Opium is produced in swamp regions, and consumed by markets in desert, plain, forest, and mountain provinces. All markets (except those in swamp regions) have some level of opium demand. However, this demand will not be visible in the market report at low levels.
Satisfying opium demand in a market will cause the next month's demand to be higher. As peasants become addicted to opium, the increased demand will be shown in the location's market report. If no opium is sold to a market, the demand will fall.
Opium adversely affects the city's tax base. The more opium the market buys, the more tax revenues will be reduced.