Either I'm uninformed about this matter and the opium problem remained present more than a century later, or China successfuly eradicated the opium "epidemic". If so, how did they do it?
More can of course be said, but you may find these answers I've written to be of interest (apologies for there being a few of them, they all cover different aspects):
To tie the whole thing together in summary form here (but please do read the linked answers to get this in detail):
The extent of the moral panic around opium was never proportionate to the scale of opium consumption. The anti-opium alarmism of the 1830s was largely the product of two factors: firstly, availability increased to the point where opium was reasonably affordable and therefore available to the general public rather than an exclusive party drug; secondly, there was a growing movement of largely Han Chinese, metropolitan bureaucrats who supported greater interventionism and oversight in provincial administration, which meant that Guangdong regional officials' profiting from opium became a major point of political contention. However, the end of the Opium War had changed the political landscape: now, both groups of officials were united in seeing the Qing defeat as the product of apathy on the part of the Manchu and Mongolian officials and aristocrats whose access to power had been considerably increased postwar. Opium having mainly been an issue of politics between the now-united factions in the Han literati, it basically faded from policy discussion. In 1858, amid costly civil wars, the Qing seem to have legalised opium quietly and uneventfully as a means of raising revenue.
There were, to be sure, some grassroots oppositions to opium: the Taiping, whom the Qing most needed those funds to fight, were firmly anti-opium, though this needs to be contextualised within a generally prohibitionist, anti-vice agenda that banned all substances with potentially mind-altering effects, including alcohol. A more specifically anti-opium discourse started to gain traction from the later years of the nineteenth century, though we should not exaggerate its scale: many farmers relied on opium as a cash crop, opium was increasingly widely used, and a large portion of the panic came from foreign missionaries with a rather complex set of moralising agendas. That being said, there was sufficient criticism of opium within the halls of power that the Qing after 1900 implemented a programme of reducing and eventually halting both the import and domestic cultivation of opium that was nearly successful. However, this process was derailed by the fall of the Qing in 1912, which led to the formation of regional warlord regimes who actively encouraged the cultivation of opium to raise revenues for their internecine conflicts – this, by the way, included the Kuomintang regime in Guangdong, which officially licensed opium houses. During Chiang Kai-Shek's rule before the outbreak of the Second Sino-Japanese War, the KMT set up provincial opium monopolies and eventually a national one – deeply unpopular among opium critics – as a revenue-raising measure, which would continue through the Second World War.
The effective end of direct opium smoking in China came about under Communist rule after 1949, during which active suppression campaigns largely succeeded in stamping out opium smoking in the newly-established People's Republic. However, the Communists, like the warlords and Nationalists before them, had profited heavily off opium revenues during their rule in Manchuria during the Chinese Civil War, a hypocrisy they never publicly admitted to; more importantly, other opiates had gained traction and so the suppression of opium in particular did not also entail the effective suppression of more concentrated derivatives such as morphine and heroin, the use of which has continued to be significant.