"During the 18th century, there was great demand for Chinese luxury goods (particularly silk, porcelain, and tea) in Europe, which created a trade imbalance quite like the one we have now. To counter this flow of European silver, the British East India Company began to grow opium in Bengal and U.P., allowing private British merchants to sell it to Chinese smugglers for illegal sale in China.
The idea of exporting opium to China started with Warren Hastings (the first governor general of British India) in 1780. It was difficult to sell it at first, but the British pushed it hard. The trade began to thrive and opium exports increased from 4,000 chests per year at the beginning of the 19th century to more than 60,000 chests by the 1880s. Narco-trade brought as much as one-fifth of all the revenue of British India."