The Brits had a trade deficit with China and was being bled of silver, so in order to make up the trade deficit, they started slanging opium.
Opium Wars - Wikipedia
The 
First Opium War broke out in 1839 between 
China and 
Britain and was fought over trading rights (including the right of 
free trade) and Britain's diplomatic status among Chinese officials. In the eighteenth century, China enjoyed a trade surplus with Europe, trading 
porcelain, 
silk, and 
tea in exchange for 
silver. By the late 17th century, the 
British East India Company (EIC) expanded the cultivation of 
opium in the 
Bengal Presidency, selling it to private merchants who transported it to China and covertly sold it on to Chinese smugglers.
[4] By 1797, the EIC was selling 4,000 chests of opium (each weighing 77 kg) to private merchants 
per annum.
[5]
In earlier centuries, opium was utilised as a medicine with 
anesthetic qualities, but new Chinese practices of smoking opium recreationally increased demand tremendously and often led to smokers developing addictions. Successive 
Chinese emperors issued edicts making opium illegal in 1729, 1799, 1814, and 1831, but imports grew as smugglers and colluding officials in China sought profit.
[6] Some American merchants entered the trade by smuggling opium from Turkey into China, including 
Warren Delano Jr., the grandfather of twentieth-century President 
Franklin D. Roosevelt, and 
Francis Blackwell Forbes; in 
American historiography this is sometimes referred to as the 
Old China Trade.
[7] By 1833, the Chinese opium trade soared to 30,000 chests.
[5] British and American merchants sent opium to warehouses in the free-trade port of 
Canton, and sold it to Chinese smugglers.
[6][8]
In 1834, the EIC's monopoly on British trade with China ceased, and the opium trade burgeoned. Partly concerned with moral issues over the consumption of opium and partly with the outflow of silver, the 
Daoguang Emperor charged Governor General 
Lin Zexu with ending the trade. In 1839, Lin published in Canton an 
open letter to Queen Victoria requesting her cooperation in halting the opium trade. The letter never reached the Queen.
[9] It was later published in 
The Times as a direct appeal to the British public for their cooperation.
[10] An edict from the Daoguang Emperor followed on 18 March,
[11] emphasising the serious penalties for opium smuggling that would now apply henceforth. Lin ordered the seizure of all opium in Canton, including that held by foreign governments and trading companies (called factories),
[12] and the companies prepared to hand over a token amount to placate him.
[13][
page needed] 
Charles Elliot, Chief Superintendent of British Trade in China, arrived 3 days after the expiry of Lin's deadline, as Chinese troops enforced a shutdown and blockade of the factories. The standoff ended after Elliot paid for all the opium on credit from the 
British government (despite lacking official authority to make the purchase) and handed the 20,000 chests (1,300 metric tons) over to Lin, who had them 
destroyed at Humen.