<human>:  When did the British start trading opium with China?
<bot>: The British started trading opium with China in the 1840s. Opium was a very popular drug in the 19th century. It was a highly addictive drug that was derived from the poppy plant, which is used to make morphine. Opium was used in a variety of ways, including as a pain reliever and cough suppressant. Opium was also used in medicines and cosmetics, and was even used as a food additive to make sweets more palatable.