Every culture has something they can't get enough of, and something they despise for seemingly no reason. In Victoria 3 this affects economies and is represented via Obsessions and Taboos.
In Victoria 3 the Opium Wars are represented through Journal Entries and Events. Qing China begins in the midst of this crisis, but it is also possible for other unrecognized countries to experience this content if the in-game conditions are appropriate.
Any culture can become obsessed with a specific Good, where Pops of that Culture, regardless of where they are in the world, will spend significantly more on Goods they are obsessed with compared to other goods in the same Pop Needs category. So in the case of Opium in China, Han pops will spend a lot more of their wealth buying Opium than they do on Liquor or Tobacco.
This naturally drives up demand for Opium and therefore makes it more expensive within the Chinese market. The foreign powers selling Opium to China are making a killing by exploiting this demand and feeding the addiction. Cultures can develop new Obsessions over time, and you’ll need to react to changes in pop demands as a result.
On the other hand, Religions have Taboos against certain goods. For instance, Muslim faiths have a Taboo against the consumption of Liquor and Wine. This has the opposite effect from a Cultural Obsession, where pops following these religions will spend much less on purchasing that Good compared to other Goods in that category.
So Muslims will typically buy Tobacco and Opium instead of Liquor, and they will buy Tea or Coffee instead of Wine. Just as in real life, not everybody completely adheres to the tenets of their faith, and so these act as powerful modifiers on purchasing decisions rather than total “bans” on consumption. Unlike Obsessions, Taboos are static throughout the game.