a large-scale moral objection to opium within China would not resurface until around the turn of the twentieth century, leading to attempts at control and prohibition such as the Anglo-Chinese opium treaty of 1906, before being frustrated by the emergence of warlord regimes and their use of opium as a means of raising revenue.

Assessing the actual quantity of opium users is, as noted, difficult, though it must be said that the situation is not as dire as a total lack of information, and having some kind of ballpark figure does help to contextualise claims about the economic and social effects of the drug. However, we run into the potential issue that the number may never have been that high. Estimates consistently seem to have put the number not much higher than about 1%: claims in the early 1830s circulated by and in response to moral crusaders like Huang Jueci put it at one in every hundred; Robert Hart in 1871 estimated it at below a third of one percent; Xue Fucheng in 1891 estimated there were 4 million users out of 400 million total population; and a 1935 survey suggested there were 3.73 million opium users out of a total national population of 479 million. For a modern comparison, a July 2019 Gallup poll saw 12% of Americans self-identifying as marijuana users. When we have that in mind, how much effect would 1% (at most) of the population consuming a particular narcotic have? Not nothing, certainly, but it's a small enough number to at least cause questions to be asked about whether the contemporary moral panic was ever proportionate to the actual material reality, detrimental as that may well have been.

Can someone recommend a good book on the Opium wars in China